//! Pruning a build cache before it is kept (oxygenflow#72, spec §5.4).

//!

//! A kept disk starts the next run, so without pruning it carries every

//! build's files: each crate's artifacts under every hash a lockfile or a

//! feature set ever gave it. Before a passing job's cache disk goes back

//! into the pool, the files this run did not use are removed, oldest

//! first, until what is on the disk is at most a share of its size (the

//! cap). The microVM's guest init prunes its disks before it powers off,

//! since only it can see inside them; the runner prunes the syscall

//! tier's directories itself. Both use this module.

//!

//! What the run used is told by the files' times against when the job

//! started: written (mtime), changed (ctime) or read (atime). Reads are

//! seen only where the file system records them: the guest mounts a cache

//! disk `strictatime`; on the runner, where a directory sits on whatever

//! file system the runner has, each file's atime is set to the epoch

//! before the job ([`forget_reads`]), so even `relatime` records its

//! first read. A file system mounted `noatime` records none, and then

//! nothing is pruned.

//!

//! A build tool does not read everything it relies on. Cargo, finding a

//! crate fresh, reads its fingerprint and only looks at its rlib and

//! binaries, so their times say nothing. Every name cargo gives a unit's

//! files carries the unit's hash (`.fingerprint/foo-<hash>/`,

//! `deps/libfoo-<hash>.rlib`, `build/foo-<hash>/`), so a file stays when

//! a file this run used carries one of the hashes in its path: a fresh

//! crate's fingerprint keeps its artifacts. A file with another link

//! that stays, or under a path the job's outputs name, stays too. What

//! goes is what this run's build did not touch, which a later build needs

//! only if an older lockfile comes back, and then cargo rebuilds it.

use serde::{Deserialize, Serialize};

use std::collections::{HashMap, HashSet};

use std::os::unix::fs::MetadataExt;

use std::path::{Path, PathBuf};

use std::time::{Duration, SystemTime};

/// The share of a cache disk a kept cache may fill when no runner says

/// otherwise: room for this run's build and as much again.

pub const DEFAULT_PERCENT: u8 = 60;

/// What pruning a cache found and did, in bytes of blocks on its disk.

#[derive(Serialize, Deserialize, Debug, Clone, Default, PartialEq, Eq)]

pub struct Pruned {

    /// The disk's size, and the share of it the cache may fill.

    pub size: u64,

    pub percent: u8,

    /// What was on the disk before pruning, and after.

    pub before: u64,

    pub after: u64,

    /// What went: bytes and files.

    pub freed: u64,

    pub files: u64,

    /// Why nothing could be pruned, when so (no access times, or an

    /// error): only the size then decides.

    #[serde(default, skip_serializing_if = "Option::is_none")]

    pub unpruned: Option<String>,

}

impl Pruned {

    /// The most the cache may keep: `percent` of `size`.

    pub fn cap(&self) -> u64 {

        cap(self.size, self.percent)

    }

    /// Whether what is left may be kept.

    pub fn fits(&self) -> bool {

        self.after <= self.cap()

    }

}

fn cap(size: u64, percent: u8) -> u64 {

    (size as u128 * percent.min(100) as u128 / 100) as u64

}

/// Now, as a job's start to compare file times against. File systems

/// stamp times from a clock that may lag this one by a tick, so the

/// moments either side of it are left to pass: nothing done before the

/// call (such as [`forget_reads`]) can look as though it came after.

pub fn start_clock() -> SystemTime {

    std::thread::sleep(Duration::from_millis(20));

    let t = SystemTime::now();

    std::thread::sleep(Duration::from_millis(20));

    t

}

/// Whether the file system holding `path` records reads (not `noatime`).

pub fn records_reads(path: &Path) -> bool {

    let Ok(c) = std::ffi::CString::new(path.as_os_str().as_encoded_bytes()) else { return false };

    let mut st: libc::statvfs = unsafe { std::mem::zeroed() };

    if unsafe { libc::statvfs(c.as_ptr(), &mut st) } != 0 {

        return false;

    }

    st.f_flag & libc::ST_NOATIME == 0

}

/// The blocks in use on the file system mounted at `path`, and its size

/// (as `df` counts them).

pub fn usage(path: &Path) -> std::io::Result<(u64, u64)> {

    let c = std::ffi::CString::new(path.as_os_str().as_encoded_bytes())?;

    let mut st: libc::statvfs = unsafe { std::mem::zeroed() };

    if unsafe { libc::statvfs(c.as_ptr(), &mut st) } != 0 {

        return Err(std::io::Error::last_os_error());

    }

    let unit = st.f_frsize as u64;

    Ok(((st.f_blocks as u64).saturating_sub(st.f_bfree as u64) * unit, st.f_blocks as u64 * unit))

}

/// Set every regular file's atime under `root` to the epoch, its mtime

/// kept, so the job's first read of each is recorded even on `relatime`

/// (which records a read only of a file read no later than it changed).

/// Links are not followed. The count of files.

pub fn forget_reads(root: &Path) -> std::io::Result<u64> {

    let mut n = 0;

    let mut err = None;

    walk(root, &mut |p, md| {

        if !md.file_type().is_file() {

            return;

        }

        let Ok(c) = std::ffi::CString::new(p.as_os_str().as_encoded_bytes()) else { return };

        let times = [libc::timespec { tv_sec: 0, tv_nsec: 0 }, libc::timespec { tv_sec: 0, tv_nsec: libc::UTIME_OMIT }];

        if unsafe { libc::utimensat(libc::AT_FDCWD, c.as_ptr(), times.as_ptr(), libc::AT_SYMLINK_NOFOLLOW) } == 0 {

            n += 1;

        } else if err.is_none() {

            err = Some(std::io::Error::last_os_error());

        }

    })?;

    match err {

        Some(e) => Err(e),

        None => Ok(n),

    }

}

/// Each entry under `root` (not `root` itself) with its metadata, parents

/// before children; links are not followed, nor other file systems

/// entered.

fn walk(root: &Path, f: &mut dyn FnMut(&Path, &std::fs::Metadata)) -> std::io::Result<()> {

    let dev = std::fs::symlink_metadata(root)?.dev();

    let mut dirs = vec![root.to_path_buf()];

    while let Some(d) = dirs.pop() {

        let Ok(entries) = std::fs::read_dir(&d) else { continue };

        for e in entries.flatten() {

            let p = e.path();

            let Ok(md) = std::fs::symlink_metadata(&p) else { continue };

            if md.dev() != dev {

                continue;

            }

            f(&p, &md);

            if md.is_dir() {

                dirs.push(p);

            }

        }

    }

    Ok(())

}

/// The unit hashes in `rel`: a 16-digit lowercase hex word after a `-`

/// in any of its names, as cargo names a unit's files.

fn hashes(rel: &Path) -> impl Iterator<Item = String> + '_ {

    rel.components().filter_map(|c| c.as_os_str().to_str()).flat_map(|name| {

        name.split('-').skip(1).filter_map(|w| {

            let w = w.split('.').next().unwrap_or("");

            (w.len() == 16 && w.bytes().all(|b| b.is_ascii_digit() || (b'a'..=b'f').contains(&b))).then(|| w.to_string())

        })

    })

}

fn nanos(s: i64, ns: i64) -> i128 {

    s as i128 * 1_000_000_000 + ns as i128

}

/// One file, however many names it has.

struct File {

    paths: Vec<PathBuf>,

    links: u64,

    bytes: u64,

    /// When it was last used as far as can be told: the later of its

    /// atime and mtime.

    last: i128,

    used: bool,

}

/// Prune the cache at `root`, a disk of `size` bytes with `before` in

/// use, down to `percent` of its size, removing only regular files this

/// run did not use (see the module's notes), those used longest ago

/// first. `started` is when the job started ([`start_clock`]); `keep` are

/// paths relative to `root` (the job's outputs) that stay whatever their

/// times. Under the cap nothing is touched. `after` is `before` less what

/// was freed.

pub fn prune(root: &Path, started: SystemTime, keep: &[String], size: u64, before: u64, percent: u8) -> std::io::Result<Pruned> {

    let mut out = Pruned { size, percent, before, after: before, ..Pruned::default() };

    if before <= out.cap() {

        return Ok(out);

    }

    let since = started.duration_since(SystemTime::UNIX_EPOCH).map(|d| d.as_nanos() as i128).unwrap_or(0);

    let keep: Vec<PathBuf> = keep.iter().map(|k| PathBuf::from(k.trim_start_matches('/'))).filter(|k| !k.as_os_str().is_empty()).collect();

    let mut files: HashMap<(u64, u64), File> = HashMap::new();

    walk(root, &mut |p, md| {

        if !md.file_type().is_file() {

            return;

        }

        let rel = p.strip_prefix(root).unwrap_or(p);

        let used = [nanos(md.atime(), md.atime_nsec()), nanos(md.mtime(), md.mtime_nsec()), nanos(md.ctime(), md.ctime_nsec())]

            .into_iter()

            .any(|t| t >= since)

            || keep.iter().any(|k| rel.starts_with(k));

        let f = files.entry((md.dev(), md.ino())).or_insert_with(|| File {

            paths: Vec::new(),

            links: md.nlink(),

            bytes: md.blocks() * 512,

            last: nanos(md.atime(), md.atime_nsec()).max(nanos(md.mtime(), md.mtime_nsec())),

            used: false,

        });

        f.paths.push(p.to_path_buf());

        f.used |= used;

    })?;

    // A hash any used file carries keeps every file carrying it.

    let live: HashSet<String> =

        files.values().filter(|f| f.used).flat_map(|f| f.paths.iter().flat_map(|p| hashes(p.strip_prefix(root).unwrap_or(p)))).collect();

    let mut gone: Vec<File> = files

        .into_values()

        .filter(|f| {

            !f.used

                // A link from outside the cache: removing it frees nothing.

                && f.links == f.paths.len() as u64

                && !f.paths.iter().any(|p| hashes(p.strip_prefix(root).unwrap_or(p)).any(|h| live.contains(&h)))

        })

        .collect();

    gone.sort_by_key(|f| f.last);

    let mut emptied: HashSet<PathBuf> = HashSet::new();

    for f in gone {

        if out.after <= out.cap() {

            break;

        }

        let mut removed = true;

        for p in &f.paths {

            if std::fs::remove_file(p).is_err() {

                removed = false;

            } else if let Some(parent) = p.parent() {

                emptied.insert(parent.to_path_buf());

            }

        }

        if removed {

            out.after = out.after.saturating_sub(f.bytes);

            out.freed += f.bytes;

            out.files += 1;

        }

    }

    // Directories pruning emptied go too, deepest first; never `root`.

    let mut dirs: Vec<PathBuf> = emptied.into_iter().collect();

    dirs.sort_by_key(|d| std::cmp::Reverse(d.components().count()));

    for d in dirs {

        let mut d = d.as_path();

        while d != root && d.starts_with(root) && std::fs::remove_dir(d).is_ok() {

            match d.parent() {

                Some(p) => d = p,

                None => break,

            }

        }

    }

    Ok(out)

}

#[cfg(test)]

mod tests {

    use super::*;

    /// Times around a job that started at `T`, ahead of the clock: a file

    /// whose times are set is changed (ctime) now, before it.

    const T: u64 = 4_000_000_000;

    fn at(secs: u64) -> SystemTime {

        SystemTime::UNIX_EPOCH + Duration::from_secs(secs)

    }

    /// A file of `kib` KiB at `rel`, last read and written at `secs`.

    fn file(root: &Path, rel: &str, kib: usize, secs: u64) -> PathBuf {

        let p = root.join(rel);

        std::fs::create_dir_all(p.parent().unwrap()).unwrap();

        let bytes: Vec<u8> = (0..kib << 10).map(|i: usize| (i.wrapping_mul(2_654_435_761) >> 13) as u8).collect();

        std::fs::write(&p, bytes).unwrap();

        let f = std::fs::File::options().write(true).open(&p).unwrap();

        f.set_times(std::fs::FileTimes::new().set_accessed(at(secs)).set_modified(at(secs))).unwrap();

        p

    }

    fn bytes(p: &Path) -> u64 {

        std::fs::metadata(p).unwrap().blocks() * 512

    }

    #[test]

    fn hashes_are_cargos_unit_hashes() {

        let h = |p: &str| hashes(Path::new(p)).collect::<Vec<_>>();

        assert_eq!(h("debug/deps/libdep-8f2b19826c505967.rlib"), ["8f2b19826c505967"]);

        assert_eq!(h("debug/.fingerprint/dep-8f2b19826c505967/lib-dep"), ["8f2b19826c505967"]);

        assert_eq!(h("debug/build/my-crate-0123456789abcdef/out/x.o"), ["0123456789abcdef"]);

        assert!(h("debug/incremental/app-2dadnjnzlrswc/s-hndl5uwdpw-1uaho4m.lock").is_empty());

        assert!(h("debug/app").is_empty() && h("debug/deps/x-0123456789ABCDEF").is_empty() && h("x-0123456789abcde").is_empty());

    }

    #[test]

    fn under_the_cap_nothing_is_touched() {

        let dir = tempfile::tempdir().unwrap();

        let old = file(dir.path(), "debug/deps/libold-1111111111111111.rlib", 64, 1_000);

        let p = prune(dir.path(), at(T), &[], 1 << 30, 64 << 10, 60).unwrap();

        assert!(old.exists() && p.fits() && p.freed == 0 && p.after == p.before, "{p:?}");

    }

    /// A cargo target as a run leaves it: this lockfile's units used this

    /// run (one rebuilt, one fresh: only its fingerprint read), an older

    /// lockfile's not, and an output the run did not touch.

    #[test]

    fn what_this_run_did_not_use_goes_oldest_first_and_its_units_stay_whole() {

        let dir = tempfile::tempdir().unwrap();

        let root = dir.path();

        let app = file(root, "debug/deps/app-aaaaaaaaaaaaaaaa", 256, T + 100);

        let app_fp = file(root, "debug/.fingerprint/app-aaaaaaaaaaaaaaaa/bin-app", 1, T + 100);

        let dep_rlib = file(root, "debug/deps/libdep-bbbbbbbbbbbbbbbb.rlib", 256, T - 5_000);

        let dep_fp = file(root, "debug/.fingerprint/dep-bbbbbbbbbbbbbbbb/lib-dep", 1, T - 5_000);

        std::fs::File::open(&dep_fp).unwrap().set_times(std::fs::FileTimes::new().set_accessed(at(T + 50))).unwrap();

        let script = file(root, "debug/build/dep-bbbbbbbbbbbbbbbb/out/gen.rs", 64, T - 5_000);

        // Uplifted: another name of the rebuilt binary.

        std::fs::hard_link(&app, root.join("debug/app")).unwrap();

        // An older lockfile's units, untouched, the oldest first to go.

        let oldest = file(root, "debug/deps/libdep-cccccccccccccccc.rlib", 256, T - 9_000);

        let oldest_fp = file(root, "debug/.fingerprint/dep-cccccccccccccccc/lib-dep", 1, T - 8_999);

        let older = file(root, "debug/deps/libdep-dddddddddddddddd.rlib", 256, T - 8_000);

        // A declared output, older still, that the run did not touch.

        let output = file(root, "release/tool", 128, T - 10_000);

        let all = [&app, &app_fp, &dep_rlib, &dep_fp, &script, &oldest, &oldest_fp, &older, &output];

        let before: u64 = all.iter().map(|p| bytes(p)).sum();

        // Room for all but the oldest unit.

        let want = before - bytes(&oldest) - bytes(&oldest_fp);

        let size = (want * 100).div_ceil(60);

        let p = prune(root, at(T), &["/release/tool".into()], size, before, 60).unwrap();

        assert!(p.fits(), "{p:?}");

        assert!(

            !oldest.exists() && !root.join("debug/.fingerprint/dep-cccccccccccccccc").exists(),

            "the oldest unit went, its emptied directory too"

        );

        assert!(older.exists(), "and pruning stopped under the cap: {p:?}");

        for kept in [&app, &app_fp, &dep_rlib, &dep_fp, &script, &output, &root.join("debug/app")] {

            assert!(kept.exists(), "{} stays", kept.display());

        }

        assert_eq!((p.files, p.freed, p.after), (2, p.before - want, want));

        // With no room at all: still only what the run did not use goes.

        let p = prune(root, at(T), &["release/tool".into()], 1, p.after, 60).unwrap();

        assert!(!older.exists() && !p.fits() && p.files == 1, "{p:?}");

        for kept in [&app, &dep_rlib, &dep_fp, &script, &output] {

            assert!(kept.exists(), "{} stays", kept.display());

        }

        assert!(root.join("debug/deps").is_dir(), "a directory still holding files stays");

    }

    #[test]

    fn reads_are_forgotten_so_the_next_is_seen() {

        let dir = tempfile::tempdir().unwrap();

        let p = file(dir.path(), "a/b", 1, 5_000);

        std::os::unix::fs::symlink("/etc/passwd", dir.path().join("link")).unwrap();

        assert_eq!(forget_reads(dir.path()).unwrap(), 1);

        let md = std::fs::metadata(&p).unwrap();

        assert_eq!((md.atime(), md.mtime()), (0, 5_000), "the mtime is kept");

        assert!(std::fs::symlink_metadata(dir.path().join("link")).unwrap().atime() > 0, "a link is not followed");

    }

}