crates/oxy-core/src/repo/sync.rs at cea2e2f860c5 · andymccall/oxygen

# cea2e2f860c5 oxygen / crates / oxy-core / src / repo / sync.rs
114.5 KiB · Rust · last changed Fall back from a checkout's own token only when the hub does not know it (401), never when it refuses it for permission (refs #155) 7 days ago source blame history raw
//! Serving and talking to remotes (spec §10.2, §16).

use super::tags::TagCheck;
use super::*;

/// What a push did, for hosts (hub spec §6.12).
#[derive(Clone, Debug, Default)]
pub struct PushReport {
    pub results: Vec<RefResult>,
    /// Accepted reference updates, in request order.
    pub updated: Vec<RefUpdated>,
    /// Changes this push stored that were not already present.
    pub imported: Vec<ChangeHash>,
}

#[derive(Clone, Debug)]
pub struct RefUpdated {
    pub name: String,
    pub old_state: ObjectRef,
    pub new_state: ObjectRef,
    pub heads: Vec<ChangeHash>,
}

/// What a fetch brought in.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct FetchReport {
    /// Bookmark name, its heads, and how many changes were new.
    pub refs: Vec<(String, Vec<ChangeHash>, usize)>,
    /// Tags adopted from the remote.
    pub new_tags: usize,
    /// The remote has never presented a policy, so nothing was verified
    /// against one (spec §14.5: warn loudly).
    pub unverified: bool,
    /// What checking the host's transparency log found short of a fork
    /// (spec §14.8): attestations not logged yet, a log that could not be
    /// read. A fork fails the fetch instead.
    pub log_notes: Vec<String>,
    /// Bookmarks left as they were because their attestation could not be
    /// verified (its signer unknown here, or its signature bad), each with
    /// why (oxygenhub#266). The current bookmark is never skipped: its
    /// fetch fails instead.
    pub skipped: Vec<(String, String)>,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct PullReport {
    /// The current bookmark's heads after the pull.
    pub heads: Vec<ChangeHash>,
    /// Whether the current bookmark changed.
    pub changed: bool,
    pub checked_out: bool,
    /// Other bookmarks the pull moved: each local bookmark tracks the
    /// remote's of the same name.
    pub others: Vec<String>,
    /// What the fetch left as it was ([`FetchReport::skipped`]).
    pub skipped: Vec<(String, String)>,
}

/// The result a push of objects alone reports (a batch of a long history):
/// not a reference.
pub const OBJECTS_RESULT: &str = "objects";

/// Graph blobs asked for in one `objects` request when a shallow clone
/// fetches its checkpoint in parts (#137): their chunk lists and chunks,
/// a few MiB a request rather than one answer of gigabytes.
const CHECKPOINT_PART: usize = 2048;

impl Repo {
    // ------------------------------------------------------------ serving (spec §16)

    fn ref_attest_path(&self, bookmark: &str) -> PathBuf {
        self.oxy.join("attest").join(name_file(bookmark))
    }

    /// The latest reference attestation stored for a bookmark.
    pub fn latest_ref_attest(&self, bookmark: &str) -> Result<Option<(ObjectRef, SignedAttest)>> {
        match fs::read_to_string(self.ref_attest_path(bookmark)) {
            Ok(h) => {
                let r = ObjectRef::from_hex(h.trim())
                    .ok_or_else(|| RepoError::Msg("corrupt attestation pointer".into()))?;
                Ok(Some((
                    r,
                    SignedAttest {
                        attest: self.store.get_attest(&r)?,
                        sigs: self.store.signatures(&r)?,
                    },
                )))
            }
            Err(e) if e.kind() == std::io::ErrorKind::NotFound => Ok(None),
            Err(e) => Err(e.into()),
        }
    }

    pub(crate) fn set_ref_attest(&self, bookmark: &str, r: &ObjectRef) -> Result<()> {
        fs::create_dir_all(self.oxy.join("attest"))?;
        write_atomic(&self.ref_attest_path(bookmark), r.to_hex().as_bytes())
    }

    pub fn remote_refs(&self) -> Result<RefsResponse> {
        let view = self.view()?;
        let mut out = Vec::new();
        for (name, state) in &view.bookmarks {
            let latest = self.latest_ref_attest(name)?.map(|(_, a)| a);
            let last_serial = latest.as_ref().and_then(|a| match a.attest.body {
                AttestBody::Ref { serial, .. } => Some(serial),
                _ => None,
            });
            let attestation = latest.filter(|a| matches!(&a.attest.body, AttestBody::Ref { new_state, .. } if new_state == state));
            let accepted = match &attestation {
                Some(a) => self.acceptance_of(&a.attest.hash())?,
                None => None,
            };
            out.push(RemoteRef {
                name: name.clone(),
                state: *state,
                heads: self.store.get_state(state)?.heads,
                attestation,
                last_serial,
                accepted,
                logged: None,
                integration: vec![],
            });
        }
        let policy = self.current_policy()?.map(|(_, sp)| sp);
        let history = self
            .policy_history()?
            .into_iter()
            .filter(|sp| {
                policy
                    .as_ref()
                    .is_some_and(|p| sp.policy.serial < p.policy.serial)
            })
            .collect();
        Ok(RefsResponse {
            refs: out,
            policy,
            tags: self.tag_refs()?,
            history,
            default_bookmark: Some(self.config.bookmark.clone()),
            log: None,
            witnesses: vec![],
        })
    }

    /// Every change reachable from `heads` that is in the store.
    fn closure_in_store(&self, heads: &[ChangeHash]) -> Result<BTreeSet<ChangeHash>> {
        let mut seen = BTreeSet::new();
        let mut stack: Vec<ChangeHash> = heads.to_vec();
        while let Some(h) = stack.pop() {
            if seen.contains(&h) || !self.store.has(&h) {
                continue;
            }
            seen.insert(h);
            stack.extend(self.store.get_change_meta(&h)?.deps);
        }
        Ok(seen)
    }

    /// The changes within `depth` dependency steps of `heads` (spec §17),
    /// nearest first; depth 1 is the heads themselves.
    pub fn shallow_changes(&self, heads: &[ChangeHash], depth: u32) -> Result<Vec<ChangeHash>> {
        let mut out = Vec::new();
        let mut seen: BTreeSet<ChangeHash> = BTreeSet::new();
        let mut layer: Vec<ChangeHash> = heads.to_vec();
        for _ in 0..depth.max(1) {
            let mut next = Vec::new();
            for h in layer {
                if !seen.insert(h) || !self.store.has(&h) {
                    continue;
                }
                let c = self.store.get_change(&h)?;
                next.extend(c.deps.iter().copied());
                out.push(h);
            }
            if next.is_empty() {
                break;
            }
            layer = next;
        }
        out.reverse();
        Ok(out)
    }

    fn shallow_path(&self) -> PathBuf {
        self.oxy.join("shallow")
    }

    /// The shallow boundary: dependencies this repository does not hold (spec §17).
    pub fn boundary(&self) -> BTreeSet<ChangeHash> {
        fs::read_to_string(self.shallow_path())
            .map(|t| t.lines().filter_map(ObjectRef::from_hex).collect())
            .unwrap_or_default()
    }

    pub fn is_shallow(&self) -> bool {
        !self.boundary().is_empty()
    }

    fn write_boundary(&self, boundary: &BTreeSet<ChangeHash>) -> Result<()> {
        if boundary.is_empty() {
            let _ = fs::remove_file(self.shallow_path());
            return Ok(());
        }
        let text: String = boundary
            .iter()
            .map(|h| format!("{}\n", h.to_hex()))
            .collect();
        write_atomic(&self.shallow_path(), text.as_bytes())
    }

    /// Recompute the boundary from what is stored: every dependency of a
    /// stored change that is not itself stored.
    fn refresh_boundary(&self, imported: &[ChangeHash]) -> Result<()> {
        let mut boundary = self.boundary();
        boundary.retain(|h| !self.store.has(h));
        for h in imported {
            for d in self.store.get_change_meta(h)?.deps {
                if !self.store.has(&d) {
                    boundary.insert(d);
                }
            }
        }
        self.write_boundary(&boundary)
    }

    /// Fetch `depth` more levels of history below the shallow boundary.
    pub fn deepen(&self, remote_name: &str, depth: u32) -> Result<usize> {
        let boundary = self.boundary();
        if boundary.is_empty() {
            return Ok(0);
        }
        let remote = self.open_named_remote(remote_name)?;
        let wanted = remote.shallow(&boundary.iter().copied().collect::<Vec<_>>(), depth)?;
        let bundle = remote.fetch(&wanted, self.config.max_blob)?;
        let imported = self.import_bundle_shallow(&bundle, false)?;
        if self.config.max_blob.is_some() {
            self.register_missing_blobs(&imported, remote_name)?;
        }
        self.refresh_boundary(&imported)?;
        Ok(imported.len())
    }

    /// `closure(want) \ closure(have)`, dependencies first (spec §16.2).
    pub fn negotiate(&self, have: &[ChangeHash], want: &[ChangeHash]) -> Result<Vec<ChangeHash>> {
        let have = self.closure_in_store(have)?;
        // In a shallow repository the walk stops at the boundary, whose
        // changes the remote is assumed to hold.
        let boundary = self.boundary();
        let mut out = Vec::new();
        let mut done: BTreeSet<ChangeHash> = have.clone();
        done.extend(boundary.iter().copied());
        // Iterative post-order DFS.
        let shallow = self.is_shallow();
        let mut stack: Vec<(ChangeHash, bool)> = want.iter().rev().map(|h| (*h, false)).collect();
        while let Some((h, expanded)) = stack.pop() {
            if done.contains(&h) {
                continue;
            }
            if !self.store.has(&h) {
                // A shallow repository holds only part of its history; a
                // change it lacks is below what it cloned (in a depth-0
                // clone, any change of the checkpoint), which the remote
                // is assumed to hold, as it is for the boundary (#137).
                if shallow {
                    done.insert(h);
                    continue;
                }
                return fail(format!("change {} is not in this repository", h.short(12)));
            }
            if expanded {
                done.insert(h);
                out.push(h);
                continue;
            }
            stack.push((h, true));
            for d in self.store.get_change_meta(&h)?.deps {
                if !done.contains(&d) {
                    stack.push((d, false));
                }
            }
        }
        Ok(out)
    }

    /// Bundle changes (in the given dependency order) with their signatures,
    /// seals, identities, blobs, chunks.
    pub fn bundle_for(&self, changes: &[ChangeHash]) -> Result<Bundle> {
        self.bundle_of(changes, None, false, false)
    }

    /// Objects by reference for the `objects` endpoint; a blob brings its chunks.
    pub fn objects_for(&self, refs: &[ObjectRef]) -> Result<Vec<(ObjectType, Vec<u8>)>> {
        // In two reads in pack order (#137): a checkpoint's graph blobs come
        // this way in parts, tens of thousands for a big state.
        let mut found = Vec::with_capacity(refs.len());
        for got in self.store.get_many(refs) {
            match got {
                Ok(x) => found.push(x),
                Err(StoreError::NotFound(_)) => {}
                Err(e) => return Err(e.into()),
            }
        }
        let mut chunks = Vec::new();
        for (ty, body) in &found {
            if *ty == ObjectType::Blob {
                chunks.extend(Blob::decode(body)?.chunks);
            }
        }
        let mut out = Vec::with_capacity(found.len() + chunks.len());
        for got in self.store.get_many(&chunks) {
            match got {
                Ok(x) => out.push(x),
                Err(StoreError::NotFound(_)) => {}
                Err(e) => return Err(e.into()),
            }
        }
        out.extend(found);
        Ok(out)
    }

    /// Like `bundle_for`, leaving out blobs larger than `max_blob` bytes
    /// (and their chunks) so the receiver fetches them lazily (spec §13.5).
    ///
    /// This is what a server answers a fetch with, so it also carries what
    /// verifies the attestations it serves beside it (see
    /// [`Repo::served_attest_signers`]), also when `changes` is empty.
    pub fn bundle_for_filtered(
        &self,
        changes: &[ChangeHash],
        max_blob: Option<u64>,
    ) -> Result<Bundle> {
        self.bundle_of(changes, max_blob, false, true)
    }

    /// Like [`Repo::bundle_for_filtered`] for a fetch between repositories
    /// on one machine: changes as they are stored ([`Bundle::raw_changes`]),
    /// with their text blobs and chunks among the objects, so neither side
    /// decodes or encodes their text.
    pub fn bundle_raw_for(&self, changes: &[ChangeHash], max_blob: Option<u64>) -> Result<Bundle> {
        self.bundle_of(changes, max_blob, true, true)
    }

    /// The keys that signed the attestations this repository serves with
    /// its references: each bookmark's latest and each tag's. A fetch
    /// bundle carries their identities and certificates (spec §16.2), so a
    /// client can check an attestation whose signer signed none of the
    /// changes it receives, a delegate or a short-lived key that only
    /// moved a bookmark (oxygenhub#266). One pointer and one signature
    /// list per reference; nothing per change.
    pub fn served_attest_signers(&self) -> Result<BTreeSet<KeyId>> {
        let view = self.view()?;
        let pointers = view
            .bookmarks
            .keys()
            .map(|b| self.ref_attest_path(b))
            .chain(view.tags.keys().map(|t| self.tag_attest_path(t)));
        let mut keys = BTreeSet::new();
        for p in pointers {
            let Ok(text) = fs::read_to_string(&p) else {
                continue;
            };
            let Some(r) = ObjectRef::from_hex(text.trim()) else {
                continue;
            };
            keys.extend(self.store.signatures(&r)?.into_iter().map(|s| s.key));
        }
        Ok(keys)
    }

    /// `serving`: a fetch's answer, which carries the identities and
    /// certificates of [`Repo::served_attest_signers`] too.
    fn bundle_of(
        &self,
        changes: &[ChangeHash],
        max_blob: Option<u64>,
        raw: bool,
        serving: bool,
    ) -> Result<Bundle> {
        let mut bundle = Bundle::default();
        let mut objects: BTreeSet<ObjectRef> = BTreeSet::new();
        let mut seals: BTreeSet<ObjectRef> = BTreeSet::new();
        let add_object = |r: &ObjectRef, objects: &mut BTreeSet<ObjectRef>| {
            if !r.is_zero() && self.store.has(r) {
                objects.insert(*r);
            }
        };
        for h in changes {
            // Raw: the stored bytes, and a skeleton for the rest.
            let (c, encoded) = if raw {
                let bytes = self.store.get_typed(h, ObjectType::Change)?;
                let (skel, texts) = Change::decode_skeleton(&bytes, *h)?;
                for t in texts {
                    if let crate::objects::TextRef::Blob(r, _, _) = t {
                        let blob = Blob::decode(&self.store.get_typed(&r, ObjectType::Blob)?)?;
                        objects.insert(r);
                        objects.extend(blob.chunks);
                    }
                }
                (skel, Some(bytes))
            } else {
                (self.store.get_change(h)?, None)
            };
            let sigs = self.store.signatures(h)?;
            add_object(&c.author, &mut objects);
            for s in &sigs {
                add_object(&s.key, &mut objects);
            }
            // A replacement's new content only: its old content is the new
            // content of a change it depends on, which the receiver holds or
            // which this bundle carries (oxygen#136: removing ~1,100 binary
            // files sent them all again).
            for e in &c.edits {
                if let Edit::Replace { new: Some(b), .. } = e {
                    if self.store.has(b) {
                        if let Ok(blob) = Blob::decode(&self.store.get_typed(b, ObjectType::Blob)?)
                        {
                            if max_blob.is_some_and(|m| blob.length > m) {
                                continue;
                            }
                            objects.insert(*b);
                            objects.extend(blob.chunks.iter().copied());
                        }
                    }
                }
            }
            for s in self.store.seals_for(h)? {
                seals.insert(s);
            }
            match encoded {
                Some(bytes) => bundle.raw_changes.push((bytes, sigs)),
                None => {
                    bundle.changes.push((c, sigs));
                    bundle.hashes.push(*h);
                }
            }
        }
        for r in seals {
            let attest = self.store.get_attest(&r)?;
            let sigs = self.store.signatures(&r)?;
            for s in &sigs {
                add_object(&s.key, &mut objects);
            }
            bundle.attests.push(SignedAttest { attest, sigs });
        }
        let signers: Vec<KeyId> = if serving {
            let signers = self.served_attest_signers()?;
            for k in &signers {
                add_object(k, &mut objects);
            }
            signers.into_iter().collect()
        } else {
            Vec::new()
        };
        for r in objects {
            let (ty, body) = self.store.get(&r)?;
            bundle.objects.push((ty, body));
        }
        self.attach_certificates_for(&mut bundle, &signers)?;
        Ok(bundle)
    }

    /// Add the certificates of short-lived keys that signed what `bundle`
    /// carries, and their issuers' identities, so a receiver can tell who
    /// they stand for (spec §14.7). `also` names more signers (a reference
    /// attestation sent beside the bundle).
    pub(crate) fn attach_certificates_for(
        &self,
        bundle: &mut Bundle,
        also: &[KeyId],
    ) -> Result<()> {
        let mut keys: BTreeSet<KeyId> = also.iter().copied().collect();
        keys.extend(
            bundle
                .changes
                .iter()
                .flat_map(|(_, s)| s.iter().map(|s| s.key)),
        );
        keys.extend(
            bundle
                .raw_changes
                .iter()
                .flat_map(|(_, s)| s.iter().map(|s| s.key)),
        );
        keys.extend(
            bundle
                .attests
                .iter()
                .flat_map(|a| a.sigs.iter().map(|s| s.key)),
        );
        for k in keys {
            let Some(r) = self.store.certificate_of(&k)? else {
                continue;
            };
            let cert = self.store.get_certificate(&r)?;
            if bundle
                .certificates
                .iter()
                .any(|c| c.cert.hash() == cert.cert.hash())
            {
                continue;
            }
            // An agent's certificate brings its grant's signer too, so the
            // receiver can check the grant (oxygen#134).
            let grantor = cert.cert.grant.as_ref().map(|g| g.grant.grantor);
            for id in [Some(k), Some(cert.cert.issuer), grantor].iter().flatten() {
                if let Ok(body) = self.store.get_typed(id, ObjectType::Identity) {
                    if !bundle
                        .objects
                        .iter()
                        .any(|(t, b)| *t == ObjectType::Identity && *b == body)
                    {
                        bundle.objects.push((ObjectType::Identity, body));
                    }
                }
            }
            bundle.certificates.push(cert);
        }
        Ok(())
    }

    /// Keep the certificates a bundle brings once each verifies with the
    /// identity of the issuer it names; one that does not refuses the bundle.
    fn adopt_certificates(&self, certs: &[crate::cert::SignedCertificate]) -> Result<()> {
        for c in certs {
            let issuer = self.store.get_identity(&c.cert.issuer).map_err(|_| {
                RepoError::Msg(format!(
                    "a certificate names issuer {}, whose identity did not come with it",
                    c.cert.issuer.short(12)
                ))
            })?;
            c.verify(&issuer)
                .map_err(|e| RepoError::Msg(format!("a certificate does not verify: {e}")))?;
            self.store.put_identity(&c.cert.key)?;
            self.store.put_certificate(c)?;
        }
        Ok(())
    }

    /// A signer authorized for `reference` by what an issuer certified it
    /// as (spec §14.7): a short-lived key with a kept certificate that
    /// verifies with its issuer, gives `scope`, covers `at_ms` (a time the
    /// signer did not choose) and names a subject the policy grants.
    pub(crate) fn certified_authorized(
        &self,
        p: &crate::policy::Policy,
        signer: &KeyId,
        reference: &str,
        scope: Scope,
        at_ms: u64,
    ) -> bool {
        let Ok(Some(r)) = self.store.certificate_of(signer) else {
            return false;
        };
        let Ok(c) = self.store.get_certificate(&r) else {
            return false;
        };
        let Some(issuer) = p
            .identity(&c.cert.issuer)
            .cloned()
            .or_else(|| self.store.get_identity(&c.cert.issuer).ok())
        else {
            return false;
        };
        c.verify(&issuer).is_ok()
            && c.cert.key_id() == *signer
            && c.cert.covers(scope, at_ms)
            && p.subject_grant(reference, &c.cert.issuer, &c.cert.subject, scope)
    }

    /// Store a bundle's contents. With `strict`, every change must
    /// authenticate (signature or seal by its author or a delegate).
    pub fn import_bundle(&self, bundle: &Bundle, strict: bool) -> Result<Vec<ChangeHash>> {
        self.import_bundle_inner(bundle, strict, false)
    }

    /// Like `import_bundle`, accepting changes whose dependencies are absent
    /// (they join the shallow boundary).
    pub fn import_bundle_shallow(&self, bundle: &Bundle, strict: bool) -> Result<Vec<ChangeHash>> {
        self.import_bundle_inner(bundle, strict, true)
    }

    fn import_bundle_inner(
        &self,
        bundle: &Bundle,
        strict: bool,
        shallow: bool,
    ) -> Result<Vec<ChangeHash>> {
        let batch = self.store.begin_batch()?;
        let result = self.import_bundle_batched(bundle, strict, shallow);
        batch.finish()?;
        result
    }

    fn import_bundle_batched(
        &self,
        bundle: &Bundle,
        strict: bool,
        shallow: bool,
    ) -> Result<Vec<ChangeHash>> {
        for (ty, _) in &bundle.objects {
            if !matches!(
                ty,
                ObjectType::Chunk
                    | ObjectType::Blob
                    | ObjectType::Identity
                    | ObjectType::State
                    | ObjectType::Checkpoint
            ) {
                return fail(format!("bundle may not carry {} objects", ty.name()));
            }
        }
        // All at once: in a large batch they are compressed on every core.
        let objects: Vec<(ObjectType, &[u8])> = bundle
            .objects
            .iter()
            .map(|(ty, body)| (*ty, body.as_slice()))
            .collect();
        self.store.put_many(&objects)?;
        self.adopt_certificates(&bundle.certificates)?;
        for sa in &bundle.attests {
            let r = self.store.put_attest(&sa.attest)?;
            for sig in &sa.sigs {
                self.store.put_signature(&r, sig)?;
            }
            if let AttestBody::Seal { hashes } = &sa.attest.body {
                for h in hashes {
                    self.store.index_seal(h, &r)?;
                }
            }
        }
        // Each change's hash: from the bundle when its maker knew it, else
        // worked out once (for a change adding a whole tree, hashing is
        // chunking all its text).
        let hashes: Vec<ChangeHash> = if bundle.hashes.len() == bundle.changes.len() {
            bundle.hashes.clone()
        } else {
            bundle.changes.iter().map(|(c, _)| c.hash()).collect()
        };
        let raw_hashes: Vec<ChangeHash> = bundle
            .raw_changes
            .iter()
            .map(|(b, _)| hash_object(ObjectType::Change, b))
            .collect();
        let incoming: BTreeSet<ChangeHash> = hashes.iter().chain(&raw_hashes).copied().collect();
        let boundary = self.boundary();
        let mut imported = Vec::new();
        // Changes as stored: checked, then stored as they are, their text
        // blobs having come with the objects.
        for ((bytes, sigs), h) in bundle.raw_changes.iter().zip(raw_hashes) {
            let (skel, texts) = Change::decode_skeleton(bytes, h)?;
            for d in &skel.deps {
                if !self.store.has(d) && !incoming.contains(d) && !shallow && !boundary.contains(d)
                {
                    return fail(format!(
                        "change {} depends on {}, which is neither stored nor in the bundle",
                        h.short(12),
                        d.short(12)
                    ));
                }
            }
            for t in texts {
                if let crate::objects::TextRef::Blob(r, _, _) = t {
                    if !self.store.has(&r) {
                        return fail(format!(
                            "change {} came without its text blob {}",
                            h.short(12),
                            r.short(12)
                        ));
                    }
                }
            }
            self.store.put(ObjectType::Change, bytes)?;
            for sig in sigs {
                self.store.put_signature(&h, sig)?;
            }
            imported.push(h);
        }
        for ((c, sigs), h) in bundle.changes.iter().zip(hashes) {
            for d in &c.deps {
                if !self.store.has(d) && !incoming.contains(d) && !shallow && !boundary.contains(d)
                {
                    return fail(format!(
                        "change {} depends on {}, which is neither stored nor in the bundle",
                        h.short(12),
                        d.short(12)
                    ));
                }
            }
            let stored = self.store.put_change(c)?;
            if stored != h {
                return fail(format!(
                    "change {} is not the change the bundle names",
                    h.short(12)
                ));
            }
            for sig in sigs {
                self.store.put_signature(&h, sig)?;
            }
            imported.push(h);
        }
        if strict {
            for h in &imported {
                if !matches!(self.authenticate_change(h)?, SignatureStatus::Valid { .. }) {
                    return fail(format!(
                        "change {} is not authenticated by its author",
                        h.short(12)
                    ));
                }
            }
        }
        Ok(imported)
    }

    /// Verify a reference attestation against an update it claims to describe.
    /// `allow_equal` lets a client re-read the attestation it already has; a
    /// server receiving a push requires a strictly greater serial.
    /// A reference attestation signed by a delegate of an identity the
    /// rule lists is authorized as that identity (spec §14.5): the identity
    /// named the delegate with the `refs` scope, still in force at `at_ms`
    /// (the attestation's wall time), and the policy names the identity.
    /// Revoked keys authorize nothing either way.
    pub(crate) fn delegate_authorized(
        &self,
        p: &crate::policy::Policy,
        signer: &KeyId,
        reference: &str,
        at_ms: u64,
    ) -> bool {
        if p.is_revoked(signer) {
            return false;
        }
        let Some(rule) = p.rule_for(reference) else {
            return false;
        };
        rule.keys.iter().any(|k| {
            !p.is_revoked(k)
                && p.identity(k)
                    .cloned()
                    .or_else(|| self.store.get_identity(k).ok())
                    .is_some_and(|id| id.delegates_to(signer, Scope::REFS, at_ms))
        })
    }

    /// A lasting attestation (a tag) signed by a key the policy has since
    /// rotated away from (spec §14.2) is authorized as the key that
    /// succeeded it: a key the rule lists reaches the signer through
    /// `previous` links, and the attestation is dated (`at_ms`) before the
    /// signer's successor was made, so a superseded key makes nothing new.
    /// Revoked keys authorize nothing.
    pub(crate) fn predecessor_authorized(
        &self,
        p: &crate::policy::Policy,
        signer: &KeyId,
        reference: &str,
        at_ms: u64,
    ) -> bool {
        if p.is_revoked(signer) {
            return false;
        }
        let Some(rule) = p.rule_for(reference) else {
            return false;
        };
        let identity = |k: &KeyId| {
            p.identity(k)
                .cloned()
                .or_else(|| self.store.get_identity(k).ok())
        };
        rule.keys.iter().any(|k| {
            if p.is_revoked(k) {
                return false;
            }
            let mut cur = identity(k);
            for _ in 0..32 {
                let Some(id) = cur else {
                    return false;
                };
                match id.previous {
                    Some(prev) if prev == *signer => return at_ms < id.created,
                    Some(prev) if !p.is_revoked(&prev) => cur = identity(&prev),
                    _ => return false,
                }
            }
            false
        })
    }

    fn check_ref_attest(
        &self,
        sa: &SignedAttest,
        name: &str,
        old_state: &ObjectRef,
        new_state: &ObjectRef,
        last_serial: Option<u64>,
        allow_equal: bool,
    ) -> std::result::Result<u64, String> {
        let serial =
            self.check_ref_attest_body(sa, name, old_state, new_state, last_serial, allow_equal)?;
        self.verify_attest_signature(sa)?;
        Ok(serial)
    }

    /// What [`Repo::check_ref_attest`] checks short of the signature: the
    /// attestation describes this update of this repository with a serial
    /// past the last seen.
    fn check_ref_attest_body(
        &self,
        sa: &SignedAttest,
        name: &str,
        old_state: &ObjectRef,
        new_state: &ObjectRef,
        last_serial: Option<u64>,
        allow_equal: bool,
    ) -> std::result::Result<u64, String> {
        let AttestBody::Ref {
            name: n,
            old_state: o,
            new_state: nw,
            serial,
        } = &sa.attest.body
        else {
            return Err("attestation is not a ref attestation".into());
        };
        if n != name || o != old_state || nw != new_state {
            return Err("attestation does not describe this update".into());
        }
        if sa.attest.repo != self.id().map_err(|e| e.to_string())? {
            return Err("attestation is for a different repository".into());
        }
        if let Some(last) = last_serial {
            if *serial < last || (*serial == last && !allow_equal) {
                return Err(format!(
                    "attestation serial {serial} is not greater than the last seen {last}"
                ));
            }
        }
        Ok(*serial)
    }

    /// An attestation's first signature verifies with its signer's
    /// identity, which must be stored here.
    fn verify_attest_signature(&self, sa: &SignedAttest) -> std::result::Result<(), String> {
        let r = sa.attest.hash();
        let sig = sa.sigs.first().ok_or("attestation carries no signature")?;
        let id = self
            .store
            .get_identity(&sig.key)
            .map_err(|_| format!("unknown key {}", sig.key.short(12)))?;
        verify_signature(&id, sig, Domain::Attest, &r).map_err(|e| e.to_string())
    }

    /// Accept a push (spec §16.4). Without a trust policy every repository
    /// is unverified, so unsigned changes and unattested updates are accepted;
    /// attestations that are present must still be valid. With a policy,
    /// every change and update is checked against it. A newer policy in the
    /// push is applied first when it is signed by the current roots.
    pub fn receive_push(
        &self,
        bundle: Bundle,
        updates: Vec<RefUpdate>,
        policy: Option<SignedPolicy>,
        tags: Vec<crate::protocol::TagRef>,
    ) -> Result<Vec<RefResult>> {
        Ok(self
            .receive_push_report(bundle, updates, policy, tags)?
            .results)
    }

    /// `receive_push` with what a host wants to log: each accepted
    /// reference's old and new state and heads, and the changes this push
    /// stored (hub spec §6.12).
    pub fn receive_push_report(
        &self,
        bundle: Bundle,
        updates: Vec<RefUpdate>,
        policy: Option<SignedPolicy>,
        tags: Vec<crate::protocol::TagRef>,
    ) -> Result<PushReport> {
        // Updates are checked against the head as it is now, not as this
        // handle first read it (oxygenhub#249): another push may have
        // landed since.
        self.refresh()?;
        let mut report = PushReport::default();
        let results = &mut report.results;
        if let Some(sp) = policy {
            match self.adopt_policy_chain(&bundle.policies, &sp, "push received") {
                Ok(()) => {
                    results.push(RefResult {
                        name: "policy".into(),
                        ok: true,
                        reason: format!("serial {}", sp.policy.serial),
                    });
                }
                Err(e) => {
                    results.push(RefResult {
                        name: "policy".into(),
                        ok: false,
                        reason: e,
                    });
                    for u in updates {
                        results.push(RefResult {
                            name: u.name,
                            ok: false,
                            reason: "policy update was rejected".into(),
                        });
                    }
                    return Ok(report);
                }
            }
        }
        let policy = self.current_policy()?.map(|(_, sp)| sp.policy);
        // A push of objects alone (one batch of a long history, oxygen#95)
        // names no reference: its outcome is reported as `objects`.
        let objects_only = updates.is_empty() && tags.is_empty();
        match self.import_bundle(&bundle, policy.is_some()) {
            Ok(imported) => {
                if objects_only {
                    results.push(RefResult {
                        name: OBJECTS_RESULT.into(),
                        ok: true,
                        reason: format!("{} changes stored", imported.len()),
                    });
                }
                report.imported = imported;
            }
            Err(e) => {
                if objects_only {
                    results.push(RefResult {
                        name: OBJECTS_RESULT.into(),
                        ok: false,
                        reason: e.to_string(),
                    });
                }
                for u in updates {
                    results.push(RefResult {
                        name: u.name,
                        ok: false,
                        reason: e.to_string(),
                    });
                }
                return Ok(report);
            }
        }
        let (op_ref, op) = self.head_op()?;
        let mut view = op.view.clone();
        let mut names = Vec::new();
        for u in updates {
            let outcome = self.apply_update(&mut view, &u, policy.as_ref());
            let (ok, reason) = match outcome {
                Ok(()) => (true, String::new()),
                Err(e) => (false, e),
            };
            if ok {
                names.push(u.name.clone());
                let heads = self
                    .store
                    .get_state(&u.new_state)
                    .map(|s| s.heads)
                    .unwrap_or_default();
                report.updated.push(RefUpdated {
                    name: u.name.clone(),
                    old_state: u.old_state,
                    new_state: u.new_state,
                    heads,
                });
            }
            results.push(RefResult {
                name: u.name,
                ok,
                reason,
            });
        }
        for t in &tags {
            let name = format!("tags/{}", t.name);
            let new = !view.tags.contains_key(&t.name);
            match self.adopt_tag(&mut view, t, policy.as_ref(), &mut TagCheck::Received) {
                Ok(()) => {
                    // A tag new here is an update too, so a hub can start
                    // its pipeline's tag jobs (oxygenflow#24); one it
                    // already had, sent again, is not.
                    if new {
                        report.updated.push(RefUpdated {
                            name: name.clone(),
                            old_state: ObjectRef::ZERO,
                            new_state: t.state,
                            heads: t.heads.clone(),
                        });
                    }
                    names.push(name.clone());
                    results.push(RefResult {
                        name,
                        ok: true,
                        reason: String::new(),
                    });
                }
                Err(e) => results.push(RefResult {
                    name,
                    ok: false,
                    reason: e.reason,
                }),
            }
        }
        if !names.is_empty() {
            self.record_op(
                &[op_ref],
                &format!("push received: {}", names.join(" ")),
                view,
            )?;
        }
        Ok(report)
    }

    /// Check an update's attestation against the latest one for its
    /// reference and store it as the new latest. Failures say what was
    /// being done, for which reference, so a refused push can be diagnosed
    /// from its reason alone.
    fn record_update_attest(
        &self,
        u: &RefUpdate,
        sa: &SignedAttest,
    ) -> std::result::Result<(), String> {
        let name = &u.name;
        let last = self
            .latest_ref_attest(name)
            .map_err(|e| format!("reading the last attestation of {name:?}: {e}"))?
            .and_then(|(_, a)| match a.attest.body {
                AttestBody::Ref { serial, .. } => Some(serial),
                _ => None,
            });
        self.check_ref_attest(sa, name, &u.old_state, &u.new_state, last, false)?;
        let store = |e: &dyn std::fmt::Display| format!("storing the attestation of {name:?}: {e}");
        let r = self.store.put_attest(&sa.attest).map_err(|e| store(&e))?;
        for sig in &sa.sigs {
            self.store.put_signature(&r, sig).map_err(|e| store(&e))?;
        }
        self.set_ref_attest(name, &r).map_err(|e| store(&e))?;
        // Signed by a short-lived key: a host records when it accepted it.
        let certified = sa
            .sigs
            .first()
            .is_some_and(|s| self.store.certificate_of(&s.key).ok().flatten().is_some());
        if certified {
            self.record_acceptance(&r).map_err(|e| store(&e))?;
        }
        Ok(())
    }

    /// Record that this host accepted attestation `r` now, signed with its
    /// acceptor key (spec §14.7); nothing without one.
    pub(crate) fn record_acceptance(&self, r: &ObjectRef) -> Result<()> {
        let Some(acceptor) = &self.acceptor else {
            return Ok(());
        };
        let (id, sk) = (&acceptor.0, &acceptor.1);
        let attest = Attest {
            repo: self.id()?,
            hlc: Hlc::now(),
            body: AttestBody::Accepted { attest: *r },
        };
        let sig = sk.sign(Domain::Attest, &id.hash(), &attest.hash());
        self.store.put_identity(id)?;
        let a = self.store.put_attest(&attest)?;
        self.store.put_signature(&a, &sig)?;
        self.store.index_add_pub("accepted", r, &a)?;
        Ok(())
    }

    /// The host's acceptance of attestation `r`, if one was recorded.
    pub fn acceptance_of(&self, r: &ObjectRef) -> Result<Option<SignedAttest>> {
        let Some(a) = self.store.index_get_pub("accepted", r)?.into_iter().next() else {
            return Ok(None);
        };
        Ok(Some(SignedAttest {
            attest: self.store.get_attest(&a)?,
            sigs: self.store.signatures(&a)?,
        }))
    }

    /// When a certified signer's attestation `sa` is checked on a fetch: the
    /// host's acceptance time, if `accepted` is signed by the issuer of the
    /// signer's certificate and accepts `sa`; else the attestation's own
    /// time, which the signer chose.
    pub(crate) fn certified_time(
        &self,
        signer: &KeyId,
        sa: &SignedAttest,
        accepted: Option<&SignedAttest>,
    ) -> u64 {
        self.issuer_acceptance(signer, sa, accepted)
            .unwrap_or(sa.attest.hlc.wall_ms)
    }

    /// The time of `accepted` when it accepts exactly `sa` and is signed by
    /// the issuer of `signer`'s kept certificate (spec §14.7); else none.
    pub(crate) fn issuer_acceptance(
        &self,
        signer: &KeyId,
        sa: &SignedAttest,
        accepted: Option<&SignedAttest>,
    ) -> Option<u64> {
        let acc = accepted?;
        let c = self.store.certificate_of(signer).ok().flatten()?;
        let cert = self.store.get_certificate(&c).ok()?;
        let AttestBody::Accepted { attest } = &acc.attest.body else {
            return None;
        };
        let sig = acc.sigs.iter().find(|s| s.key == cert.cert.issuer)?;
        let issuer = self.store.get_identity(&cert.cert.issuer).ok()?;
        (*attest == sa.attest.hash()
            && verify_signature(&issuer, sig, Domain::Attest, &acc.attest.hash()).is_ok())
        .then_some(acc.attest.hlc.wall_ms)
    }

    /// Keep an acceptance this repository was given for attestation `r`
    /// (one [`Repo::issuer_acceptance`] took), so it is served again with
    /// what it accepts.
    pub(crate) fn keep_acceptance(&self, r: &ObjectRef, acc: &SignedAttest) -> Result<()> {
        let a = self.store.put_attest(&acc.attest)?;
        for sig in &acc.sigs {
            self.store.put_signature(&a, sig)?;
        }
        self.store.index_add_pub("accepted", r, &a)?;
        Ok(())
    }

    /// A fetched bookmark's attestation by a signer the policy does not
    /// list itself: a delegate or a short-lived key, whose authority
    /// depends on when it signed (spec §14.5, §14.8). Checked at the
    /// attestation's integrated time when the host's log gives one; else
    /// as before, at the host's acceptance for a certified signer, when it
    /// countersigned one, or at the attestation's own time, and said so
    /// when the host keeps a log. Failing at the integrated time but not
    /// as before is a note while logs are new; a large gap between the
    /// claimed and the integrated time is a note.
    #[allow(clippy::too_many_arguments)]
    pub(crate) fn authorize_in_time(
        &self,
        p: &crate::policy::Policy,
        remote_name: &str,
        r: &crate::protocol::RemoteRef,
        sa: &SignedAttest,
        signer: &KeyId,
        log: &super::transparency::LogCheck,
        notes: &mut Vec<String>,
    ) -> Result<()> {
        use super::transparency::{gap_note, judge, Timed};
        let own = sa.attest.hlc.wall_ms;
        let at = self.certified_time(signer, sa, r.accepted.as_ref());
        let time = log.time_of(&r.name);
        let check = |t_delegate: u64, t_cert: u64| {
            self.delegate_authorized(p, signer, &r.name, t_delegate)
                || self.certified_authorized(p, signer, &r.name, Scope::REFS, t_cert)
        };
        let before = if at != own {
            "the host's acceptance"
        } else {
            "the time it claims"
        };
        let what = format!("{remote_name}/{}", r.name);
        match judge(time.as_ref().ok().map(|i| i.ms()), |t| check(t, t), || check(own, at)) {
            Timed::Refused => {
                return fail(format!(
                    "{what}: attested by {}, which the policy does not authorize for this bookmark; a policy owner can grant it with `oxy policy grant`",
                    signer.short(12)
                ))
            }
            Timed::Integrated => {}
            Timed::Disagrees => notes.push(format!(
                "{what}: {}'s authority had lapsed when witness {} first saw its attestation, though not at {before}; accepted while transparency logs are new",
                signer.short(12),
                time.as_ref().map(|i| i.witness.as_str()).unwrap_or("?")
            )),
            Timed::Fallback if log.kept => notes.push(format!(
                "{what}: {} checked at {before}, not at a witnessed time: {}",
                signer.short(12),
                time.as_ref().err().map(String::as_str).unwrap_or("none")
            )),
            Timed::Fallback => {}
        }
        if let Some(n) = time.ok().and_then(|i| gap_note(own, i)) {
            notes.push(format!("{what}'s attestation {n}"));
        }
        Ok(())
    }

    /// A deletion (spec §16.4): `new_state` is zero. The bookmark must exist
    /// at `old_state`, is never the bookmark a non-bare repository has
    /// checked out (a bare one's default is its host's to protect, see
    /// `oxy_server::Backend::default_bookmark`), and under a policy needs
    /// an attestation by a key that may update it.
    /// The deletion's attestation stays the bookmark's latest, so an older
    /// one cannot be replayed to bring it back.
    fn apply_deletion(
        &self,
        view: &mut View,
        u: &RefUpdate,
        policy: Option<&crate::policy::Policy>,
    ) -> std::result::Result<(), String> {
        let Some(current) = view.bookmarks.get(&u.name).copied() else {
            return Err(format!("no bookmark {:?} to delete", u.name));
        };
        if current != u.old_state {
            return Err("stale: the remote bookmark has moved; pull first".into());
        }
        if !self.is_bare() && u.name == self.config.bookmark {
            return Err(format!(
                "{:?} is this repository's default bookmark (checked out there) and cannot be deleted",
                u.name
            ));
        }
        if let Some(p) = policy {
            let Some(sa) = &u.attestation else {
                return Err(format!(
                    "deleting {:?} requires a signed reference attestation under this repository's policy",
                    u.name
                ));
            };
            let signer = sa
                .sigs
                .first()
                .map(|s| s.key)
                .ok_or("attestation carries no signature")?;
            // Received now: the receiver's own clock is the trusted time.
            let now = Hlc::now().wall_ms;
            if !p.authorizes(&signer, &u.name)
                && !self.delegate_authorized(p, &signer, &u.name, sa.attest.hlc.wall_ms)
                && !self.certified_authorized(p, &signer, &u.name, Scope::REFS, now)
            {
                return Err(format!(
                    "key {} may not delete {:?} under this repository's policy",
                    signer.short(12),
                    u.name
                ));
            }
        }
        if let Some(sa) = &u.attestation {
            self.record_update_attest(u, sa)?;
        }
        view.bookmarks.remove(&u.name);
        Ok(())
    }

    fn apply_update(
        &self,
        view: &mut View,
        u: &RefUpdate,
        policy: Option<&crate::policy::Policy>,
    ) -> std::result::Result<(), String> {
        if u.name == "policy" {
            return Err(
                "the policy reference is updated by sending a policy, not a bookmark update".into(),
            );
        }
        let empty = self
            .store
            .put_state(&StateObject::new(vec![]))
            .map_err(|e| e.to_string())?;
        if u.new_state.is_zero() {
            return self.apply_deletion(view, u, policy);
        }
        let current = view.bookmarks.get(&u.name).copied().unwrap_or(empty);
        if current != u.old_state {
            return Err("stale: the remote bookmark has moved; pull first".into());
        }
        let new_state = self
            .store
            .get_state(&u.new_state)
            .map_err(|_| "new state object is missing".to_string())?;
        for h in &new_state.heads {
            if !self.store.has(h) {
                return Err(format!("head {} is missing", h.short(12)));
            }
        }
        if !u.force {
            let old_heads = self
                .store
                .get_state(&current)
                .map_err(|e| e.to_string())?
                .heads;
            let closure = self
                .closure_in_store(&new_state.heads)
                .map_err(|e| e.to_string())?;
            if let Some(h) = old_heads.iter().find(|h| !closure.contains(h)) {
                return Err(format!(
                    "not a fast-forward: current head {} would be dropped (use --force)",
                    h.short(12)
                ));
            }
        }
        if let Some(p) = policy {
            let Some(sa) = &u.attestation else {
                return Err(format!(
                    "{:?} requires a signed reference attestation under this repository's policy",
                    u.name
                ));
            };
            let signer = sa
                .sigs
                .first()
                .map(|s| s.key)
                .ok_or("attestation carries no signature")?;
            // Received now: the receiver's own clock is the trusted time.
            let now = Hlc::now().wall_ms;
            if !p.authorizes(&signer, &u.name)
                && !self.delegate_authorized(p, &signer, &u.name, sa.attest.hlc.wall_ms)
                && !self.certified_authorized(p, &signer, &u.name, Scope::REFS, now)
            {
                return Err(format!(
                    "key {} may not update {:?} under this repository's policy",
                    signer.short(12),
                    u.name
                ));
            }
            let old_heads = self
                .store
                .get_state(&current)
                .map_err(|e| e.to_string())?
                .heads;
            self.state_allowed(&old_heads, &new_state.heads, p, &u.name)?;
        }
        if let Some(sa) = &u.attestation {
            self.record_update_attest(u, sa)?;
        }
        view.bookmarks.insert(u.name.clone(), u.new_state);
        Ok(())
    }

    // ------------------------------------------------------------ client side

    fn remote_url(&self, name: &str) -> Result<String> {
        self.config.remotes.get(name).cloned().ok_or_else(|| {
            let names: Vec<&str> = self.config.remotes.keys().map(String::as_str).collect();
            RepoError::Msg(match names.as_slice() {
                [] => format!("no remote named {name:?}; add one with `oxy remote add`"),
                _ if name == "origin" && names.len() > 1 && self.config.default_remote.is_none() => format!(
                    "this repository has no remote named origin, and several others ({}): name one (`oxy pull {}`), or make one the default with `oxy config core.remote {}`",
                    names.join(", "),
                    names[0],
                    names[0]
                ),
                _ => format!(
                    "no remote named {name:?}; this repository has {}",
                    names.join(", ")
                ),
            })
        })
    }

    /// Refuse a remote that is another repository. An **unbound** remote
    /// (id all zero: a hosted repository nobody has pushed to) is accepted
    /// for pushes, which bind it to this repository's id (spec §16.1).
    fn check_remote_id(
        &self,
        remote: &dyn crate::protocol::Remote,
        remote_name: &str,
        allow_unbound: bool,
    ) -> Result<()> {
        let theirs = remote.repo_id()?;
        if theirs == self.id()? {
            return Ok(());
        }
        if theirs == crate::protocol::UNBOUND_ID {
            if allow_unbound {
                return Ok(());
            }
            return fail(format!(
                "remote {remote_name:?} is empty and unbound: push to it first"
            ));
        }
        fail(format!(
            "remote {remote_name:?} is a different repository (repository IDs differ); check its URL with `oxy remote list`"
        ))
    }

    /// Credentials and trust for a remote: `remote.<name>.token` and
    /// `remote.<name>.ca`, else the defaults for its URL
    /// ([`Repo::fill_credentials`]).
    pub(crate) fn remote_options(&self, name: &str) -> RemoteOptions {
        let url = self.config.remotes.get(name).cloned().unwrap_or_default();
        let mut options = RemoteOptions {
            repo_id: self.id().ok(),
            token: self.config.remote_tokens.get(name).cloned(),
            ca: self.config.remote_cas.get(name).map(PathBuf::from),
            cache_dir: Some(self.oxy.join("remotes").join(name).join("strata")),
            ..RemoteOptions::default()
        };
        self.fill_credentials(&url, &mut options);
        options
    }

    /// The user's credential store (`credentials.toml` beside the user
    /// configuration file).
    pub fn credentials(&self) -> crate::credentials::Credentials {
        crate::credentials::Credentials::at(
            self.user_config
                .parent()
                .map(Path::to_path_buf)
                .unwrap_or_else(default_home),
        )
    }

    /// Fill in what `options` lacks for `url`: `$OXY_TOKEN` and `$OXY_CA`,
    /// then the user's credential store for its hub, then a remote on the
    /// same hub in an enclosing checkout (a subrepo borrows its parent's).
    /// When the token is not this remote's own or the environment's, the
    /// other tokens enclosing checkouts hold for the hub are kept to try
    /// if it is refused: a stale stored login must not hide a parent's
    /// good token (oxygen#112).
    fn fill_credentials(&self, url: &str, options: &mut RemoteOptions) {
        if options.token.is_some() && options.ca.is_some() {
            return;
        }
        let own = options.token.is_some();
        let env = std::env::var("OXY_TOKEN").is_ok_and(|t| !t.is_empty());
        let explicit = own || env;
        let stored = self.credentials().options_for(url);
        // A checkout's own token refused as expired or unknown (401) falls
        // back to this computer's sign-in for the hub, never to another
        // checkout's, and never when it is refused for permission (#155).
        if own && !env {
            if let Some(t) = stored
                .token
                .clone()
                .filter(|t| options.token.as_ref() != Some(t))
            {
                options.fallback_tokens.push(t);
                options.fallback_on_401_only = true;
            }
        }
        options.token = options.token.take().or(stored.token);
        options.ca = options.ca.take().or(stored.ca);
        let enclosing = self.enclosing_credentials(url);
        if options.token.is_none() {
            options.token = enclosing.iter().find_map(|(t, _)| t.clone());
        }
        if options.ca.is_none() {
            options.ca = enclosing.iter().find_map(|(_, c)| c.clone());
        }
        if !explicit {
            for t in enclosing.into_iter().filter_map(|(t, _)| t) {
                if options.token.as_ref() != Some(&t) && !options.fallback_tokens.contains(&t) {
                    options.fallback_tokens.push(t);
                }
            }
        }
    }

    /// What to do when `remote`'s own token was refused (`message` says
    /// HTTP 401) and so was this computer's sign-in for its hub, which it
    /// falls back to (oxygen#155).
    pub fn stale_token_note(&self, remote: &str, message: &str) -> Option<String> {
        if !message.contains("HTTP 401") {
            return None;
        }
        let own = self.config.remote_tokens.get(remote)?;
        let url = self.config.remotes.get(remote)?;
        let stored = self.credentials().options_for(url).token?;
        (stored != *own).then(|| {
            format!(
                "the hub refused both this checkout's own token for {remote} (remote.{remote}.token) and your sign-in: \
                 `oxy auth login HUB`, then `oxy config --unset remote.{remote}.token` so the sign-in is used"
            )
        })
    }

    /// The tokens and CAs of remotes on `url`'s hub in enclosing
    /// checkouts, nearest first.
    fn enclosing_credentials(&self, url: &str) -> Vec<(Option<String>, Option<PathBuf>)> {
        let mut out = Vec::new();
        let Some(hub) = crate::credentials::hub_of(url) else {
            return out;
        };
        let mut at = self.root.parent();
        while let Some(dir) = at {
            let Ok(cfg) = read_config_file(&dir.join(OXY_DIR).join("config")) else {
                break;
            };
            for (key, remote_url) in &cfg {
                let Some(name) = key
                    .strip_prefix("remote.")
                    .and_then(|k| k.strip_suffix(".url"))
                else {
                    continue;
                };
                if crate::credentials::hub_of(remote_url).as_deref() != Some(hub.as_str()) {
                    continue;
                }
                out.push((
                    cfg.get(&format!("remote.{name}.token")).cloned(),
                    cfg.get(&format!("remote.{name}.ca")).map(PathBuf::from),
                ));
            }
            at = dir.parent();
        }
        out
    }

    /// Push seal attestations on their own (hub spec §7.4): a bundle with
    /// no changes and no reference updates, which the receiver stores.
    pub fn push_seals(&self, remote_name: &str, seals: &[ObjectRef]) -> Result<()> {
        let remote = self.open_named_remote(remote_name)?;
        self.check_remote_id(remote.as_ref(), remote_name, false)?;
        let mut bundle = Bundle::default();
        for r in seals {
            let attest = self.store.get_attest(r)?;
            let sigs = self.store.signatures(r)?;
            bundle
                .attests
                .push(crate::protocol::SignedAttest { attest, sigs });
        }
        let results = remote.push(bundle, vec![], None, vec![])?;
        if let Some(r) = results.iter().find(|r| !r.ok) {
            return fail(format!("{}: {}", r.name, r.reason));
        }
        Ok(())
    }

    /// The remote whose URL starts with `base` (a hub), if configured.
    pub fn remote_for_url(&self, base: &str) -> Option<String> {
        let base = base.trim_end_matches('/');
        self.config
            .remotes
            .iter()
            .find(|(_, url)| url.trim_end_matches('/') == base)
            .map(|(name, _)| name.clone())
    }

    /// Open a remote by name or, failing that, by URL with the default
    /// credentials (see [`Repo::credentials_for`]).
    pub fn open_remote_at(&self, url: &str) -> Result<(String, Box<dyn crate::protocol::Remote>)> {
        if let Some(name) = self.remote_for_url(url) {
            return Ok((name.clone(), self.open_named_remote(&name)?));
        }
        let options = RemoteOptions {
            repo_id: self.id().ok(),
            ..self.credentials_for(url)
        };
        Ok((url.to_string(), open_remote(url, &options)?))
    }

    /// Credentials for a URL: those of the configured remote with that
    /// URL, else the environment's.
    pub fn options_for_url(&self, url: &str) -> RemoteOptions {
        match self.remote_for_url(url) {
            Some(name) => self.remote_options(&name),
            None => RemoteOptions {
                repo_id: self.id().ok(),
                ..self.credentials_for(url)
            },
        }
    }

    pub(crate) fn open_named_remote(&self, name: &str) -> Result<Box<dyn crate::protocol::Remote>> {
        open_remote(&self.remote_url(name)?, &self.remote_options(name))
    }

    fn serial_path(&self, remote: &str, bookmark: &str) -> PathBuf {
        self.oxy
            .join("remotes")
            .join(remote)
            .join(format!("{bookmark}.serial"))
    }

    fn last_seen_serial(&self, remote: &str, bookmark: &str) -> Option<u64> {
        fs::read_to_string(self.serial_path(remote, bookmark))
            .ok()
            .and_then(|s| s.trim().parse().ok())
    }

    fn record_serial(&self, remote: &str, bookmark: &str, serial: u64) -> Result<()> {
        let p = self.serial_path(remote, bookmark);
        fs::create_dir_all(p.parent().unwrap())?;
        write_atomic(&p, serial.to_string().as_bytes())
    }

    fn policy_serial_path(&self, remote: &str) -> PathBuf {
        self.oxy.join("remotes").join(remote).join("policy.serial")
    }

    /// The highest policy serial `remote` has ever presented.
    fn remote_policy_serial(&self, remote: &str) -> Option<u64> {
        fs::read_to_string(self.policy_serial_path(remote))
            .ok()
            .and_then(|s| s.trim().parse().ok())
    }

    fn record_policy_serial(&self, remote: &str, serial: u64) -> Result<()> {
        let p = self.policy_serial_path(remote);
        fs::create_dir_all(p.parent().unwrap())?;
        write_atomic(&p, serial.to_string().as_bytes())
    }

    fn lazy_dir(&self) -> PathBuf {
        self.oxy.join("lazy")
    }

    /// Remember that `blob` was withheld by `remote` (spec §13.5).
    fn register_lazy(&self, blob: &ObjectRef, remote: &str) -> Result<()> {
        fs::create_dir_all(self.lazy_dir())?;
        write_atomic(&self.lazy_dir().join(blob.to_hex()), remote.as_bytes())
    }

    /// Which remote promised an absent blob, if any.
    pub fn lazy_remote(&self, blob: &ObjectRef) -> Option<String> {
        fs::read_to_string(self.lazy_dir().join(blob.to_hex()))
            .ok()
            .map(|s| s.trim().to_string())
    }

    /// Fetch an absent blob (and its chunks) from the remote that promised it.
    pub(crate) fn fetch_lazy_blob(&self, blob: &ObjectRef) -> Result<()> {
        let Some(remote_name) = self.lazy_remote(blob) else {
            return Err(RepoError::Store(StoreError::NotFound(*blob)));
        };
        let remote = self.open_named_remote(&remote_name)?;
        let objects = remote.objects(&[*blob])?;
        for (ty, body) in objects {
            if matches!(ty, ObjectType::Blob | ObjectType::Chunk) {
                self.store.put(ty, &body)?;
            }
        }
        if !self.store.has(blob) {
            return fail(format!(
                "{remote_name} did not send blob {}",
                blob.short(12)
            ));
        }
        let _ = fs::remove_file(self.lazy_dir().join(blob.to_hex()));
        Ok(())
    }

    /// After importing from `remote`, record every blob the changes reference
    /// that the store lacks as lazily available from that remote.
    fn register_missing_blobs(&self, changes: &[ChangeHash], remote: &str) -> Result<usize> {
        let mut n = 0;
        for h in changes {
            let c = self.store.get_change(h)?;
            for e in &c.edits {
                if let Edit::Replace { old, new, .. } = e {
                    for b in [old, new].into_iter().flatten() {
                        if !self.store.has(b) {
                            self.register_lazy(b, remote)?;
                            n += 1;
                        }
                    }
                }
            }
        }
        Ok(n)
    }

    /// Every head this repository knows: bookmarks, remote tracking, working copy.
    pub fn known_heads(&self, view: &View) -> Result<Vec<ChangeHash>> {
        let mut have: BTreeSet<ChangeHash> = BTreeSet::new();
        for st in view
            .bookmarks
            .values()
            .chain(view.remotes.values().flat_map(|b| b.values()))
        {
            if let Ok(s) = self.store.get_state(st) {
                have.extend(s.heads);
            }
        }
        have.extend(view.workspaces.values().copied());
        Ok(have.into_iter().collect())
    }

    /// Fetch every bookmark of a remote into `remotes/<name>/bookmarks/*` (spec §10.2).
    pub fn fetch(&self, remote_name: &str) -> Result<FetchReport> {
        let remote = self.open_named_remote(remote_name)?;
        self.check_remote_id(remote.as_ref(), remote_name, false)?;
        let resp = remote.refs()?;
        // The host's transparency log (spec §14.8): a fork fails here.
        let log = self.check_log(
            &self.remote_url(remote_name)?,
            self.config.remote_cas.get(remote_name).map(Path::new),
            &resp,
        )?;
        let mut log_notes = log.notes.clone();
        // Trust policy (spec §14.5). Adopt the remote's policy when it is newer
        // and signed by roots we trust, or on first contact. A remote that
        // once presented a policy may never present none or an older one:
        // that is a downgrade. A remote that never presented one is
        // unverified, and this fetch is checked only as far as possible.
        let local_policy = self.current_policy()?.map(|(_, sp)| sp.policy);
        let seen = self.remote_policy_serial(remote_name);
        let mut unverified = false;
        match &resp.policy {
            Some(sp) => {
                if seen.is_some_and(|s| sp.policy.serial < s) {
                    return fail(format!("{remote_name} presents policy serial {} but presented {} before: refusing the downgrade", sp.policy.serial, seen.unwrap()));
                }
                match &local_policy {
                    Some(l) if sp.policy.serial < l.serial => {}
                    Some(l) if sp.policy.serial == l.serial => {
                        if sp.policy.hash() != l.hash() {
                            return fail(format!(
                                "{remote_name} presents a different policy with the same serial {}",
                                l.serial
                            ));
                        }
                    }
                    _ => {
                        self.adopt_policy_chain(
                            &resp.history,
                            sp,
                            &format!("oxy fetch {remote_name}"),
                        )
                        .map_err(|e| RepoError::Msg(format!("policy from {remote_name}: {e}")))?;
                    }
                }
                self.record_policy_serial(remote_name, sp.policy.serial)?;
            }
            None => {
                if seen.is_some() {
                    return fail(format!("{remote_name} presented a policy before but has none now: refusing the downgrade"));
                }
                unverified = true;
            }
        }
        let policy = if unverified {
            None
        } else {
            self.current_policy()?.map(|(_, sp)| sp.policy)
        };
        let (op_ref, op) = self.head_op()?;
        let mut view = op.view.clone();
        let have = self.known_heads(&view)?;
        let mut report = Vec::new();
        let mut skipped = Vec::new();
        let max_blob = self.config.max_blob;
        for r in resp.refs {
            let needed = {
                let _t = crate::timing::phase("fetch: negotiate");
                remote.negotiate(&have, &r.heads)?
            };
            let bundle = {
                let _t = crate::timing::phase("fetch: bundle");
                remote.fetch(&needed, max_blob)?
            };
            // Changes are not individually re-verified here: the bookmark's
            // attestation (checked below) binds its whole state, and the
            // receiver of the push verified the added changes at that time.
            // A shallow repository takes changes whose dependencies it never
            // held (below what it cloned), as its clone did (#137).
            let shallow = self.is_shallow();
            let imported = {
                let _t = crate::timing::phase("fetch: import");
                if shallow {
                    self.import_bundle_shallow(&bundle, false)?
                } else {
                    self.import_bundle(&bundle, false)?
                }
            };
            if shallow {
                self.refresh_boundary(&imported)?;
            }
            if max_blob.is_some() {
                self.register_missing_blobs(&imported, remote_name)?;
            }
            let boundary = if shallow {
                self.boundary()
            } else {
                BTreeSet::new()
            };
            for h in &r.heads {
                if !self.store.has(h) && !boundary.contains(h) {
                    return fail(format!("remote did not send head {}", h.short(12)));
                }
            }
            let state = self.store.put_state(&StateObject::new(r.heads.clone()))?;
            if state != r.state {
                return fail(format!(
                    "remote state {} does not match its heads",
                    r.state.short(12)
                ));
            }
            if let Some(p) = &policy {
                let Some(sa) = &r.attestation else {
                    return fail(format!(
                        "{remote_name}/{} has no attestation, which its policy requires",
                        r.name
                    ));
                };
                let signer = sa
                    .sigs
                    .first()
                    .map(|s| s.key)
                    .ok_or_else(|| RepoError::Msg("attestation carries no signature".into()))?;
                if !p.authorizes(&signer, &r.name) {
                    self.authorize_in_time(p, remote_name, &r, sa, &signer, &log, &mut log_notes)?;
                }
            }
            if let Some(sa) = &r.attestation {
                let AttestBody::Ref {
                    old_state,
                    new_state,
                    ..
                } = &sa.attest.body
                else {
                    return fail("remote sent a non-ref attestation for a bookmark");
                };
                let (old_state, new_state) = (*old_state, *new_state);
                let serial = self
                    .check_ref_attest_body(
                        sa,
                        &r.name,
                        &old_state,
                        &new_state,
                        self.last_seen_serial(remote_name, &r.name),
                        true,
                    )
                    .map_err(|e| {
                        RepoError::Msg(format!("attestation for {}/{}: {e}", remote_name, r.name))
                    })?;
                if new_state != r.state {
                    return fail(format!(
                        "attestation for {}/{} does not cover its current state",
                        remote_name, r.name
                    ));
                }
                // A signature this client cannot verify (its signer unknown
                // here, as from a host that did not send the identity, or
                // bad) fails that bookmark alone: it stays as it was and
                // the rest are fetched (oxygenhub#266). The current
                // bookmark, which a pull would take, still fails the fetch.
                // A policy's requirements were checked above and still fail.
                if let Err(e) = self.verify_attest_signature(sa) {
                    let why = format!("attestation for {remote_name}/{}: {e}", r.name);
                    if r.name == self.config.bookmark {
                        return fail(why);
                    }
                    skipped.push((r.name, why));
                    continue;
                }
                let ar = self.store.put_attest(&sa.attest)?;
                for sig in &sa.sigs {
                    self.store.put_signature(&ar, sig)?;
                }
                self.record_serial(remote_name, &r.name, serial)?;
            }
            view.remotes
                .entry(remote_name.to_string())
                .or_default()
                .insert(r.name.clone(), state);
            report.push((r.name, r.heads, imported.len()));
        }
        let mut new_tags = 0;
        for t in &resp.tags {
            if view.tags.contains_key(&t.name) {
                continue;
            }
            // The tag's changes must be here: fetch them if the bookmarks did not bring them.
            let missing: Vec<ChangeHash> = t
                .heads
                .iter()
                .filter(|h| !self.store.has(h))
                .copied()
                .collect();
            if !missing.is_empty() {
                let needed = remote.negotiate(&self.known_heads(&view)?, &missing)?;
                let bundle = remote.fetch(&needed, max_blob)?;
                self.import_bundle(&bundle, false)?;
            }
            self.store.put_state(&StateObject::new(t.heads.clone()))?;
            let mut check = TagCheck::Fetched {
                remote: remote_name,
                log: &log,
                notes: &mut log_notes,
            };
            match self.adopt_tag(&mut view, t, policy.as_ref(), &mut check) {
                Ok(()) => new_tags += 1,
                // Nothing says when a short-lived key signed it: the tag is
                // left out, said so, and the rest is fetched (oxygen#129).
                Err(e) if e.untimed => {
                    skipped.push((format!("tags/{}", t.name), e.reason));
                }
                Err(e) => return fail(format!("{remote_name} {}: {}", t.name, e.reason)),
            }
        }
        if view != op.view {
            self.record_op(&[op_ref], &format!("oxy fetch {remote_name}"), view)?;
        }
        let _ = self.run_hook(
            "post-fetch",
            &format!(
                "{{\"event\":\"post-fetch\",\"remote\":{},\"bookmarks\":{}}}",
                json_str(remote_name),
                json_array(report.iter().map(|(n, heads, _)| format!(
                    "{{\"name\":{},\"heads\":{}}}",
                    json_str(n),
                    json_array(heads.iter().map(|h| json_str(&h.to_hex())))
                )))
            ),
        );
        Ok(FetchReport {
            refs: report,
            unverified,
            new_tags,
            log_notes,
            skipped,
        })
    }

    /// Shallow fetch of the remote's bookmark: its checkpoint as our base,
    /// the changes within `depth` steps of its heads, and the boundary below.
    pub fn fetch_shallow(&self, remote_name: &str, depth: u32) -> Result<usize> {
        let remote = self.open_named_remote(remote_name)?;
        self.check_remote_id(remote.as_ref(), remote_name, false)?;
        let resp = remote.refs()?;
        let bookmark = self.config.bookmark.clone();
        if let Some(sp) = &resp.policy {
            self.adopt_policy_chain(&resp.history, sp, "oxy clone --depth")
                .map_err(|e| RepoError::Msg(format!("policy from {remote_name}: {e}")))?;
            self.record_policy_serial(remote_name, sp.policy.serial)?;
        }
        let Some(r) = resp.refs.into_iter().find(|r| r.name == bookmark) else {
            return fail(format!("{remote_name} has no bookmark {bookmark:?}"));
        };
        if r.heads.is_empty() {
            return Ok(0);
        }
        // Base: the remote's checkpoint of exactly these heads, without its
        // graph blobs, which come in parts below (#137).
        let cp_bundle = {
            let _t = crate::timing::phase("shallow: checkpoint request");
            remote.checkpoint_index(&r.heads)?
        };
        // In one batch: a big state's checkpoint is tens of thousands of
        // graph blobs and chunks, which stored one by one were each written
        // loose and flushed (a VS Code clone ran at 27 objects a second).
        let objects: Vec<(ObjectType, &[u8])> = cp_bundle
            .objects
            .iter()
            .map(|(ty, body)| (*ty, body.as_slice()))
            .collect();
        let t_store = crate::timing::phase("shallow: store the checkpoint");
        let batch = self.store.begin_batch()?;
        let stored = self.store.put_many(&objects);
        batch.finish()?;
        drop(t_store);
        let cp_ref = objects
            .iter()
            .zip(stored?)
            .find(|((ty, _), _)| *ty == ObjectType::Checkpoint)
            .map(|(_, r)| r);
        let Some(cp_ref) = cp_ref else {
            return fail(format!("{remote_name} sent no checkpoint"));
        };
        // The graph blobs the payload names, with their chunks, a part at
        // a time (#137): one answer for a big state was gigabytes, held
        // whole on both sides. A remote that sent the whole checkpoint has
        // left none to ask for.
        let cp = crate::objects::Checkpoint::decode(
            &self.store.get_typed(&cp_ref, ObjectType::Checkpoint)?,
        )?;
        let graphs: Vec<ObjectRef> = self
            .checkpoint_graph_blobs(&cp)?
            .into_iter()
            .filter(|b| !self.store.has(b))
            .collect();
        {
            let _t = crate::timing::phase("shallow: the checkpoint's graphs, in parts");
            for part in graphs.chunks(CHECKPOINT_PART) {
                let objects = remote.objects(part)?;
                let objects: Vec<(ObjectType, &[u8])> = objects
                    .iter()
                    .filter(|(ty, _)| matches!(ty, ObjectType::Blob | ObjectType::Chunk))
                    .map(|(ty, body)| (*ty, body.as_slice()))
                    .collect();
                let batch = self.store.begin_batch()?;
                let stored = self.store.put_many(&objects);
                batch.finish()?;
                stored?;
            }
            if let Some(b) = graphs.iter().find(|b| !self.store.has(b)) {
                return fail(format!(
                    "{remote_name} did not send graph blob {}",
                    b.short(12)
                ));
            }
        }
        self.install_checkpoint(&bookmark, &cp_ref)?;
        let _t_blobs = crate::timing::phase("shallow: the binary files' blobs");
        // The blobs of the state's opaque files come with no change when the
        // window holds none of the changes that set them (#137): fetch them
        // now, or under `filter.max-blob` leave them to be fetched on read.
        let base = State::from_checkpoint_history(&self.read_blob(&cp.payload)?)?;
        let mut missing: Vec<ObjectRef> = base
            .opaque_nodes()
            .iter()
            .flat_map(|n| base.live_blobs(n))
            .filter(|b| !self.store.has(b))
            .collect();
        missing.sort();
        missing.dedup();
        if self.config.max_blob.is_some() {
            for b in &missing {
                self.register_lazy(b, remote_name)?;
            }
        } else {
            for part in missing.chunks(512) {
                let objects = remote.objects(part)?;
                let objects: Vec<(ObjectType, &[u8])> = objects
                    .iter()
                    .filter(|(ty, _)| matches!(ty, ObjectType::Blob | ObjectType::Chunk))
                    .map(|(ty, body)| (*ty, body.as_slice()))
                    .collect();
                let batch = self.store.begin_batch()?;
                let stored = self.store.put_many(&objects);
                batch.finish()?;
                stored?;
            }
            if let Some(b) = missing.iter().find(|b| !self.store.has(b)) {
                return fail(format!("{remote_name} did not send blob {}", b.short(12)));
            }
        }
        // History window. Depth 0 takes none (#137): the checkpoint holds
        // every change's metadata, and its heads become the boundary, so a
        // state of many heads (33,835 in the VS Code import) costs nothing
        // to fetch.
        let imported = if depth == 0 {
            let mut boundary = self.boundary();
            boundary.extend(r.heads.iter().filter(|h| !self.store.has(h)).copied());
            self.write_boundary(&boundary)?;
            Vec::new()
        } else {
            let wanted = remote.shallow(&r.heads, depth)?;
            let bundle = remote.fetch(&wanted, self.config.max_blob)?;
            self.import_bundle_shallow(&bundle, false)?
        };
        if self.config.max_blob.is_some() {
            self.register_missing_blobs(&imported, remote_name)?;
        }
        self.refresh_boundary(&imported)?;
        if let Some(sa) = &r.attestation {
            let ar = self.store.put_attest(&sa.attest)?;
            for sig in &sa.sigs {
                self.store.put_signature(&ar, sig)?;
            }
            if let AttestBody::Ref { serial, .. } = sa.attest.body {
                self.record_serial(remote_name, &r.name, serial)?;
            }
        }
        let state = self.store.put_state(&StateObject::new(r.heads.clone()))?;
        let (op_ref, op) = self.head_op()?;
        let mut view = op.view.clone();
        view.remotes
            .entry(remote_name.to_string())
            .or_default()
            .insert(r.name.clone(), state);
        self.record_op(
            &[op_ref],
            &format!("oxy fetch {remote_name} --depth {depth}"),
            view,
        )?;
        Ok(imported.len())
    }

    /// Fetch, then take the union of the local bookmark's state with the
    /// remote's: no merge change, conflicts become reified (spec §9.4).
    pub fn pull(&self, remote_name: &str) -> Result<PullReport> {
        let _wc = self.lock_working_copy()?;
        let skipped = self.fetch(remote_name)?.skipped;
        let (op_ref, op) = self.head_op()?;
        let current = self.config.bookmark.clone();
        let mut local_current = self.heads_of(&op.view, &current)?;
        local_current.sort();
        let mut view = op.view.clone();
        let mut current_heads = local_current.clone();
        let mut changed = false;
        let mut others = Vec::new();
        let tracked = op
            .view
            .remotes
            .get(remote_name)
            .cloned()
            .unwrap_or_default();
        for (name, remote_state) in tracked {
            // Every local bookmark tracks the remote's of the same name;
            // the current one may be new here.
            let local_state = op.view.bookmarks.get(&name).copied();
            if local_state.is_none() && name != current {
                continue;
            }
            if local_state == Some(remote_state) {
                continue;
            }
            let local = self.heads_of(&op.view, &name)?;
            let remote_heads = self.store.get_state(&remote_state)?.heads;
            if remote_heads.iter().all(|h| local.contains(h)) {
                continue;
            }
            let mut all = local.clone();
            all.extend(remote_heads);
            let st = self.load_state_history(&all)?;
            let mut heads = st.maximal(&all);
            heads.sort();
            let mut sorted_local = local;
            sorted_local.sort();
            if heads == sorted_local {
                continue;
            }
            view.bookmarks.insert(
                name.clone(),
                self.store.put_state(&StateObject::new(heads.clone()))?,
            );
            if name == current {
                changed = true;
                current_heads = heads;
            } else {
                others.push(name);
            }
        }
        if changed || !others.is_empty() {
            self.record_op(&[op_ref], &format!("oxy pull {remote_name}"), view)?;
        }
        // The working copy follows its bookmark whenever nothing would be
        // lost, also when this pull brought nothing new but it had been
        // left behind (oxygen#89).
        let checked_out = self.follow_bookmark(&current_heads, &current, "oxy pull")?;
        Ok(PullReport {
            heads: if changed {
                current_heads
            } else {
                local_current
            },
            changed,
            checked_out,
            others,
            skipped,
        })
    }

    /// How many changes the working copy's bookmark has that its base
    /// lacks, when the base is part of the bookmark's state (it fell behind,
    /// and `oxy checkout` or `oxy pull` brings it up); `None` when it is on
    /// the bookmark or holds changes the bookmark lacks. History is read
    /// only when the base and the bookmark's heads differ (oxygen#89).
    pub fn working_copy_behind(&self) -> Result<Option<usize>> {
        let (_, wc) = self.working_copy()?;
        let mut base = wc.deps.clone();
        base.sort();
        let mut heads = self.bookmark_heads()?;
        heads.sort();
        if base == heads || heads.is_empty() {
            return Ok(None);
        }
        let state = self.load_state_history(&heads)?;
        if !base.iter().all(|h| state.contains(h)) {
            return Ok(None);
        }
        let had = if base.is_empty() {
            0
        } else {
            self.load_state_history(&base)?.len()
        };
        Ok(Some(state.len().saturating_sub(had)).filter(|n| *n > 0))
    }

    /// Bring the working copy to `heads`, the state of its bookmark
    /// `bookmark`, when nothing would be lost: its base is part of that
    /// state, so the bookmark only adds to it. Clean, it is checked out;
    /// with uncommitted work, the work comes along when the new changes
    /// leave its paths alone (as `git pull` does), else it stays. Before,
    /// it followed only from exactly the bookmark's old heads, so a working
    /// copy already behind (the bookmark moved by `bookmark set`, a landing
    /// or a pull from another workspace) never caught up, and nothing said
    /// so (oxygen#74, #89). Returns whether it moved.
    pub fn follow_bookmark(
        &self,
        heads: &[ChangeHash],
        bookmark: &str,
        command: &str,
    ) -> Result<bool> {
        let snap = self.snapshot(command)?;
        let clean = snap.wc.tree_ops.is_empty() && snap.wc.edits.is_empty();
        let mut base = snap.base.maximal(&snap.wc.deps);
        base.sort();
        let mut target = heads.to_vec();
        target.sort();
        if base == target {
            return Ok(false);
        }
        let state = self.load_state_history(heads)?;
        if !base.iter().all(|h| state.contains(h)) {
            // The working copy holds changes the bookmark lacks: not ours to move.
            return Ok(false);
        }
        if clean {
            self.checkout(heads, false, &format!("{command} (checkout)"))?;
            Ok(true)
        } else {
            Ok(self
                .checkout_carry(heads, bookmark, &format!("{command} (checkout --carry)"))
                .is_ok())
        }
    }

    /// Push the current bookmark (spec §16.3), sealing this author's unsigned
    /// changes and attesting the reference when a key is available.
    /// Delete `bookmark` on a remote (spec §16.4): an update to the zero
    /// state, attested when we have an identity. The local bookmark, if
    /// any, is kept; only the remote-tracking entry goes.
    pub fn push_delete(&self, remote_name: &str, bookmark: &str) -> Result<Vec<RefResult>> {
        let remote = self.open_named_remote(remote_name)?;
        self.check_remote_id(remote.as_ref(), remote_name, true)?;
        let resp = remote.refs()?;
        let Some(r) = resp.refs.iter().find(|r| r.name == bookmark) else {
            return fail(format!("{remote_name} has no bookmark {bookmark}"));
        };
        if resp.policy.is_some() && self.secret_key()?.is_none() {
            return fail(format!("{remote_name} enforces a trust policy; deleting needs an identity (`oxy identity create`)"));
        }
        let attestation = match self.secret_key()? {
            Some(_) => Some(self.sign_attest(&Attest {
                repo: self.id()?,
                hlc: Hlc::now(),
                body: AttestBody::Ref {
                    name: bookmark.to_string(),
                    old_state: r.state,
                    new_state: ObjectRef::ZERO,
                    serial: r.last_serial.unwrap_or(0) + 1,
                },
            })?),
            None => None,
        };
        let results = remote.push(
            Bundle::default(),
            vec![RefUpdate {
                name: bookmark.to_string(),
                old_state: r.state,
                new_state: ObjectRef::ZERO,
                force: true,
                attestation,
            }],
            None,
            vec![],
        )?;
        if results.iter().any(|x| x.ok && x.name == bookmark) {
            let (op_ref, op) = self.head_op()?;
            let mut view = op.view.clone();
            if let Some(m) = view.remotes.get_mut(remote_name) {
                m.remove(bookmark);
            }
            self.record_op(
                &[op_ref],
                &format!("oxy push {remote_name} --delete {bookmark}"),
                view,
            )?;
        }
        Ok(results)
    }

    /// Remove remote `name` (oxygen#86): every `remote.NAME.*` key of this
    /// repository's config (its URL, token, CA and store settings), its
    /// tracking bookmarks in the view, and a store remote's mirror under
    /// `.oxy/remotes/NAME`. Nothing stays to be read as if it were current.
    pub fn remove_remote(&mut self, name: &str) -> Result<()> {
        if !self.config.remotes.contains_key(name) {
            return fail(format!("no remote {name}: `oxy remote list` shows them"));
        }
        let prefix = format!("remote.{name}.");
        let keys: Vec<String> = self
            .config
            .repo_layer()
            .keys()
            .filter(|k| k.starts_with(&prefix))
            .cloned()
            .collect();
        for k in keys {
            self.config.set(&k, "")?;
        }
        self.config.remotes.remove(name);
        self.save_config()?;
        let (op_ref, op) = self.head_op()?;
        if op.view.remotes.contains_key(name) {
            let mut view = op.view.clone();
            view.remotes.remove(name);
            self.record_op(&[op_ref], &format!("oxy remote remove {name}"), view)?;
        }
        let mirror = self.oxy.join("remotes").join(name);
        if mirror.is_dir() {
            fs::remove_dir_all(&mirror)?;
        }
        Ok(())
    }

    /// Send `bundle`'s blobs and chunks ahead of it, in parts of about
    /// `push.batch-bytes` of objects alone, when the whole would be larger
    /// (oxygen#133): one change can bring more blobs than one request may
    /// carry, and a proxy in front of a hub refuses a request past its limit
    /// (Cloudflare's is 100 MB). What is left, the changes with their
    /// identities and states, is returned to send as before. A receiver
    /// stores objects as they come, and checks a change's blobs are stored.
    fn send_objects_ahead(
        &self,
        remote: &dyn crate::protocol::Remote,
        remote_name: &str,
        mut bundle: Bundle,
        progress: crate::store::Progress,
    ) -> Result<Bundle> {
        let limit = self.config.push_batch_bytes;
        let changes: u64 = bundle
            .changes
            .iter()
            .map(|(c, _)| c.encode().len() as u64)
            .sum();
        let objects: u64 = bundle.objects.iter().map(|(_, b)| b.len() as u64).sum();
        if changes + objects <= limit {
            return Ok(bundle);
        }
        let (mut kept, mut parts, mut part, mut bytes) = (Vec::new(), Vec::new(), Vec::new(), 0u64);
        for (ty, body) in std::mem::take(&mut bundle.objects) {
            if !matches!(ty, ObjectType::Blob | ObjectType::Chunk) {
                kept.push((ty, body));
                continue;
            }
            if !part.is_empty() && bytes + body.len() as u64 > limit {
                parts.push(std::mem::take(&mut part));
                bytes = 0;
            }
            bytes += body.len() as u64;
            part.push((ty, body));
        }
        if !part.is_empty() {
            parts.push(part);
        }
        let n = parts.len();
        for (i, objects) in parts.into_iter().enumerate() {
            progress(&format!(
                "objects {} of {n}: {} blobs and chunks",
                i + 1,
                objects.len()
            ));
            let results = remote.push(
                Bundle {
                    objects,
                    ..Default::default()
                },
                vec![],
                None,
                vec![],
            )?;
            match results.iter().find(|r| r.name == OBJECTS_RESULT) {
                Some(r) if r.ok => {}
                Some(r) => {
                    return fail(format!(
                        "{remote_name} refused objects part {} of {n}: {}",
                        i + 1,
                        r.reason
                    ))
                }
                None => {
                    return fail(format!(
                        "{remote_name} does not take a push in parts, and this one is too large for one: update the server's oxy"
                    ))
                }
            }
        }
        bundle.objects = kept;
        Ok(bundle)
    }

    /// `changes`, in dependency order, cut into batches of about
    /// `push.batch-bytes` by what each change and its texts and blobs weigh.
    /// There is always one, perhaps empty.
    fn push_batches(&self, changes: &[ChangeHash]) -> Result<Vec<Vec<ChangeHash>>> {
        let blob_len = |r: &ObjectRef| -> u64 {
            self.store
                .get_typed(r, ObjectType::Blob)
                .ok()
                .and_then(|b| Blob::decode(&b).ok())
                .map_or(0, |b| b.length)
        };
        let (mut out, mut batch, mut bytes) = (Vec::new(), Vec::new(), 0u64);
        for h in changes {
            let raw = self.store.get_typed(h, ObjectType::Change)?;
            let (c, texts) = Change::decode_skeleton(&raw, *h)?;
            let mut weight = raw.len() as u64;
            for t in &texts {
                if let crate::objects::TextRef::Blob(r, _, _) = t {
                    weight += blob_len(r);
                }
            }
            for e in &c.edits {
                if let Edit::Replace { new: Some(b), .. } = e {
                    weight += blob_len(b);
                }
            }
            if !batch.is_empty() && bytes + weight > self.config.push_batch_bytes {
                out.push(std::mem::take(&mut batch));
                bytes = 0;
            }
            batch.push(*h);
            bytes += weight;
        }
        out.push(batch);
        Ok(out)
    }

    pub fn push(&self, remote_name: &str, force: bool) -> Result<Vec<RefResult>> {
        let bookmark = self.config.bookmark.clone();
        self.push_bookmark(remote_name, &bookmark, force)
    }

    /// `push` for a named bookmark (`oxy push -b NAME`).
    pub fn push_bookmark(
        &self,
        remote_name: &str,
        bookmark: &str,
        force: bool,
    ) -> Result<Vec<RefResult>> {
        self.push_bookmark_with(remote_name, bookmark, force, &mut |_| {})
    }

    /// `push_bookmark`, saying how a long push is going: one line per
    /// batch sent (oxygen#95).
    /// What a push may take the remote to hold already: the heads of the
    /// bookmark pushed, and of every other bookmark it lists that are held
    /// here, so a new bookmark made from main sends only what main lacks
    /// (oxygen#142), not its whole history again.
    pub(crate) fn remote_holds(
        &self,
        refs: &[crate::protocol::RemoteRef],
        bookmark_heads: &[ChangeHash],
    ) -> Vec<ChangeHash> {
        let mut have: Vec<ChangeHash> = bookmark_heads.to_vec();
        for h in refs.iter().flat_map(|r| &r.heads) {
            if !have.contains(h) && self.store.has(h) {
                have.push(*h);
            }
        }
        have
    }

    pub fn push_bookmark_with(
        &self,
        remote_name: &str,
        bookmark: &str,
        force: bool,
        progress: crate::store::Progress,
    ) -> Result<Vec<RefResult>> {
        let (op_ref, op) = self.head_op()?;
        if !op.view.bookmarks.contains_key(bookmark) {
            return fail(format!("no bookmark {bookmark}"));
        }
        let bookmark = bookmark.to_string();
        let local_heads = self.heads_of(&op.view, &bookmark)?;
        // pre-push hook: may veto before the remote is contacted.
        self.run_hook(
            "pre-push",
            &format!(
                "{{\"event\":\"pre-push\",\"remote\":{},\"bookmark\":{},\"heads\":{}}}",
                json_str(remote_name),
                json_str(&bookmark),
                json_array(local_heads.iter().map(|h| json_str(&h.to_hex())))
            ),
        )?;
        let remote = self.open_named_remote(remote_name)?;
        self.check_remote_id(remote.as_ref(), remote_name, true)?;
        let resp = remote.refs()?;
        let local_policy = self.current_policy()?.map(|(_, sp)| sp);
        let resp_policy_serial = resp.policy.as_ref().map(|p| p.policy.serial);
        let policy_to_send = match (&local_policy, &resp.policy) {
            (Some(l), Some(r)) if l.policy.serial > r.policy.serial => Some(l.clone()),
            (Some(l), None) => Some(l.clone()),
            (Some(l), Some(r)) if l.policy.serial < r.policy.serial => {
                return fail(format!(
                    "{remote_name} has a newer policy (serial {}); fetch first",
                    r.policy.serial
                ));
            }
            _ => None,
        };
        let remote_has_policy = resp.policy.is_some() || policy_to_send.is_some();
        if remote_has_policy && self.secret_key()?.is_none() {
            return fail(format!("{remote_name} enforces a trust policy; pushing needs an identity (`oxy identity create`)"));
        }
        let remote_tags: BTreeSet<String> = resp.tags.iter().map(|t| t.name.clone()).collect();
        let tags_to_send: Vec<crate::protocol::TagRef> = self
            .tag_refs()?
            .into_iter()
            .filter(|t| !remote_tags.contains(&t.name))
            .collect();
        let refs = resp.refs;
        let empty = self.store.put_state(&StateObject::new(vec![]))?;
        let (remote_state, remote_heads, last_serial) =
            match refs.iter().find(|r| r.name == bookmark) {
                Some(r) => (r.state, r.heads.clone(), r.last_serial),
                None => (empty, vec![], None),
            };
        let new_state = self
            .store
            .put_state(&StateObject::new(local_heads.clone()))?;
        // A push that carries a policy re-attests the bookmark even when its
        // state is unchanged, so the current attestation is always by a key
        // the current policy authorizes (a revoked key's attestation would
        // otherwise be refused by every later fetch).
        if new_state == remote_state && policy_to_send.is_none() && tags_to_send.is_empty() {
            return Ok(vec![RefResult {
                name: bookmark,
                ok: true,
                reason: "already up to date".into(),
            }]);
        }
        // A shallow repository knows the changes on its boundary without
        // holding them: a depth-0 clone's are the heads it cloned (#137).
        let boundary = self.boundary();
        for h in &remote_heads {
            if !self.store.has(h) && !boundary.contains(h) {
                return fail(format!(
                    "the remote has change {} which you do not; pull first (`oxy pull {remote_name}`), then push again",
                    h.short(12)
                ));
            }
        }
        if !force {
            let closure = self.closure_in_store(&local_heads)?;
            if let Some(h) = remote_heads
                .iter()
                .find(|h| !closure.contains(h) && !boundary.contains(h))
            {
                return fail(format!("not a fast-forward: remote head {} is not in your state; pull first or use --force", h.short(12)));
            }
        }
        let to_send = self.negotiate(&self.remote_holds(&refs, &remote_heads), &local_heads)?;
        let key = self.secret_key()?.map(|(k, _)| k);
        if let Some(key) = key {
            // Seal this author's changes that carry no valid authenticator,
            // and adopt anonymous ones (zero author, spec §14.4). The seals
            // are stored, so each batch's bundle carries those it needs.
            let mut unsigned = Vec::new();
            for h in &to_send {
                let c = self.store.get_change_meta(h)?;
                if self.seals_for_author(&c.author, &key)
                    && !matches!(self.authenticate_change(h)?, SignatureStatus::Valid { .. })
                {
                    unsigned.push(*h);
                }
            }
            for chunk in unsigned.chunks(crate::attest::MAX_SEAL_HASHES) {
                self.sign_attest(&Attest::seal(self.id()?, chunk.to_vec()))?;
            }
        }
        // A long history goes in batches, in dependency order, so neither
        // side holds it whole and no request passes the remote's limit:
        // all but the last carry objects alone; the last moves the bookmark
        // (oxygen#95).
        let batches = self.push_batches(&to_send)?;
        let (last, earlier) = batches.split_last().expect("at least one batch");
        for (i, batch) in earlier.iter().enumerate() {
            progress(&format!(
                "part {} of {}: {} changes",
                i + 1,
                batches.len(),
                batch.len()
            ));
            let bundle = self.send_objects_ahead(
                remote.as_ref(),
                remote_name,
                self.bundle_for(batch)?,
                &mut *progress,
            )?;
            let results = remote.push(bundle, vec![], None, vec![])?;
            match results.iter().find(|r| r.name == OBJECTS_RESULT) {
                Some(r) if r.ok => {}
                Some(r) => {
                    return fail(format!(
                        "{remote_name} refused part {} of the push: {}",
                        i + 1,
                        r.reason
                    ))
                }
                None => {
                    return fail(format!(
                        "{remote_name} does not take a push in parts, and this one is too long for one: update the server's oxy"
                    ))
                }
            }
        }
        if batches.len() > 1 {
            progress(&format!(
                "part {} of {}: {} changes",
                batches.len(),
                batches.len(),
                last.len()
            ));
        }
        let mut bundle = self.send_objects_ahead(
            remote.as_ref(),
            remote_name,
            self.bundle_for(last)?,
            &mut *progress,
        )?;
        bundle.objects.push((
            ObjectType::State,
            StateObject::new(local_heads.clone()).encode(),
        ));
        let attestation = match key {
            Some(_) => Some(self.sign_attest(&Attest {
                repo: self.id()?,
                hlc: Hlc::now(),
                body: AttestBody::Ref {
                    name: bookmark.clone(),
                    old_state: remote_state,
                    new_state,
                    serial: next_serial(last_serial),
                },
            })?),
            None => None,
        };
        // Whoever signed the move, and whom a delegate acts for, travel with
        // it: a receiver that has never seen a change by this delegate could
        // not otherwise check its signature (oxygenhub#129).
        if attestation.is_some() {
            for k in [self.signing_key(), self.config.author_key]
                .into_iter()
                .flatten()
            {
                if let Ok(id) = self.identity(&k) {
                    let body = id.encode();
                    if !bundle
                        .objects
                        .iter()
                        .any(|(t, b)| *t == ObjectType::Identity && *b == body)
                    {
                        bundle.objects.push((ObjectType::Identity, body));
                    }
                }
            }
            // A short-lived signing key's certificate goes with it (§14.7).
            let signers: Vec<KeyId> = self.signing_key().into_iter().collect();
            self.attach_certificates_for(&mut bundle, &signers)?;
        }
        let serial = attestation.as_ref().and_then(|a| match a.attest.body {
            AttestBody::Ref { serial, .. } => Some(serial),
            _ => None,
        });
        let sent_serial = policy_to_send.as_ref().map(|p| p.policy.serial);
        // Tag states must be present on the remote: bundle their changes too.
        for t in &tags_to_send {
            let extra = self.negotiate(&remote_heads, &t.heads)?;
            let extra: Vec<ChangeHash> =
                extra.into_iter().filter(|h| !to_send.contains(h)).collect();
            if !extra.is_empty() {
                let more = self.bundle_for(&extra)?;
                bundle.changes.extend(more.changes);
                bundle.hashes.extend(more.hashes);
                bundle.attests.extend(more.attests);
                bundle.objects.extend(more.objects);
            }
            bundle.objects.push((
                ObjectType::State,
                StateObject::new(t.heads.clone()).encode(),
            ));
        }
        // A tag's signer travels with it, as the bookmark's does: a tag made
        // by `oxy tag create` under a short-lived key was signed by a key the
        // receiver has never seen, so its identity and certificate go too.
        let tag_signers: Vec<KeyId> = tags_to_send
            .iter()
            .filter_map(|t| t.attestation.as_ref())
            .flat_map(|a| a.sigs.iter().map(|s| s.key))
            .collect();
        for k in &tag_signers {
            if let Ok(id) = self.identity(k) {
                let body = id.encode();
                if !bundle
                    .objects
                    .iter()
                    .any(|(t, b)| *t == ObjectType::Identity && *b == body)
                {
                    bundle.objects.push((ObjectType::Identity, body));
                }
            }
        }
        if !tag_signers.is_empty() {
            self.attach_certificates_for(&mut bundle, &tag_signers)?;
        }
        if let Some(l) = &policy_to_send {
            let remote_serial = resp_policy_serial;
            bundle.policies = self
                .policy_history()?
                .into_iter()
                .filter(|sp| {
                    remote_serial.is_none_or(|r| sp.policy.serial > r)
                        && sp.policy.serial < l.policy.serial
                })
                .collect();
        }
        let results = remote.push(
            bundle,
            vec![RefUpdate {
                name: bookmark.clone(),
                old_state: remote_state,
                new_state,
                force,
                attestation,
            }],
            policy_to_send,
            tags_to_send,
        )?;
        if let (Some(serial), true) = (
            sent_serial,
            results.iter().any(|r| r.ok && r.name == "policy"),
        ) {
            self.record_policy_serial(remote_name, serial)?;
        }
        if results.iter().any(|r| r.ok && r.name == bookmark) {
            let mut view = op.view.clone();
            view.remotes
                .entry(remote_name.to_string())
                .or_default()
                .insert(bookmark.clone(), new_state);
            self.record_op(&[op_ref], &format!("oxy push {remote_name}"), view)?;
            if let Some(s) = serial {
                self.record_serial(remote_name, &bookmark, s)?;
            }
        }
        Ok(results)
    }

    /// Clone a remote into `dest`: same repository ID, `origin` remote,
    /// bookmark set to the remote's, working copy checked out.
    pub fn clone(url: &str, dest: &Path) -> Result<Repo> {
        Self::clone_with(url, dest, None, &[])
    }

    /// Clone, leaving blobs above `max_blob` bytes to be fetched on first
    /// read, and materializing only `sparse` paths (which implies lazy blobs
    /// unless a size is given, so out-of-scope binaries are never fetched).
    pub fn clone_with(
        url: &str,
        dest: &Path,
        max_blob: Option<u64>,
        sparse: &[String],
    ) -> Result<Repo> {
        Self::clone_full(url, dest, max_blob, sparse, None)
    }

    /// Shallow clone (spec §17): a checkpoint of the bookmark's heads plus the
    /// changes within `depth` steps of them; the rest is the boundary.
    pub fn clone_shallow(
        url: &str,
        dest: &Path,
        depth: u32,
        max_blob: Option<u64>,
        sparse: &[String],
    ) -> Result<Repo> {
        Self::clone_full(url, dest, max_blob, sparse, Some(depth))
    }

    fn clone_full(
        url: &str,
        dest: &Path,
        max_blob: Option<u64>,
        sparse: &[String],
        depth: Option<u32>,
    ) -> Result<Repo> {
        Ok(Self::clone_reporting(url, dest, max_blob, sparse, depth)?.0)
    }

    /// Clone as [`Repo::clone_with`] (or [`Repo::clone_shallow`] with a
    /// `depth`), also returning the bookmarks the fetch left out because
    /// their attestation could not be verified ([`FetchReport::skipped`]),
    /// for the caller to warn about. The bookmark checked out is never one
    /// of them: that fails the clone.
    pub fn clone_reporting(
        url: &str,
        dest: &Path,
        max_blob: Option<u64>,
        sparse: &[String],
        depth: Option<u32>,
    ) -> Result<(Repo, Vec<(String, String)>)> {
        let options = crate::credentials::Credentials::user().options_for(url);
        Self::clone_using(url, dest, max_blob, sparse, depth, &options)
    }

    /// Clone a subrepo with the credentials the parent would use for its
    /// hub (see [`Repo::credentials_for`]). Nothing is recorded in the
    /// subrepo: it finds the same ones again through its parent.
    pub(crate) fn clone_as_subrepo(&self, url: &str, dest: &Path) -> Result<Repo> {
        let options = self.credentials_for(url);
        Ok(Self::clone_using(url, dest, None, &[], None, &options)?.0)
    }

    /// The name of a remote of this repository on the same hub (scheme,
    /// host and port) as `url`.
    fn remote_on_same_hub(&self, url: &str) -> Option<String> {
        let origin = |u: &str| -> Option<String> {
            let (scheme, rest) = u.split_once("://")?;
            if !matches!(scheme, "http" | "https") {
                return None;
            }
            let host = rest.split('/').next()?;
            Some(format!(
                "{}://{}",
                scheme.to_ascii_lowercase(),
                host.to_ascii_lowercase()
            ))
        };
        let want = origin(url)?;
        self.config
            .remotes
            .iter()
            .find(|(_, u)| origin(u).as_deref() == Some(want.as_str()))
            .map(|(name, _)| name.clone())
    }

    /// The token and CA to reach `url` with: those of a remote of this
    /// repository on the same hub (scheme, host and port), else the
    /// environment's (`OXY_TOKEN`, `OXY_CA`), the user's credential store
    /// and enclosing checkouts', as a clone would use them.
    pub(crate) fn credentials_for(&self, url: &str) -> RemoteOptions {
        match self.remote_on_same_hub(url) {
            Some(name) => RemoteOptions {
                repo_id: None,
                cache_dir: None,
                ..self.remote_options(&name)
            },
            None => {
                let mut options = RemoteOptions::default();
                self.fill_credentials(url, &mut options);
                options
            }
        }
    }

    fn clone_using(
        url: &str,
        dest: &Path,
        max_blob: Option<u64>,
        sparse: &[String],
        depth: Option<u32>,
        options: &RemoteOptions,
    ) -> Result<(Repo, Vec<(String, String)>)> {
        let remote = open_remote(url, options)?;
        let id = remote.repo_id()?;
        if id == crate::protocol::UNBOUND_ID {
            return fail(format!(
                "{url} is empty and unbound: push a repository to it first"
            ));
        }
        let mut repo = Repo::init_with_id(dest, id)?;
        repo.config
            .set("remote.origin.url", &crate::protocol::stored_url(url))?;
        if let Some(m) = max_blob {
            repo.config.set("filter.max-blob", &m.to_string())?;
        } else if !sparse.is_empty() {
            repo.config.set("filter.max-blob", "0")?;
        }
        if !sparse.is_empty() {
            repo.config.set("filter.paths", &sparse.join(","))?;
        }
        repo.save_config()?;
        let skipped = if let Some(depth) = depth {
            repo.fetch_shallow("origin", depth)?;
            Vec::new()
        } else {
            repo.fetch("origin")?.skipped
        };
        let (op_ref, op) = repo.head_op()?;
        // The remote's default bookmark, else ours (main), else its only
        // one; a bookmark the fetch skipped counts, so the clone fails
        // rather than check out another (oxygenhub#266).
        let mut theirs = op.view.remotes.get("origin").cloned().unwrap_or_default();
        for (name, _) in &skipped {
            theirs.insert(name.clone(), ObjectRef::ZERO);
        }
        // Asked of the remote as now configured, so a store-backed one
        // answers from the mirror the fetch just filled.
        let advertised = repo
            .open_named_remote("origin")
            .ok()
            .and_then(|r| r.refs().ok())
            .and_then(|r| r.default_bookmark);
        let bookmark = advertised
            .filter(|b| theirs.contains_key(b))
            .or_else(|| {
                theirs
                    .contains_key(&repo.config.bookmark)
                    .then(|| repo.config.bookmark.clone())
            })
            .or_else(|| {
                (theirs.len() == 1)
                    .then(|| theirs.keys().next().cloned())
                    .flatten()
            })
            .unwrap_or_else(|| repo.config.bookmark.clone());
        if let Some((_, why)) = skipped.iter().find(|(name, _)| *name == bookmark) {
            return fail(format!("cannot check out {bookmark}: {why}"));
        }
        let initial = repo.config.bookmark.clone();
        if bookmark != repo.config.bookmark {
            fs::create_dir_all(repo.workspace_bookmark_path().parent().unwrap())?;
            write_atomic(&repo.workspace_bookmark_path(), bookmark.as_bytes())?;
            repo.config.bookmark = bookmark.clone();
        }
        if let Some(state) = theirs.get(&bookmark).copied() {
            let heads = repo.store.get_state(&state)?.heads;
            let mut view = op.view.clone();
            // The empty bookmark `init` made goes when another is checked out.
            if initial != bookmark && repo.heads_of(&view, &initial)?.is_empty() {
                view.bookmarks.remove(&initial);
            }
            view.bookmarks.insert(bookmark, state);
            repo.record_op(&[op_ref], &format!("oxy clone {url}"), view)?;
            repo.checkout(&heads, false, "oxy clone (checkout)")?;
        }
        Ok((repo, skipped))
    }
}

/// The serial of a reference's next attestation: one more than the last the
/// remote reports, or, for a bookmark the remote does not have, the clock in
/// milliseconds, so a bookmark created again after a deletion goes past the
/// deletion's serial the remote keeps.
fn next_serial(last: Option<u64>) -> u64 {
    match last {
        Some(l) => l + 1,
        None => Hlc::now().wall_ms.max(1),
    }
}
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