epicenter_cli/

prune.rs

//! Garbage collection for the shared cache.
//!
//! The key idea: every artifact in the cache belongs to a build "unit"
//! identified by a `name-hash` key (Cargo appends a metadata hash to
//! filenames). Deleting a *whole unit* keeps Cargo's bookkeeping consistent;
//! and if anything still needed got deleted, Cargo simply rebuilds it —
//! pruning is safe by design.
//!

use std::collections::BTreeMap;
use std::fs;
use std::io::ErrorKind;
use std::path::Component;
use std::path::{Path, PathBuf};
use std::time::SystemTime;

use anyhow::Result;
use walkdir::WalkDir;

/// One build unit: all files in the cache sharing a `name-hash` key.
pub struct Group {
    /// e.g. `serde-deadbeef`
    pub key: String,
    /// Every file belonging to the unit (may span `deps/` and `.fingerprint/`).
    pub files: Vec<PathBuf>,
    /// For `build/`, `incremental/`, `.fingerprint/` units: the directory that
    /// backs the whole unit, deleted as a whole when stale.
    pub anchor: Option<PathBuf>,
    /// Newest mtime seen among the files ("last used").
    pub newest: Option<SystemTime>,
    /// Total size of all files.
    pub bytes: u64,
}

/// Extract `name-hash` from a Cargo artifact name.
fn name_hash(name: &str, strip_ext: bool) -> Option<String> {
    // Final component only (survives both `path\to\name` and `path/to/name`).
    let file = name.replace('\\', "/");
    let file = file.split('/').next_back()?;

    let stem = if strip_ext {
        Path::new(file).file_stem()?.to_str()?
    } else {
        file
    };

    // Split at the LAST `-`; no dash or an empty hash is unhashable.
    let (name, hash) = stem.rsplit_once('-')?;
    if hash.is_empty() {
        return None;
    }

    // `.fingerprint/.../<dir>` anchors have no `lib` prefix — dep files do.
    let name = if strip_ext {
        name.strip_prefix("lib").unwrap_or(name)
    } else {
        name
    };

    // Normalize dashes to underscores in the name part so that
    // `deps/libtokio_util-abc.rlib` and `.fingerprint/tokio-util-abc/`
    // land on the SAME key and get deleted together.
    Some(format!("{}-{}", name.replace('-', "_"), hash))
}

/// Classify a file inside the cache into its build unit.
pub fn artifact_key(root: &Path, path: &Path) -> Option<String> {
    if !path.starts_with(root) {
        return None;
    }
    // Only the components RELATIVE to the root are scanned — otherwise the
    // root's own name can trigger keywords (the cache root is literally
    // named `.../epicenter/build`, which would classify every file as
    // an `anchor` unit and silently empty the whole scan!).
    let components: Vec<&str> = path
        .strip_prefix(root)
        .ok()?
        .components()
        .filter(|c| !matches!(c, Component::RootDir))
        .filter_map(|c| c.as_os_str().to_str())
        .collect();

    if components.len() < 2 || components.iter().any(|c| *c == ".cargo-lock") {
        return None;
    }

    for (i, comp) in components.iter().enumerate() {
        match *comp {
            // deps/<libname-hash>.rlib|.rmeta|.d ... — key from the file name
            "deps" => {
                let file = components.last()?;
                return name_hash(file, true);
            }
            // build|incremental|.fingerprint followed by <crate-hash>= anchor
            "build" | "incremental" | ".fingerprint" => {
                let anchor_dir = components.get(i + 1)?;
                return name_hash(anchor_dir, false);
            }
            _ => {}
        }
    }
    None
}

/// For `build/`, `incremental/` and `.fingerprint/` units: the directory
/// backing the unit (everything up to and including the component AFTER the
/// keyword, joined onto `root`).
fn anchor_for(root: &Path, path: &Path) -> Option<PathBuf> {
    let rel = path.strip_prefix(root).ok()?;
    let comps: Vec<Component> = rel.components().collect();
    for (i, comp) in comps.iter().enumerate() {
        let name = comp.as_os_str().to_str()?;
        if matches!(name, "build" | "incremental" | ".fingerprint") {
            let anchor: PathBuf = comps.iter().take(i + 2).collect();
            return Some(root.join(anchor));
        }
    }
    None
}

/// Walk `root` and group every file into units.
pub fn scan(root: &Path) -> Result<BTreeMap<String, Group>> {
    let mut groups: BTreeMap<String, Group> = BTreeMap::new();
    for entry in WalkDir::new(root).min_depth(1) {
        // Unreadable entries are skipped rather than failing the whole scan.
        let entry = match entry {
            Ok(e) => e,
            Err(_) => continue,
        };
        if !entry.file_type().is_file() {
            continue;
        }
        // `artifact_key` -> None means "never touch" (locks, root files...).
        let Some(key) = artifact_key(root, entry.path()) else {
            continue;
        };
        let meta = entry.metadata()?;
        let group = groups.entry(key.clone()).or_insert_with(|| Group {
            key,
            files: Vec::new(),
            anchor: None,
            newest: None,
            bytes: 0,
        });
        group.bytes += meta.len();
        group.files.push(entry.into_path());
        if let Ok(mtime) = meta.modified() {
            if group.newest.map_or(false, |n| mtime > n) || group.newest.is_none() {
                group.newest = Some(mtime);
            }
        }
    }
    for group in groups.values_mut() {
        group.anchor = group.files.iter().find_map(|f| anchor_for(root, f));
    }
    Ok(groups)
}

#[derive(Debug, Default)]
pub struct PruneReport {
    pub removed_groups: u64,
    pub kept_groups: u64,
    pub files_freed: u64,
    pub bytes_freed: u64,
    pub errors: u64,
}

/// The garbage collector.
pub fn prune(root: &Path, cutoff: SystemTime, delete: bool) -> Result<PruneReport> {
    let groups = scan(root)?;
    let mut report = PruneReport::default();
    for group in groups.into_values() {
        // No usable mtime -> keep (conservative).
        let Some(newest) = group.newest else {
            report.kept_groups += 1;
            continue;
        };
        if newest >= cutoff {
            report.kept_groups += 1;
            continue;
        }

        // Count everything BEFORE deleting so dry-runs report identically.
        report.removed_groups += 1;
        report.files_freed += group.files.len() as u64;
        report.bytes_freed += group.bytes;
        if !delete {
            continue;
        }

        match &group.anchor {
            Some(anchor) => {
                if let Err(e) = fs::remove_dir_all(anchor) {
                    if e.kind() != ErrorKind::NotFound {
                        report.errors += 1;
                        eprintln!("could not remove {}: {e}", anchor.display());
                    }
                }
                // One unit can span both `.fingerprint/<dir>` and `deps/`: the
                // anchor covers the first family; files outside it go individually.
                for file in &group.files {
                    if file.starts_with(anchor) {
                        continue;
                    }
                    if let Err(e) = fs::remove_file(file) {
                        if e.kind() != ErrorKind::NotFound {
                            report.errors += 1;
                            eprintln!("could not remove {}: {e}", file.display());
                        }
                    }
                }
            }
            None => {
                for file in &group.files {
                    if let Err(e) = fs::remove_file(file) {
                        if e.kind() != ErrorKind::NotFound {
                            report.errors += 1;
                            eprintln!("could not remove {}: {e}", file.display());
                        }
                    }
                }
            }
        }
    }
    Ok(report)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn name_hash_strips_and_normalizes() {
        assert_eq!(
            name_hash(".fingerprints\\libserde-deadbeef.rlib", true),
            Some("serde-deadbeef".into())
        );
        assert_eq!(
            name_hash("sada/libtokio_util-abc123.rmeta", true),
            Some("tokio_util-abc123".into())
        );
        assert_eq!(
            name_hash("mybin-abc123.d", true),
            Some("mybin-abc123".into())
        );
        // Anchor dirs never have the `lib` prefix, but dashes -> underscores applies.
        assert_eq!(
            name_hash("tokio-util-abc123", false),
            Some("tokio_util-abc123".into())
        );
        assert_eq!(
            name_hash("serde-deadbeef", false),
            Some("serde-deadbeef".into())
        );
    }

    #[test]
    fn name_hash_rejects_unhashable_names() {
        assert_eq!(name_hash("nometa", false), None);
        assert_eq!(name_hash("weird", false), None);
    }

    #[test]
    fn keys_deps_and_fingerprints_together() {
        let root = Path::new("/cache");
        assert_eq!(
            artifact_key(root, Path::new("/cache/debug/deps/libserde-deadbeef.rlib")),
            Some("serde-deadbeef".into()),
        );
        assert_eq!(
            artifact_key(
                root,
                Path::new("/cache/debug/.fingerprint/serde-deadbeef/root-output")
            ),
            Some("serde-deadbeef".into()),
            "same unit must share one key so they get deleted together",
        );
    }

    #[test]
    fn keys_build_and_incremental_by_anchor_dir() {
        let root = Path::new("/cache");
        assert_eq!(
            artifact_key(
                root,
                Path::new("/cache/debug/build/tokio-util-abc/out/main.rs")
            ),
            Some("tokio_util-abc".into()),
        );
        assert_eq!(
            artifact_key(
                root,
                Path::new("/cache/debug/incremental/myproj-1a2b/s-gx/cache.bin")
            ),
            Some("myproj-1a2b".into()),
        );
    }

    #[test]
    fn leaves_protected_files_alone() {
        let root = Path::new("/cache");
        assert_eq!(artifact_key(root, Path::new("/cache/.cargo-lock")), None);
        assert_eq!(
            artifact_key(root, Path::new("/cache/.rustc_info.json")),
            None
        );
        assert_eq!(artifact_key(root, Path::new("/cache/debug/mybin.d")), None);
        assert_eq!(artifact_key(root, Path::new("/elsewhere/x")), None);
    }

    #[test]
    fn scans_units_and_anchors() -> Result<()> {
        let root = tempfile::tempdir()?;
        let deps = root.path().join("debug/deps");
        let fingerprint = root.path().join("debug/.fingerprint/anyhow-abc");
        fs::create_dir_all(&deps)?;
        fs::create_dir_all(&fingerprint)?;
        fs::write(deps.join("libanyhow-abc.rlib"), [0u8; 10])?;
        fs::write(deps.join("libanyhow-abc.rmeta"), [0u8; 5])?;
        fs::write(fingerprint.join("root-output"), [0u8; 1])?;
        fs::write(root.path().join("debug/.cargo-lock"), [0u8; 2])?;

        let groups = scan(root.path())?;
        assert_eq!(
            groups.len(),
            1,
            "lock file is not a unit; got {:?}",
            groups.keys()
        );
        let group = &groups["anyhow-abc"];
        assert_eq!(group.files.len(), 3);
        assert_eq!(group.bytes, 16);
        assert_eq!(group.anchor.as_deref(), Some(fingerprint.as_path()));
        Ok(())
    }

    #[test]
    fn prune_reports_then_deletes_stale_units() -> Result<()> {
        let root = tempfile::tempdir()?;
        let deps = root.path().join("debug/deps");
        let fingerprint = root.path().join("debug/.fingerprint/x-abc");
        fs::create_dir_all(&deps)?;
        fs::create_dir_all(&fingerprint)?;
        fs::write(deps.join("libx-abc.rlib"), [0u8; 7])?;
        fs::write(fingerprint.join("root-output"), [0u8; 2])?;

        // Cutoff in the future makes everything look stale — deterministic.
        let cutoff = SystemTime::now() + std::time::Duration::from_secs(3_600);

        // Dry run: full report, but nothing actually deleted.
        let report = prune(root.path(), cutoff, false)?;
        assert_eq!(report.removed_groups, 1);
        assert_eq!(report.files_freed, 2);
        assert_eq!(report.bytes_freed, 9);
        assert!(
            deps.join("libx-abc.rlib").exists(),
            "dry run must not delete"
        );

        // Real run: files AND the .fingerprint dir are gone.
        let report = prune(root.path(), cutoff, true)?;
        assert_eq!(report.bytes_freed, 9);
        assert!(!deps.join("libx-abc.rlib").exists());
        assert!(!fingerprint.exists());
        Ok(())
    }
}