crates/oxy-core/src/repo/sync.rs at 76f752503e7e · andymccall/oxygen

# 76f752503e7e oxygen / crates / oxy-core / src / repo / sync.rs
53.1 KiB · Rust · last changed Challenge signatures and a push report for hosts 1 month ago source blame history raw
//! Serving and talking to remotes (spec §10.2, §16).

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,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct PullReport {
    pub heads: Vec<ChangeHash>,
    pub changed: bool,
    pub checked_out: bool,
}

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

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

    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));
            out.push(RemoteRef {
                name: name.clone(),
                state: *state,
                heads: self.store.get_state(state)?.heads,
                attestation,
                last_serial,
            });
        }
        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,
        })
    }

    /// 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(&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(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 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) {
                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(&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_for_filtered(changes, None)
    }

    /// Objects by reference for the `objects` endpoint; a blob brings its chunks.
    pub fn objects_for(&self, refs: &[ObjectRef]) -> Result<Vec<(ObjectType, Vec<u8>)>> {
        let mut out = Vec::new();
        for r in refs {
            let (ty, body) = match self.store.get(r) {
                Ok(x) => x,
                Err(StoreError::NotFound(_)) => continue,
                Err(e) => return Err(e.into()),
            };
            if ty == ObjectType::Blob {
                for c in Blob::decode(&body)?.chunks {
                    if let Ok((cty, cb)) = self.store.get(&c) {
                        out.push((cty, cb));
                    }
                }
            }
            out.push((ty, body));
        }
        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).
    pub fn bundle_for_filtered(
        &self,
        changes: &[ChangeHash],
        max_blob: Option<u64>,
    ) -> 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 {
            let c = self.store.get_change(h)?;
            let sigs = self.store.signatures(h)?;
            add_object(&c.author, &mut objects);
            for s in &sigs {
                add_object(&s.key, &mut objects);
            }
            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) {
                            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);
            }
            bundle.changes.push((c, sigs));
        }
        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 });
        }
        for r in objects {
            let (ty, body) = self.store.get(&r)?;
            bundle.objects.push((ty, body));
        }
        Ok(bundle)
    }

    /// 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, body) in &bundle.objects {
            match ty {
                ObjectType::Chunk
                | ObjectType::Blob
                | ObjectType::Identity
                | ObjectType::State
                | ObjectType::Checkpoint => {
                    self.store.put(*ty, body)?;
                }
                other => return fail(format!("bundle may not carry {} objects", other.name())),
            }
        }
        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)?;
                }
            }
        }
        let incoming: BTreeSet<ChangeHash> = bundle.changes.iter().map(|(c, _)| c.hash()).collect();
        let boundary = self.boundary();
        let mut imported = Vec::new();
        for (c, sigs) in &bundle.changes {
            let h = c.hash();
            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)
                    ));
                }
            }
            self.store.put_change(c)?;
            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.
    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 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}"
                ));
            }
        }
        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())?;
        Ok(*serial)
    }

    /// 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> {
        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);
        match self.import_bundle(&bundle, policy.is_some()) {
            Ok(imported) => report.imported = imported,
            Err(e) => {
                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);
            match self.adopt_tag(&mut view, t, policy.as_ref()) {
                Ok(()) => {
                    names.push(name.clone());
                    results.push(RefResult {
                        name,
                        ok: true,
                        reason: String::new(),
                    });
                }
                Err(e) => results.push(RefResult {
                    name,
                    ok: false,
                    reason: e,
                }),
            }
        }
        if !names.is_empty() {
            self.record_op(
                &[op_ref],
                &format!("push received: {}", names.join(" ")),
                view,
            )?;
        }
        Ok(report)
    }

    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())?;
        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")?;
            if !p.authorizes(&signer, &u.name) {
                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 {
            let last = self
                .latest_ref_attest(&u.name)
                .map_err(|e| e.to_string())?
                .and_then(|(_, a)| match a.attest.body {
                    AttestBody::Ref { serial, .. } => Some(serial),
                    _ => None,
                });
            self.check_ref_attest(sa, &u.name, &u.old_state, &u.new_state, last, false)?;
            let r = self
                .store
                .put_attest(&sa.attest)
                .map_err(|e| e.to_string())?;
            for sig in &sa.sigs {
                self.store
                    .put_signature(&r, sig)
                    .map_err(|e| e.to_string())?;
            }
            self.set_ref_attest(&u.name, &r)
                .map_err(|e| e.to_string())?;
        }
        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(|| {
            RepoError::Msg(format!(
                "no remote named {name:?}; add one with `oxy remote add`"
            ))
        })
    }

    /// Credentials and trust for a remote: `remote.<name>.token` (else
    /// `$OXY_TOKEN`) and `remote.<name>.ca` (else `$OXY_CA`).
    /// 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)"
        ))
    }

    fn remote_options(&self, name: &str) -> RemoteOptions {
        RemoteOptions {
            repo_id: self.id().ok(),
            token: self
                .config
                .remote_tokens
                .get(name)
                .cloned()
                .or_else(|| std::env::var("OXY_TOKEN").ok()),
            ca: self
                .config
                .remote_cas
                .get(name)
                .map(PathBuf::from)
                .or_else(|| std::env::var("OXY_CA").ok().map(PathBuf::from)),
        }
    }

    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.
    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()?;
        // 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 max_blob = self.config.max_blob;
        for r in resp.refs {
            let needed = remote.negotiate(&have, &r.heads)?;
            let 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.
            let imported = self.import_bundle(&bundle, false)?;
            if max_blob.is_some() {
                self.register_missing_blobs(&imported, remote_name)?;
            }
            for h in &r.heads {
                if !self.store.has(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) {
                    return fail(format!(
                        "{remote_name}/{}: attested by {}, which the policy does not authorize",
                        r.name,
                        signer.short(12)
                    ));
                }
            }
            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(
                        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
                    ));
                }
                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()))?;
            self.adopt_tag(&mut view, t, policy.as_ref())
                .map_err(|e| RepoError::Msg(format!("{remote_name} {}: {e}", t.name)))?;
            new_tags += 1;
        }
        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,
        })
    }

    /// 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.
        let cp_bundle = remote.checkpoint(&r.heads)?;
        let mut cp_ref = None;
        for (ty, body) in &cp_bundle.objects {
            let put = self.store.put(*ty, body)?;
            if *ty == ObjectType::Checkpoint {
                cp_ref = Some(put);
            }
        }
        let Some(cp_ref) = cp_ref else {
            return fail(format!("{remote_name} sent no checkpoint"));
        };
        self.install_checkpoint(&bookmark, &cp_ref)?;
        // History window.
        let wanted = remote.shallow(&r.heads, 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)?;
        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> {
        self.fetch(remote_name)?;
        let (op_ref, op) = self.head_op()?;
        let bookmark = self.config.bookmark.clone();
        let local = self.heads_of(&op.view, &bookmark)?;
        let Some(remote_state) = op
            .view
            .remotes
            .get(remote_name)
            .and_then(|b| b.get(&bookmark))
            .copied()
        else {
            return Ok(PullReport {
                heads: local,
                changed: false,
                checked_out: false,
            });
        };
        let remote_heads = self.store.get_state(&remote_state)?.heads;
        let mut all = local.clone();
        all.extend(remote_heads);
        let st = self.load_state(&all)?;
        let mut heads = st.maximal(&all);
        heads.sort();
        let mut sorted_local = local.clone();
        sorted_local.sort();
        if heads == sorted_local {
            return Ok(PullReport {
                heads,
                changed: false,
                checked_out: false,
            });
        }
        let mut view = op.view.clone();
        view.bookmarks.insert(
            bookmark.clone(),
            self.store.put_state(&StateObject::new(heads.clone()))?,
        );
        self.record_op(&[op_ref], &format!("oxy pull {remote_name}"), view)?;
        // Follow along if the working copy was clean and on the old heads.
        let snap = self.snapshot("oxy pull")?;
        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 checked_out = false;
        if clean && base == sorted_local {
            self.checkout(&heads, false, "oxy pull (checkout)")?;
            checked_out = true;
        }
        Ok(PullReport {
            heads,
            changed: true,
            checked_out,
        })
    }

    /// Push the current bookmark (spec §16.3), sealing this author's unsigned
    /// changes and attesting the reference when a key is available.
    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>> {
        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(),
            }]);
        }
        for h in &remote_heads {
            if !self.store.has(h) {
                return fail(format!(
                    "the remote has change {} which you do not; pull first",
                    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)) {
                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(&remote_heads, &local_heads)?;
        let mut bundle = self.bundle_for(&to_send)?;
        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).
            let mut unsigned = Vec::new();
            for h in &to_send {
                let c = self.store.get_change(h)?;
                if (c.author == key || c.author.is_zero())
                    && !matches!(self.authenticate_change(h)?, SignatureStatus::Valid { .. })
                {
                    unsigned.push(*h);
                }
            }
            for chunk in unsigned.chunks(crate::attest::MAX_SEAL_HASHES) {
                let sa = self.sign_attest(&Attest::seal(self.id()?, chunk.to_vec()))?;
                bundle.attests.push(sa);
            }
            bundle = self.bundle_for(&to_send)?;
        }
        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: last_serial.unwrap_or(0) + 1,
                },
            })?),
            None => None,
        };
        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.attests.extend(more.attests);
                bundle.objects.extend(more.objects);
            }
            bundle.objects.push((
                ObjectType::State,
                StateObject::new(t.heads.clone()).encode(),
            ));
        }
        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> {
        let options = RemoteOptions {
            token: std::env::var("OXY_TOKEN").ok(),
            ca: std::env::var("OXY_CA").ok().map(PathBuf::from),
            repo_id: None,
        };
        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", 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()?;
        if let Some(depth) = depth {
            repo.fetch_shallow("origin", depth)?;
        } else {
            repo.fetch("origin")?;
        }
        let (op_ref, op) = repo.head_op()?;
        let bookmark = repo.config.bookmark.clone();
        if let Some(state) = op
            .view
            .remotes
            .get("origin")
            .and_then(|b| b.get(&bookmark))
            .copied()
        {
            let heads = repo.store.get_state(&state)?.heads;
            let mut view = op.view.clone();
            view.bookmarks.insert(bookmark, state);
            repo.record_op(&[op_ref], &format!("oxy clone {url}"), view)?;
            repo.checkout(&heads, false, "oxy clone (checkout)")?;
        }
        Ok(repo)
    }
}
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