qk_cache/

evict.rs

//! Keeps the cache under its size limit by evicting the least recently used
//! entries, as Nx's `maxCacheSize` does: once over the limit, down to 90% of
//! it. As in Nx, entries unused for a week go whatever the limit.
//!
//! An entry's last use is its manifest's modification time, refreshed on every
//! hit. Blobs are shared between entries, so a blob is deleted only once no
//! remaining manifest cites it. A blob no manifest cites may belong to an entry
//! another run is still publishing, so it is kept until it is an hour old.
//! Blobs a fetch relies on are [pinned](pin) until it publishes its record,
//! and kept whatever their age or how many manifests cite them.

use std::collections::{BTreeMap, BTreeSet};
use std::fs::{self, File};
use std::io::{BufRead, Write};
use std::path::{Path, PathBuf};
use std::time::{Duration, SystemTime};

use anyhow::{Context, Result, bail};
use fs2::FileExt;
use serde::{Deserialize, Serialize};
use serde_json::Value;

/// How long an unreferenced blob or scratch file may be part of a publish.
const IN_FLIGHT: Duration = Duration::from_secs(60 * 60);

/// How long an entry may go unused before it is evicted, as Nx's
/// `remove_old_cache_records` evicts records not accessed in 7 days.
const UNUSED: Duration = Duration::from_secs(7 * 24 * 60 * 60);

#[derive(Debug, Default, PartialEq, Eq)]
pub struct Pruned {
    /// Bytes the cache holds afterwards.
    pub size: u64,
    pub entries: usize,
    pub blobs: usize,
    pub freed: u64,
}

/// Nx's `parseMaxCacheSize`: a number of bytes with an optional `KB`, `MB` or
/// `GB` unit, in powers of 1024.
pub fn parse_size(text: &str) -> Result<u64> {
    let text = text.trim();
    let (number, unit) = match text.find(|c: char| !(c.is_ascii_digit() || c == '.')) {
        Some(at) => (&text[..at], text[at..].trim_start()),
        None => (text, ""),
    };
    let factor: u64 = match unit {
        "" | "B" => 1,
        "KB" => 1 << 10,
        "MB" => 1 << 20,
        "GB" => 1 << 30,
        _ => bail!("invalid cache size {text:?}: use a number with an optional KB, MB or GB"),
    };
    if number.matches('.').count() > 1 || number.is_empty() {
        bail!("invalid cache size {text:?}");
    }
    let number: f64 = number
        .parse()
        .with_context(|| format!("invalid cache size {text:?}"))?;
    Ok((number * factor as f64) as u64)
}

/// The size limit: `QK_MAX_CACHE_SIZE`, qk's alone, then `NX_MAX_CACHE_SIZE`,
/// then nx.json `maxCacheSize`, then, as in Nx, a tenth of the disk holding the
/// cache, or 100GB when the disk's size cannot be read. `None` when the limit
/// is 0, which means unlimited.
pub fn max_size(configured: Option<&Value>, cache: &Path) -> Result<Option<u64>> {
    let configured =
        match std::env::var("QK_MAX_CACHE_SIZE").or_else(|_| std::env::var("NX_MAX_CACHE_SIZE")) {
            Ok(value) => Some(value),
            Err(_) => configured.map(|value| match value {
                Value::String(text) => text.clone(),
                other => other.to_string(),
            }),
        };
    let limit = match configured {
        Some(text) => parse_size(&text)?,
        None => cache
            .ancestors()
            .find(|path| path.exists())
            .and_then(|existing| fs2::total_space(existing).ok())
            .map_or(100 << 30, |total| total / 10),
    };
    Ok((limit > 0).then_some(limit))
}

/// Where pins live: under `locks/`, whose dot-named items are not lock files.
fn pins(root: &Path) -> PathBuf {
    root.join("locks").join(".pins")
}

/// Held shared while a pin is made, and exclusively by a prune while it
/// deletes, so a pin made while a prune runs waits until it is done.
fn pin_gate(root: &Path) -> Result<File> {
    let directory = pins(root);
    fs::create_dir_all(&directory)?;
    Ok(File::options()
        .create(true)
        .truncate(false)
        .write(true)
        .open(directory.join(".gate"))?)
}

/// Blobs kept from eviction while this is alive. Each pin is two files: one
/// its holder keeps locked, so a prune can tell a live pin from an abandoned
/// one, and one naming the blobs, which a prune can read even where a lock
/// keeps other processes from reading the file it covers.
#[derive(Debug)]
pub(crate) struct Pin {
    lock: Option<File>,
    path: PathBuf,
}

/// Keeps `blobs` from being pruned until the returned [`Pin`] is dropped. A
/// prune already deleting finishes first, so a caller that checks which blobs
/// exist after pinning sees every deletion.
pub(crate) fn pin(root: &Path, blobs: &BTreeSet<String>) -> Result<Pin> {
    let gate = pin_gate(root)?;
    gate.lock_shared()?;
    let directory = pins(root);
    let name = format!(
        "{}-{}",
        std::process::id(),
        &blake3::hash(
            format!("{:?}{:?}", std::thread::current().id(), SystemTime::now()).as_bytes()
        )
        .to_hex()[..16]
    );
    let path = directory.join(name);
    let lock = File::options()
        .create_new(true)
        .write(true)
        .open(path.with_extension("lock"))?;
    lock.lock_exclusive()?;
    let pin = Pin {
        lock: Some(lock),
        path,
    };
    let mut listing = String::new();
    for blob in blobs {
        listing.push_str(blob);
        listing.push('\n');
    }
    let mut file = File::create(pin.path.with_extension("blobs"))?;
    file.write_all(listing.as_bytes())?;
    drop(gate);
    Ok(pin)
}

impl Drop for Pin {
    fn drop(&mut self) {
        let _ = fs::remove_file(self.path.with_extension("blobs"));
        // Unlock last: a prune that finds the lock free removes what is left.
        drop(self.lock.take());
        let _ = fs::remove_file(self.path.with_extension("lock"));
    }
}

/// The blobs live pins hold, removing pins whose holder is gone.
fn pinned(root: &Path) -> BTreeSet<String> {
    let mut held = BTreeSet::new();
    for item in fs::read_dir(pins(root)).into_iter().flatten().flatten() {
        let path = item.path();
        if path.extension().is_none_or(|extension| extension != "lock") {
            continue;
        }
        let Ok(lock) = File::open(&path) else {
            continue;
        };
        if lock.try_lock_exclusive().is_ok() {
            let _ = fs::remove_file(path.with_extension("blobs"));
            let _ = fs::remove_file(&path);
            continue;
        }
        if let Ok(listing) = File::open(path.with_extension("blobs")) {
            held.extend(
                std::io::BufReader::new(listing)
                    .lines()
                    .map_while(Result::ok),
            );
        }
    }
    held
}

/// Refreshes an entry's last use.
pub(crate) fn touch(manifest: &Path) {
    if let Ok(file) = File::options().write(true).open(manifest) {
        let _ = file.set_modified(SystemTime::now());
    }
}

struct Entry {
    path: std::path::PathBuf,
    used: SystemTime,
    size: u64,
    blobs: BTreeSet<String>,
}

/// Evicts entries unused for a week and, when the cache holds more than
/// `limit` bytes, least recently used entries until it holds at most 90% of
/// it, and removes what no entry needs. Another prune in progress makes
/// this one return without doing anything.
///
/// Explicit pruning always remeasures the cache, including files changed in
/// place by external tools. Normal runs use [`prune_automatic`].
pub fn prune(root: &Path, limit: u64) -> Result<Pruned> {
    prune_inner(root, limit, false)
}

/// Enforces the limit after a run, reusing size accounting while the cache's
/// directories are unchanged. Normal publication replaces records atomically
/// and blobs are immutable, so directory changes invalidate the memo. A full
/// reconciliation still runs at least once an hour.
pub fn prune_automatic(root: &Path, limit: u64) -> Result<Pruned> {
    prune_inner(root, limit, true)
}

fn prune_inner(root: &Path, limit: u64, automatic: bool) -> Result<Pruned> {
    if !root.join("entries").is_dir() {
        return Ok(Pruned::default());
    }
    fs::create_dir_all(root.join("locks"))?;
    let marker = root.join("locks").join(".prune");
    let recent = fs::metadata(&marker)
        .ok()
        .and_then(|metadata| metadata.modified().ok())
        .and_then(|modified| SystemTime::now().duration_since(modified).ok())
        .is_some_and(|age| age < IN_FLIGHT);
    if automatic && recent {
        let size = cached_size(root)?;
        if size <= limit {
            return Ok(Pruned {
                size,
                ..Pruned::default()
            });
        }
    }
    let guard = File::options()
        .create(true)
        .truncate(false)
        .write(true)
        .open(&marker)?;
    if guard.try_lock_exclusive().is_err() {
        return Ok(Pruned::default());
    }
    // Never associate a computed post-eviction size with stamps taken only
    // after eviction: a concurrent publisher could have added uncounted bytes.
    let _ = fs::remove_file(root.join("locks").join(".size"));
    let _ = guard.set_modified(SystemTime::now());
    // Held until deletion ends, so no pin is made while blobs are deleted.
    let gate = pin_gate(root)?;
    gate.lock_exclusive()?;
    let now = SystemTime::now();
    let age = |metadata: &fs::Metadata| {
        metadata
            .modified()
            .ok()
            .and_then(|modified| now.duration_since(modified).ok())
            .unwrap_or_default()
    };
    let mut entries = Vec::new();
    let mut references: BTreeMap<String, usize> = BTreeMap::new();
    // Warm state records cite blobs as entries do, and age out the same way.
    // Path sets cite none, but take space and age out like entries.
    let warm = fs::read_dir(root.join("warm")).into_iter().flatten();
    let pathsets = fs::read_dir(root.join("pathsets")).into_iter().flatten();
    for item in fs::read_dir(root.join("entries"))?
        .chain(warm)
        .chain(pathsets)
    {
        let item = item?;
        let metadata = item.metadata()?;
        // Unreadable manifests are misses already; they only take space.
        let blobs = fs::read(item.path())
            .ok()
            .and_then(|bytes| crate::record::StoredRecord::read(&bytes).ok())
            .map(|record| record.blobs())
            .unwrap_or_default();
        for blob in &blobs {
            *references.entry(blob.clone()).or_default() += 1;
        }
        entries.push(Entry {
            path: item.path(),
            used: metadata.modified().unwrap_or(SystemTime::UNIX_EPOCH),
            size: metadata.len(),
            blobs,
        });
    }
    // A pin is a reference no eviction removes.
    for blob in pinned(root) {
        *references.entry(blob).or_default() += 1;
    }
    let mut blob_sizes = BTreeMap::new();
    let mut pruned = Pruned::default();
    for item in fs::read_dir(root.join("blobs"))? {
        let item = item?;
        let metadata = item.metadata()?;
        let name = item.file_name().to_string_lossy().into_owned();
        if !references.contains_key(&name) && age(&metadata) > IN_FLIGHT {
            if fs::remove_file(item.path()).is_ok() {
                pruned.blobs += 1;
                pruned.freed += metadata.len();
            }
            continue;
        }
        blob_sizes.insert(name, metadata.len());
    }
    let mut size: u64 =
        entries.iter().map(|entry| entry.size).sum::<u64>() + blob_sizes.values().sum::<u64>();
    entries.sort_by_key(|entry| entry.used);
    // Nx stops a tenth below the limit, so the next run does not evict again.
    let target = if size > limit {
        limit - limit / 10
    } else {
        limit
    };
    for entry in entries {
        let unused = now.duration_since(entry.used).is_ok_and(|age| age > UNUSED);
        if size <= target && !unused {
            break;
        }
        if fs::remove_file(&entry.path).is_err() {
            continue;
        }
        pruned.entries += 1;
        pruned.freed += entry.size;
        size -= entry.size;
        for blob in entry.blobs {
            let count = references.get_mut(&blob).expect("counted above");
            *count -= 1;
            if *count == 0
                && let Some(blob_size) = blob_sizes.remove(&blob)
                && fs::remove_file(root.join("blobs").join(&blob)).is_ok()
            {
                pruned.blobs += 1;
                pruned.freed += blob_size;
                size -= blob_size;
            }
        }
    }
    drop(gate);
    // Scratch space of runs that ended without cleaning up.
    for item in fs::read_dir(root.join("tmp"))
        .into_iter()
        .flatten()
        .flatten()
    {
        if item
            .metadata()
            .is_ok_and(|metadata| age(&metadata) > IN_FLIGHT)
        {
            let _ = fs::remove_dir_all(item.path()).or_else(|_| fs::remove_file(item.path()));
        }
    }
    // Locks of keys without an entry, unless another run holds them.
    for item in fs::read_dir(root.join("locks"))?.flatten() {
        let name = item.file_name().to_string_lossy().into_owned();
        if name.starts_with('.') || root.join("entries").join(format!("{name}.json")).exists() {
            continue;
        }
        if item
            .metadata()
            .is_ok_and(|metadata| age(&metadata) > IN_FLIGHT)
            && let Ok(file) = File::open(item.path())
            && file.try_lock_exclusive().is_ok()
        {
            let _ = fs::remove_file(item.path());
        }
    }
    pruned.size = if automatic { cached_size(root)? } else { size };
    Ok(pruned)
}

// Recent timestamps may have coarse resolution on the backing filesystem.
const STAMP_SETTLE: Duration = Duration::from_secs(2);

#[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
struct DirectoryStamp {
    modified: SystemTime,
    len: u64,
    #[cfg(unix)]
    identity: (u64, u64, i64, i64),
}

#[derive(Debug, PartialEq, Eq, Serialize, Deserialize)]
struct SizeMemo {
    measured: SystemTime,
    stamps: Vec<Option<DirectoryStamp>>,
    size: u64,
}

fn directory_stamps(root: &Path) -> Result<Vec<Option<DirectoryStamp>>> {
    ["entries", "blobs", "warm", "pathsets"]
        .iter()
        .map(|name| {
            let metadata = match fs::metadata(root.join(name)) {
                Ok(metadata) => metadata,
                Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(None),
                Err(error) => return Err(error.into()),
            };
            #[cfg(unix)]
            use std::os::unix::fs::MetadataExt;
            Ok(Some(DirectoryStamp {
                modified: metadata.modified()?,
                len: metadata.len(),
                #[cfg(unix)]
                identity: (
                    metadata.dev(),
                    metadata.ino(),
                    metadata.ctime(),
                    metadata.ctime_nsec(),
                ),
            }))
        })
        .collect()
}

fn settled(stamps: &[Option<DirectoryStamp>], now: SystemTime) -> bool {
    stamps.iter().flatten().all(|stamp| {
        let old_enough = |time| {
            now.duration_since(time)
                .is_ok_and(|age| age >= STAMP_SETTLE)
        };
        if !old_enough(stamp.modified) {
            return false;
        }
        #[cfg(unix)]
        {
            let (_, _, seconds, nanos) = stamp.identity;
            if seconds < 0 || !(0..1_000_000_000).contains(&nanos) {
                return false;
            }
            let changed = SystemTime::UNIX_EPOCH + Duration::new(seconds as u64, nanos as u32);
            if !old_enough(changed) {
                return false;
            }
        }
        true
    })
}

fn cached_size(root: &Path) -> Result<u64> {
    let memo_path = root.join("locks").join(".size");
    let now = SystemTime::now();
    let before = directory_stamps(root)?;
    if settled(&before, now)
        && let Some(memo) = fs::read(&memo_path)
            .ok()
            .and_then(|bytes| serde_json::from_slice::<SizeMemo>(&bytes).ok())
        && now
            .duration_since(memo.measured)
            .is_ok_and(|age| age < IN_FLIGHT)
        && memo.stamps == before
    {
        return Ok(memo.size);
    }
    let size = ["entries", "blobs", "warm"]
        .iter()
        .zip(&before)
        .filter(|(_, stamp)| stamp.is_some())
        .try_fold(0u64, |total, (name, _)| {
            directory_size(&root.join(name)).map(|size| total.saturating_add(size))
        })?;
    let after = directory_stamps(root)?;
    if before == after && settled(&after, now) {
        let memo = SizeMemo {
            measured: now,
            stamps: after,
            size,
        };
        // Concurrent writers can replace this memo safely: readers still compare
        // its stamps with the current directories. Partial writes are misses.
        let mut temporary = tempfile::NamedTempFile::new_in(root.join("locks"))?;
        serde_json::to_writer(temporary.as_file_mut(), &memo)?;
        temporary.persist(&memo_path).map_err(|error| error.error)?;
    } else {
        let _ = fs::remove_file(memo_path);
    }
    Ok(size)
}

fn directory_size(directory: &Path) -> Result<u64> {
    let mut size = 0;
    for item in fs::read_dir(directory)? {
        size += item?.metadata()?.len();
    }
    Ok(size)
}

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

    fn fixture() -> tempfile::TempDir {
        let root = tempfile::tempdir().unwrap();
        for name in ["entries", "blobs", "locks"] {
            fs::create_dir(root.path().join(name)).unwrap();
        }
        fs::write(root.path().join("entries/a.json"), b"bad record").unwrap();
        fs::write(root.path().join("blobs/a"), [0; 100]).unwrap();
        fs::write(root.path().join("locks/.prune"), []).unwrap();
        root
    }

    /// A cache whose one entry cites one blob, dated two hours ago.
    fn cited() -> (tempfile::TempDir, String) {
        let root = fixture();
        let blob = "b".repeat(64);
        fs::write(root.path().join("blobs").join(&blob), [0; 100]).unwrap();
        fs::write(
            root.path().join("entries/a.json"),
            serde_json::to_vec(&serde_json::json!({"log": blob})).unwrap(),
        )
        .unwrap();
        let old = SystemTime::now() - Duration::from_secs(2 * 60 * 60);
        for path in [format!("blobs/{blob}"), "entries/a.json".into()] {
            File::options()
                .write(true)
                .open(root.path().join(path))
                .unwrap()
                .set_modified(old)
                .unwrap();
        }
        (root, blob)
    }

    /// Prunes until `blob` is gone. A prune finds `.prune` held, and does
    /// nothing, while a child another test is starting has not yet exec'd
    /// away its copy of the lock.
    fn prune_away(root: &Path, limit: u64, blob: &str) {
        let deadline = std::time::Instant::now() + Duration::from_secs(5);
        while root.join("blobs").join(blob).exists() && std::time::Instant::now() < deadline {
            prune(root, limit).unwrap();
            std::thread::sleep(Duration::from_millis(10));
        }
        assert!(!root.join("blobs").join(blob).exists());
    }

    #[test]
    fn a_pinned_blob_survives_evicting_the_last_entry_that_cites_it() {
        let (root, blob) = cited();
        let root = root.path();
        let pin = pin(root, &BTreeSet::from([blob.clone()])).unwrap();
        assert_eq!(prune(root, 0).unwrap().entries, 1);
        assert!(root.join("blobs").join(&blob).exists());
        drop(pin);
        prune_away(root, 0, &blob);
    }

    #[test]
    fn a_pinned_old_blob_no_entry_cites_survives() {
        let (root, blob) = cited();
        let root = root.path();
        fs::remove_file(root.join("entries/a.json")).unwrap();
        let pin = pin(root, &BTreeSet::from([blob.clone()])).unwrap();
        prune(root, 1 << 30).unwrap();
        assert!(root.join("blobs").join(&blob).exists());
        drop(pin);
        assert!(
            fs::read_dir(pins(root))
                .unwrap()
                .all(|item| { item.unwrap().file_name() == ".gate" })
        );
        prune_away(root, 1 << 30, &blob);
    }

    #[test]
    fn an_abandoned_pin_neither_keeps_blobs_nor_stays() {
        let (root, blob) = cited();
        let root = root.path();
        fs::create_dir_all(pins(root)).unwrap();
        fs::write(pins(root).join("gone.lock"), []).unwrap();
        fs::write(pins(root).join("gone.blobs"), format!("{blob}\n")).unwrap();
        prune(root, 0).unwrap();
        assert!(!root.join("blobs").join(&blob).exists());
        assert!(!pins(root).join("gone.lock").exists());
        assert!(!pins(root).join("gone.blobs").exists());
    }

    #[test]
    fn a_pin_waits_for_a_prune_that_is_deleting() {
        let (root, blob) = cited();
        let root = root.path().to_owned();
        let gate = pin_gate(&root).unwrap();
        gate.lock_exclusive().unwrap();
        let (sender, receiver) = std::sync::mpsc::channel();
        let pinning = std::thread::spawn({
            let root = root.clone();
            move || {
                let pin = pin(&root, &BTreeSet::from([blob])).unwrap();
                sender.send(()).unwrap();
                pin
            }
        });
        assert!(receiver.recv_timeout(Duration::from_millis(200)).is_err());
        drop(gate);
        receiver.recv_timeout(Duration::from_secs(5)).unwrap();
        drop(pinning.join().unwrap());
    }

    #[test]
    fn automatic_size_memo_reuses_and_reconciles() {
        let root = fixture();
        let root = root.path();
        // No memo can be trusted while directory timestamps are recent.
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 110);
        assert!(!root.join("locks/.size").exists());
        std::thread::sleep(STAMP_SETTLE + Duration::from_millis(100));
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 110);
        assert!(root.join("locks/.size").exists());

        // Unsupported external in-place changes intentionally demonstrate that
        // the automatic path reads only directory stamps, rather than each file.
        fs::write(root.join("blobs/a"), [0; 200]).unwrap();
        touch(&root.join("entries/a.json"));
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 110);
        assert_eq!(prune(root, 1_000).unwrap().size, 210);
        assert!(!root.join("locks/.size").exists());

        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 210);
        fs::write(root.join("locks/.size"), b"corrupt").unwrap();
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 210);
        let mut memo: SizeMemo =
            serde_json::from_slice(&fs::read(root.join("locks/.size")).unwrap()).unwrap();
        memo.measured = SystemTime::UNIX_EPOCH;
        memo.size = 0;
        fs::write(root.join("locks/.size"), serde_json::to_vec(&memo).unwrap()).unwrap();
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 210);

        // An atomic publication invalidates accounting immediately.
        let temp = root.join("entries/new.tmp");
        fs::write(&temp, [0; 30]).unwrap();
        fs::rename(temp, root.join("entries/new.json")).unwrap();
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 240);
        fs::create_dir(root.join("warm")).unwrap();
        fs::write(root.join("warm/a.json"), [0; 40]).unwrap();
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 280);
        std::thread::sleep(STAMP_SETTLE + Duration::from_millis(100));
        assert_eq!(prune_automatic(root, 1_000).unwrap().size, 280);
        assert!(root.join("locks/.size").exists());
        // A lower limit must still trigger eviction with an unchanged memo.
        let result = prune_automatic(root, 250).unwrap();
        assert!(result.entries > 0);
        assert!(result.size <= 250);
    }

    #[test]
    fn old_prune_marker_forces_full_reconciliation() {
        let root = fixture();
        let root = root.path();
        let marker = File::options()
            .write(true)
            .open(root.join("locks/.prune"))
            .unwrap();
        marker.set_modified(SystemTime::UNIX_EPOCH).unwrap();
        let result = prune_automatic(root, 100).unwrap();
        assert_eq!(result.entries, 1);
        assert_eq!(result.size, 100);
    }

    #[test]
    fn entries_unused_for_a_week_go_and_eviction_stops_a_tenth_below_the_limit() {
        let root = tempfile::tempdir().unwrap();
        let root = root.path();
        for name in ["entries", "blobs", "locks"] {
            fs::create_dir(root.join(name)).unwrap();
        }
        let now = SystemTime::now();
        for (name, age_days, size) in [
            ("stale", 8, 10),
            ("old", 3, 15),
            ("middle", 2, 100),
            ("new", 1, 180),
        ] {
            let path = root.join("entries").join(format!("{name}.json"));
            fs::write(&path, vec![0; size]).unwrap();
            File::options()
                .write(true)
                .open(path)
                .unwrap()
                .set_modified(now - Duration::from_secs(age_days * 24 * 60 * 60))
                .unwrap();
        }
        let left = || {
            let mut names: Vec<String> = fs::read_dir(root.join("entries"))
                .unwrap()
                .map(|item| item.unwrap().file_name().to_string_lossy().into_owned())
                .collect();
            names.sort();
            names
        };
        // A prune that finds `.prune` held by another test's child does
        // nothing; try again until one runs.
        let prune = |limit: u64| {
            let deadline = std::time::Instant::now() + Duration::from_secs(5);
            loop {
                let pruned = prune(root, limit).unwrap();
                if pruned != Pruned::default() || std::time::Instant::now() > deadline {
                    return pruned;
                }
                std::thread::sleep(Duration::from_millis(10));
            }
        };
        // Without a limit, only the week-old entry goes.
        assert_eq!(prune(u64::MAX).entries, 1);
        assert_eq!(left(), ["middle.json", "new.json", "old.json"]);
        // Over a limit of 290 bytes, eviction goes on to 261: evicting `old`
        // alone would leave 280.
        assert_eq!(prune(290).entries, 2);
        assert_eq!(left(), ["new.json"]);
    }

    #[test]
    fn parses_sizes_like_nx() {
        assert_eq!(parse_size("1024").unwrap(), 1024);
        assert_eq!(parse_size("1KB").unwrap(), 1024);
        assert_eq!(parse_size("1.5 MB").unwrap(), 1_572_864);
        assert_eq!(parse_size("2GB").unwrap(), 2 << 30);
        assert_eq!(parse_size("0").unwrap(), 0);
        for bad in ["", "1TB", "1.2.3MB", "MB", "-1"] {
            assert!(parse_size(bad).is_err(), "{bad}");
        }
    }
}