a3s_box_runtime/cache/

layer_cache.rs

//! Content-addressed cache for extracted OCI layers.
//!
//! Each layer is stored by its digest (SHA256), so identical layers
//! shared across different images are only stored once on disk.

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

use a3s_box_core::error::{BoxError, Result};
use a3s_box_core::traits::{CacheBackend, CacheEntry};
use serde::{Deserialize, Serialize};

/// Metadata for a cached layer entry.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct LayerMeta {
    /// Layer digest (e.g., "sha256:abc123...")
    pub digest: String,
    /// Size of the extracted layer in bytes
    pub size_bytes: u64,
    /// When this layer was cached (Unix timestamp)
    pub cached_at: i64,
    /// Last time this layer was accessed (Unix timestamp)
    pub last_accessed: i64,
}

/// Content-addressed cache for extracted OCI layers.
///
/// Layers are stored by digest under `cache_dir/layers/<digest>/`.
/// Metadata is stored alongside as `<digest>.meta.json`.
pub struct LayerCache {
    /// Root directory for layer cache (e.g., ~/.a3s/cache/layers)
    cache_dir: PathBuf,
}

impl LayerCache {
    /// Create a new layer cache at the given directory.
    pub fn new(cache_dir: &Path) -> Result<Self> {
        std::fs::create_dir_all(cache_dir).map_err(|e| {
            BoxError::CacheError(format!(
                "Failed to create layer cache directory {}: {}",
                cache_dir.display(),
                e
            ))
        })?;

        Ok(Self {
            cache_dir: cache_dir.to_path_buf(),
        })
    }

    /// Get the path to a cached layer by digest.
    ///
    /// Returns `None` if the layer is not cached or the cache entry is invalid.
    pub fn get(&self, digest: &str) -> Result<Option<PathBuf>> {
        super::validate_cache_key(digest, "layer digest")?;
        let safe_name = Self::digest_to_dirname(digest);
        let layer_dir = self.cache_dir.join(&safe_name);
        let meta_path = self.cache_dir.join(format!("{}.meta.json", safe_name));

        if !super::is_real_directory(&layer_dir) || !super::is_regular_file(&meta_path) {
            return Ok(None);
        }

        // Update last_accessed timestamp
        if let Ok(content) = std::fs::read_to_string(&meta_path) {
            if let Ok(mut meta) = serde_json::from_str::<LayerMeta>(&content) {
                meta.last_accessed = chrono::Utc::now().timestamp();
                if let Err(e) =
                    write_meta_atomically(&meta_path, &serde_json::to_string_pretty(&meta)?)
                {
                    tracing::warn!(path = %meta_path.display(), error = %e, "Failed to update layer cache metadata");
                }
            }
        }

        Ok(Some(layer_dir))
    }

    /// Store an extracted layer directory in the cache.
    ///
    /// Copies the contents of `source_dir` into the cache keyed by `digest`.
    /// Returns the path to the cached layer directory.
    pub fn put(&self, digest: &str, source_dir: &Path) -> Result<PathBuf> {
        super::validate_cache_key(digest, "layer digest")?;
        let safe_name = Self::digest_to_dirname(digest);
        let layer_dir = self.cache_dir.join(&safe_name);
        let meta_path = self.cache_dir.join(format!("{}.meta.json", safe_name));

        // Already fully cached (content-addressed ⇒ identical): nothing to do.
        // Returning early also makes concurrent puts of the same layer idempotent.
        if super::is_real_directory(&layer_dir) && super::is_regular_file(&meta_path) {
            return Ok(layer_dir);
        }

        // Atomically publish the extracted layer (staging dir + rename) so a
        // concurrent pull of the same layer cannot corrupt the cache by
        // removing/interleaving a half-copied directory.
        publish_dir_atomically(source_dir, &layer_dir, &self.cache_dir)?;

        // Calculate size (from whichever copy landed — they are identical).
        let size_bytes = dir_size(&layer_dir).unwrap_or(0);

        // Write metadata atomically (unique temp + rename).
        let now = chrono::Utc::now().timestamp();
        let meta = LayerMeta {
            digest: digest.to_string(),
            size_bytes,
            cached_at: now,
            last_accessed: now,
        };
        write_meta_atomically(&meta_path, &serde_json::to_string_pretty(&meta)?)?;

        tracing::debug!(
            digest = %digest,
            size_bytes,
            path = %layer_dir.display(),
            "Cached OCI layer"
        );

        Ok(layer_dir)
    }

    /// Remove a cached layer by digest.
    pub fn invalidate(&self, digest: &str) -> Result<()> {
        super::validate_cache_key(digest, "layer digest")?;
        let safe_name = Self::digest_to_dirname(digest);
        let layer_dir = self.cache_dir.join(&safe_name);
        let meta_path = self.cache_dir.join(format!("{}.meta.json", safe_name));

        super::remove_path_no_follow(&layer_dir)?;
        super::remove_path_no_follow(&meta_path)?;

        Ok(())
    }

    /// Prune the cache to stay within the given byte limit.
    ///
    /// Evicts least-recently-accessed entries first.
    /// Returns the number of entries evicted.
    pub fn prune(&self, max_bytes: u64) -> Result<usize> {
        let mut entries = self.list_entries()?;

        // Calculate total size
        let total_size: u64 = entries.iter().map(|e| e.size_bytes).sum();
        if total_size <= max_bytes {
            return Ok(0);
        }

        // Sort by last_accessed ascending (oldest first)
        entries.sort_by_key(|e| e.last_accessed);

        let mut current_size = total_size;
        let mut evicted = 0;

        for entry in &entries {
            if current_size <= max_bytes {
                break;
            }
            self.invalidate(&entry.digest)?;
            current_size = current_size.saturating_sub(entry.size_bytes);
            evicted += 1;

            tracing::debug!(
                digest = %entry.digest,
                size_bytes = entry.size_bytes,
                "Evicted cached layer"
            );
        }

        Ok(evicted)
    }

    /// List all cached layer entries with their metadata.
    pub fn list_entries(&self) -> Result<Vec<LayerMeta>> {
        let mut entries = Vec::new();

        let read_dir = std::fs::read_dir(&self.cache_dir).map_err(|e| {
            BoxError::CacheError(format!(
                "Failed to read cache directory {}: {}",
                self.cache_dir.display(),
                e
            ))
        })?;

        for entry in read_dir {
            let entry = entry.map_err(|e| {
                BoxError::CacheError(format!("Failed to read directory entry: {}", e))
            })?;
            let path = entry.path();

            // Only process .meta.json files
            if path.extension().and_then(|e| e.to_str()) == Some("json") {
                if let Some(name) = path.file_name().and_then(|n| n.to_str()) {
                    if name.ends_with(".meta.json") {
                        if let Ok(content) = std::fs::read_to_string(&path) {
                            if let Ok(meta) = serde_json::from_str::<LayerMeta>(&content) {
                                entries.push(meta);
                            }
                        }
                    }
                }
            }
        }

        Ok(entries)
    }

    /// Get the total size of all cached layers in bytes.
    pub fn total_size(&self) -> Result<u64> {
        Ok(self.list_entries()?.iter().map(|e| e.size_bytes).sum())
    }

    /// Convert a digest string to a safe directory name.
    ///
    /// Replaces ':' with '_' to avoid filesystem issues.
    /// e.g., "sha256:abc123" → "sha256_abc123"
    fn digest_to_dirname(digest: &str) -> String {
        digest.replace(':', "_")
    }
}

/// Recursively copy a directory and its contents.
/// Copy `src`'s uid/gid onto `dst` (no symlink follow), best-effort, root only.
///
/// `std::fs::copy`/`create_dir_all` do not carry ownership, so a rootfs copied
/// from extracted layers would collapse to root and lose `COPY --chown` (and
/// base-image) ownership. Only root can chown to arbitrary ids, so this is a
/// no-op otherwise.
#[cfg(unix)]
fn preserve_owner(meta: &std::fs::Metadata, dst: &Path) {
    use std::os::unix::ffi::OsStrExt;
    use std::os::unix::fs::MetadataExt;
    if unsafe { libc::geteuid() } != 0 {
        return;
    }
    if let Ok(c_path) = std::ffi::CString::new(dst.as_os_str().as_bytes()) {
        // lchown so a symlink's own ownership is set, not its target's.
        unsafe {
            libc::lchown(c_path.as_ptr(), meta.uid(), meta.gid());
        }
    }
}

#[cfg(not(unix))]
fn preserve_owner(_meta: &std::fs::Metadata, _dst: &Path) {}

pub(crate) fn copy_dir_recursive(src: &Path, dst: &Path) -> Result<()> {
    #[cfg(target_os = "macos")]
    {
        let dst_preexisted = dst.exists();
        // copyfile(3) gives an existing destination directory different
        // semantics from this helper: it creates `dst/src.file_name()` instead
        // of copying the source directory's contents directly into `dst`.
        // APFS rootfs mountpoints already exist, so taking that fast path would
        // restore a snapshot below `.a3s-rootfs/rootfs/`. Fall back to the
        // entry-by-entry copy for an existing destination; nested directories
        // can still use APFS clones once their destination does not exist.
        if !dst_preexisted && copy_dir_recursive_cow(src, dst).unwrap_or(false) {
            return Ok(());
        }
        if !dst_preexisted && dst.exists() {
            let _ = std::fs::remove_dir_all(dst);
        }
    }

    std::fs::create_dir_all(dst).map_err(|e| {
        BoxError::CacheError(format!(
            "Failed to create directory {}: {}",
            dst.display(),
            e
        ))
    })?;
    let src_meta = std::fs::symlink_metadata(src).map_err(|e| {
        BoxError::CacheError(format!(
            "Failed to read directory metadata for {}: {}",
            src.display(),
            e
        ))
    })?;
    // Mirror the source directory's ownership onto the destination (root only).
    preserve_owner(&src_meta, dst);

    for entry in std::fs::read_dir(src).map_err(|e| {
        BoxError::CacheError(format!("Failed to read directory {}: {}", src.display(), e))
    })? {
        let entry = entry
            .map_err(|e| BoxError::CacheError(format!("Failed to read directory entry: {}", e)))?;
        let src_path = entry.path();
        let dst_path = dst.join(entry.file_name());

        // Use symlink_metadata so is_symlink() works correctly (does not follow links).
        let meta = std::fs::symlink_metadata(&src_path).map_err(|e| {
            BoxError::CacheError(format!(
                "Failed to read metadata for {}: {}",
                src_path.display(),
                e
            ))
        })?;

        if meta.file_type().is_symlink() {
            let target = std::fs::read_link(&src_path).map_err(|e| {
                BoxError::CacheError(format!(
                    "Failed to read symlink {}: {}",
                    src_path.display(),
                    e
                ))
            })?;
            #[cfg(unix)]
            {
                std::os::unix::fs::symlink(&target, &dst_path).map_err(|e| {
                    BoxError::CacheError(format!(
                        "Failed to create symlink {} -> {}: {}",
                        dst_path.display(),
                        target.display(),
                        e
                    ))
                })?;
                preserve_owner(&meta, &dst_path);
            }
            #[cfg(windows)]
            {
                use std::os::windows::fs::MetadataExt;

                // Do not follow the link to determine its type. Absolute Linux
                // targets such as `/bin/busybox` are intentionally broken from
                // the Windows host's perspective but valid inside the guest.
                const FILE_ATTRIBUTE_DIRECTORY: u32 = 0x10;
                let result = if meta.file_attributes() & FILE_ATTRIBUTE_DIRECTORY != 0 {
                    std::os::windows::fs::symlink_dir(&target, &dst_path)
                } else {
                    std::os::windows::fs::symlink_file(&target, &dst_path)
                };
                result.map_err(|e| {
                    BoxError::CacheError(format!(
                        "Failed to create symlink {} -> {}: {}",
                        dst_path.display(),
                        target.display(),
                        e
                    ))
                })?;
            }
            #[cfg(not(any(unix, windows)))]
            {
                return Err(BoxError::CacheError(format!(
                    "Symlink copy is not supported on this platform: {}",
                    src_path.display()
                )));
            }
        } else if meta.is_dir() {
            copy_dir_recursive(&src_path, &dst_path)?;
        } else {
            copy_file_cow(&src_path, &dst_path).map_err(|e| {
                BoxError::CacheError(format!(
                    "Failed to copy {} to {}: {}",
                    src_path.display(),
                    dst_path.display(),
                    e
                ))
            })?;
            preserve_owner(&meta, &dst_path);
        }
    }

    // Apply directory permissions only after copying its children. This both
    // preserves image modes in the cache and avoids making a read-only source
    // directory's destination unwritable before recursion is complete.
    std::fs::set_permissions(dst, src_meta.permissions()).map_err(|e| {
        BoxError::CacheError(format!(
            "Failed to preserve directory permissions on {}: {}",
            dst.display(),
            e
        ))
    })?;

    Ok(())
}

#[cfg(target_os = "macos")]
fn copy_dir_recursive_cow(src: &Path, dst: &Path) -> std::io::Result<bool> {
    use std::os::unix::ffi::OsStrExt;

    let src_c = std::ffi::CString::new(src.as_os_str().as_bytes()).map_err(|_| {
        std::io::Error::new(
            std::io::ErrorKind::InvalidInput,
            "source path contains a NUL byte",
        )
    })?;
    let dst_c = std::ffi::CString::new(dst.as_os_str().as_bytes()).map_err(|_| {
        std::io::Error::new(
            std::io::ErrorKind::InvalidInput,
            "destination path contains a NUL byte",
        )
    })?;
    let flags = libc::COPYFILE_CLONE | libc::COPYFILE_RECURSIVE;
    // SAFETY: both C strings are valid NUL-terminated paths for the duration of
    // the call. A non-zero result means the system fast path could not handle
    // this tree, so callers fall back to the portable recursive copy.
    let rc = unsafe { libc::copyfile(src_c.as_ptr(), dst_c.as_ptr(), std::ptr::null_mut(), flags) };
    Ok(rc == 0)
}

/// Copy a regular file, preferring copy-on-write cloning so a new box's rootfs
/// shares blocks with the cached image — instant, no extra disk — on capable
/// filesystems (Linux FICLONE, macOS APFS clonefile). Falls back to a plain byte
/// copy when cloning is unsupported or the source and destination are on
/// different filesystems. Overlay is preferred on Linux, so this mostly helps
/// macOS/HVF and Linux `CopyProvider` fallback paths.
pub(crate) fn copy_file_cow(src: &Path, dst: &Path) -> std::io::Result<()> {
    #[cfg(target_os = "linux")]
    {
        use std::os::unix::io::AsRawFd;
        let reflinked = (|| -> std::io::Result<bool> {
            let s = std::fs::File::open(src)?;
            let d = std::fs::OpenOptions::new()
                .write(true)
                .create(true)
                .truncate(true)
                .open(dst)?;
            // SAFETY: FICLONE's argument is the source fd; both fds are valid for
            // the call. A non-zero return (unsupported FS / cross-device) just
            // means "fall back to a byte copy".
            // `libc::Ioctl` is `c_ulong` on glibc and `c_int` on musl. Use
            // libc's target-specific constant instead of fixing the request to
            // one ABI's integer type.
            let rc = unsafe { libc::ioctl(d.as_raw_fd(), libc::FICLONE as _, s.as_raw_fd()) };
            if rc != 0 {
                return Ok(false);
            }
            // FICLONE clones data only — copy the permission bits like fs::copy.
            if let Ok(perm) = s.metadata().map(|m| m.permissions()) {
                let _ = d.set_permissions(perm);
            }
            Ok(true)
        })()
        .unwrap_or(false);
        if reflinked {
            return Ok(());
        }
    }

    #[cfg(target_os = "macos")]
    {
        use std::os::unix::ffi::OsStrExt;

        let cloned = (|| -> std::io::Result<bool> {
            let src_c = std::ffi::CString::new(src.as_os_str().as_bytes()).map_err(|_| {
                std::io::Error::new(
                    std::io::ErrorKind::InvalidInput,
                    "source path contains a NUL byte",
                )
            })?;
            let dst_c = std::ffi::CString::new(dst.as_os_str().as_bytes()).map_err(|_| {
                std::io::Error::new(
                    std::io::ErrorKind::InvalidInput,
                    "destination path contains a NUL byte",
                )
            })?;

            // SAFETY: both C strings are valid NUL-terminated paths for the
            // duration of the call. A non-zero result means clonefile is not
            // available for this path/filesystem and we fall back to byte copy.
            let rc = unsafe { libc::clonefile(src_c.as_ptr(), dst_c.as_ptr(), 0) };
            if rc != 0 {
                return Ok(false);
            }
            if let Ok(perm) = std::fs::metadata(src).map(|m| m.permissions()) {
                let _ = std::fs::set_permissions(dst, perm);
            }
            Ok(true)
        })()
        .unwrap_or(false);
        if cloned {
            return Ok(());
        }
    }

    std::fs::copy(src, dst).map(|_| ())
}

/// Atomically publish `source_dir`'s contents as the content-addressed cache
/// entry `dest_dir`. Returns `true` if THIS call created the entry, `false` if
/// an entry was already present (a concurrent put of the same key won).
///
/// Copying straight into `dest_dir` (and `remove_dir_all`-ing a pre-existing
/// one) corrupts the cache when two processes pull the same layer at once: one
/// deletes the other's half-copied directory, or both interleave files into the
/// same path. Instead, copy into a unique staging dir under `staging_parent`,
/// then `rename` it into place. Because both writers use the same key (a
/// content digest), an entry that already exists is byte-identical, so a lost
/// rename race is harmless — we keep the winner and drop our staging copy. The
/// rename is atomic on the same filesystem, so `dest_dir` is only ever absent or
/// fully populated, never partial.
pub(crate) fn publish_dir_atomically(
    source_dir: &Path,
    dest_dir: &Path,
    staging_parent: &Path,
) -> Result<bool> {
    match std::fs::symlink_metadata(dest_dir) {
        Ok(metadata) if metadata.is_dir() && !metadata.file_type().is_symlink() => {
            return Ok(false)
        }
        Ok(_) => {
            return Err(BoxError::CacheError(format!(
                "Cache destination is not a real directory: {}",
                dest_dir.display()
            )))
        }
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
        Err(error) => {
            return Err(BoxError::CacheError(format!(
                "Failed to inspect cache destination {}: {error}",
                dest_dir.display()
            )))
        }
    }
    let staging = tempfile::Builder::new()
        .prefix(".staging-")
        .tempdir_in(staging_parent)
        .map_err(|e| BoxError::CacheError(format!("Failed to create staging dir: {e}")))?;
    // copy_dir_recursive create_dir_all's its destination, so copy into a fresh
    // subpath of the (already-existing) staging dir, then rename that subpath.
    let staged = staging.path().join("d");
    copy_dir_recursive(source_dir, &staged)?;

    match std::fs::rename(&staged, dest_dir) {
        Ok(()) => Ok(true),
        // Lost the race: a concurrent put populated dest_dir first. Same key ⇒
        // identical content, so keep the winner (staging auto-removes on drop).
        Err(_) if super::is_real_directory(dest_dir) => Ok(false),
        Err(e) => Err(BoxError::CacheError(format!(
            "Failed to publish cache entry {}: {e}",
            dest_dir.display()
        ))),
    }
}

/// Atomically write `json` to `meta_path` (unique temp + rename), so a
/// concurrent reader never sees a half-written metadata file.
pub(crate) fn write_meta_atomically(meta_path: &Path, json: &str) -> Result<()> {
    let parent = meta_path.parent().ok_or_else(|| {
        BoxError::CacheError(format!("meta path has no parent: {}", meta_path.display()))
    })?;
    let _lock = crate::file_lock::FileLock::acquire(meta_path).map_err(|e| {
        BoxError::CacheError(format!(
            "Failed to lock cache metadata {}: {e}",
            meta_path.display()
        ))
    })?;
    let mut tmp = tempfile::NamedTempFile::new_in(parent)
        .map_err(|e| BoxError::CacheError(format!("Failed to stage metadata: {e}")))?;
    use std::io::Write as _;
    tmp.write_all(json.as_bytes())
        .map_err(|e| BoxError::CacheError(format!("Failed to write metadata: {e}")))?;
    persist_meta_file(tmp, meta_path)
}

#[cfg(windows)]
fn persist_meta_file(mut tmp: tempfile::NamedTempFile, meta_path: &Path) -> Result<()> {
    use std::time::Duration;

    const ERROR_ACCESS_DENIED: i32 = 5;
    const ERROR_SHARING_VIOLATION: i32 = 32;
    const ERROR_LOCK_VIOLATION: i32 = 33;
    const MAX_RETRIES: usize = 200;

    let mut attempts = 0;
    loop {
        match tmp.persist(meta_path) {
            Ok(_) => return Ok(()),
            Err(error)
                if attempts < MAX_RETRIES
                    && matches!(
                        error.error.raw_os_error(),
                        Some(ERROR_ACCESS_DENIED | ERROR_SHARING_VIOLATION | ERROR_LOCK_VIOLATION)
                    ) =>
            {
                tmp = error.file;
                attempts += 1;
                std::thread::sleep(Duration::from_millis(10));
            }
            Err(error) => {
                return Err(BoxError::CacheError(format!(
                    "Failed to persist metadata: {error}"
                )));
            }
        }
    }
}

#[cfg(not(windows))]
fn persist_meta_file(tmp: tempfile::NamedTempFile, meta_path: &Path) -> Result<()> {
    tmp.persist(meta_path)
        .map_err(|e| BoxError::CacheError(format!("Failed to persist metadata: {e}")))?;
    Ok(())
}

/// Calculate the total size of a directory recursively.
pub(crate) fn dir_size(path: &Path) -> std::io::Result<u64> {
    let metadata = match std::fs::symlink_metadata(path) {
        Ok(metadata) => metadata,
        Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(0),
        Err(error) => return Err(error),
    };
    if metadata.file_type().is_symlink() || metadata.is_file() {
        return Ok(metadata.len());
    }
    if !metadata.is_dir() {
        return Ok(0);
    }

    let mut total = 0_u64;
    for entry in std::fs::read_dir(path)? {
        total = total.saturating_add(dir_size(&entry?.path())?);
    }
    Ok(total)
}

impl CacheBackend for LayerCache {
    fn get(&self, key: &str) -> Result<Option<PathBuf>> {
        self.get(key)
    }

    fn put(&self, key: &str, source_dir: &Path, _description: &str) -> Result<PathBuf> {
        self.put(key, source_dir)
    }

    fn invalidate(&self, key: &str) -> Result<()> {
        self.invalidate(key)
    }

    fn prune(&self, _max_entries: usize, max_bytes: u64) -> Result<usize> {
        self.prune(max_bytes)
    }

    fn list(&self) -> Result<Vec<CacheEntry>> {
        self.list_entries().map(|entries| {
            entries
                .into_iter()
                .map(|m| CacheEntry {
                    key: m.digest,
                    description: String::new(),
                    size_bytes: m.size_bytes,
                    cached_at: m.cached_at,
                    last_accessed: m.last_accessed,
                })
                .collect()
        })
    }
}

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

    fn create_test_layer(dir: &Path, files: &[(&str, &str)]) {
        std::fs::create_dir_all(dir).unwrap();
        for (name, content) in files {
            let file_path = dir.join(name);
            if let Some(parent) = file_path.parent() {
                std::fs::create_dir_all(parent).unwrap();
            }
            std::fs::write(&file_path, content).unwrap();
        }
    }

    #[test]
    fn test_layer_cache_new_creates_directory() {
        let tmp = TempDir::new().unwrap();
        let cache_dir = tmp.path().join("layers");

        assert!(!cache_dir.exists());
        let _cache = LayerCache::new(&cache_dir).unwrap();
        assert!(cache_dir.is_dir());
    }

    #[test]
    fn test_layer_cache_get_miss() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let result = cache.get("sha256:nonexistent").unwrap();
        assert!(result.is_none());
    }

    #[test]
    fn test_layer_cache_put_and_get() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Create a source layer directory
        let source = tmp.path().join("source_layer");
        create_test_layer(
            &source,
            &[("file.txt", "hello"), ("sub/nested.txt", "world")],
        );

        // Put into cache
        let digest = "sha256:abc123def456";
        let cached_path = cache.put(digest, &source).unwrap();

        assert!(cached_path.is_dir());
        assert!(cached_path.join("file.txt").is_file());
        assert!(cached_path.join("sub/nested.txt").is_file());

        // Get from cache
        let result = cache.get(digest).unwrap();
        assert!(result.is_some());
        assert_eq!(result.unwrap(), cached_path);
    }

    #[test]
    fn test_layer_cache_put_same_digest_is_idempotent() {
        // A layer digest IS the hash of its content, so the same digest can only
        // ever map to identical content — re-putting it must be a no-op that
        // keeps the first entry, not a remove-and-recopy (which corrupts the
        // cache when two pulls of the same layer race). The "different content"
        // below is an impossible-in-reality stand-in to prove the first write wins.
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:idempotent_test";

        let source1 = tmp.path().join("v1");
        create_test_layer(&source1, &[("v1.txt", "version 1")]);
        let first = cache.put(digest, &source1).unwrap();

        let source2 = tmp.path().join("v2");
        create_test_layer(&source2, &[("v2.txt", "version 2")]);
        let second = cache.put(digest, &source2).unwrap();

        // Same cache path, first content preserved (idempotent, no overwrite).
        assert_eq!(first, second);
        assert!(second.join("v1.txt").is_file());
        assert!(!second.join("v2.txt").exists());
    }

    #[test]
    fn test_layer_cache_concurrent_put_same_digest_no_corruption() {
        use std::sync::Arc;

        let tmp = TempDir::new().unwrap();
        let cache = Arc::new(LayerCache::new(tmp.path()).unwrap());
        let digest = "sha256:concurrent_test";

        // Several identical source layers (like the same layer extracted by N
        // racing pulls), each with the same multi-file content.
        let files: &[(&str, &str)] = &[("a.txt", "alpha"), ("sub/b.txt", "beta")];
        let handles: Vec<_> = (0..12)
            .map(|i| {
                let cache = Arc::clone(&cache);
                let src = tmp.path().join(format!("src{i}"));
                create_test_layer(&src, files);
                std::thread::spawn(move || cache.put(digest, &src).unwrap())
            })
            .collect();
        let paths: Vec<_> = handles.into_iter().map(|h| h.join().unwrap()).collect();

        // Every put returns the same cache dir, and it is COMPLETE (no half-copied
        // / interleaved layer): all files present with the right content.
        for p in &paths {
            assert_eq!(p, &paths[0]);
            assert_eq!(std::fs::read_to_string(p.join("a.txt")).unwrap(), "alpha");
            assert_eq!(
                std::fs::read_to_string(p.join("sub/b.txt")).unwrap(),
                "beta"
            );
        }
        assert!(cache.get(digest).unwrap().is_some());
    }

    #[cfg(windows)]
    #[test]
    fn test_write_meta_atomically_retries_windows_sharing_conflict() {
        use std::os::windows::fs::OpenOptionsExt;
        use std::time::{Duration, Instant};

        let tmp = TempDir::new().unwrap();
        let meta_path = tmp.path().join("entry.meta.json");
        std::fs::write(&meta_path, r#"{"state":"old"}"#).unwrap();

        let blocker = std::fs::OpenOptions::new()
            .read(true)
            .share_mode(0)
            .open(&meta_path)
            .unwrap();
        let writer_path = meta_path.clone();
        let writer =
            std::thread::spawn(move || write_meta_atomically(&writer_path, r#"{"state":"new"}"#));

        let deadline = Instant::now() + Duration::from_secs(1);
        let staged_file_seen = loop {
            let entry_count = std::fs::read_dir(tmp.path()).unwrap().count();
            if entry_count >= 3 {
                break true;
            }
            if writer.is_finished() || Instant::now() >= deadline {
                break false;
            }
            std::thread::sleep(Duration::from_millis(5));
        };
        assert!(
            staged_file_seen,
            "metadata writer should stage a temporary file before retrying"
        );
        assert!(
            !writer.is_finished(),
            "metadata writer should wait for the Windows sharing conflict"
        );

        drop(blocker);
        writer.join().unwrap().unwrap();
        assert_eq!(
            std::fs::read_to_string(meta_path).unwrap(),
            r#"{"state":"new"}"#
        );
    }

    #[test]
    fn test_layer_cache_invalidate() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:to_invalidate";

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("data.bin", "binary data")]);
        cache.put(digest, &source).unwrap();

        // Verify it exists
        assert!(cache.get(digest).unwrap().is_some());

        // Invalidate
        cache.invalidate(digest).unwrap();

        // Should be gone
        assert!(cache.get(digest).unwrap().is_none());
    }

    #[test]
    fn test_layer_cache_invalidate_nonexistent() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Should not error on nonexistent digest
        cache.invalidate("sha256:does_not_exist").unwrap();
    }

    #[test]
    fn test_layer_cache_list_entries() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Empty cache
        assert_eq!(cache.list_entries().unwrap().len(), 0);

        // Add two layers
        let s1 = tmp.path().join("s1");
        create_test_layer(&s1, &[("a.txt", "aaa")]);
        cache.put("sha256:layer1", &s1).unwrap();

        let s2 = tmp.path().join("s2");
        create_test_layer(&s2, &[("b.txt", "bbb")]);
        cache.put("sha256:layer2", &s2).unwrap();

        let entries = cache.list_entries().unwrap();
        assert_eq!(entries.len(), 2);

        let digests: Vec<&str> = entries.iter().map(|e| e.digest.as_str()).collect();
        assert!(digests.contains(&"sha256:layer1"));
        assert!(digests.contains(&"sha256:layer2"));
    }

    #[test]
    fn test_layer_cache_total_size() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        assert_eq!(cache.total_size().unwrap(), 0);

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("data.txt", "hello world")]);
        cache.put("sha256:sized", &source).unwrap();

        let total = cache.total_size().unwrap();
        assert!(total > 0);
    }

    #[test]
    fn test_layer_cache_prune_under_limit() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("small.txt", "tiny")]);
        cache.put("sha256:small", &source).unwrap();

        // Prune with a large limit — nothing should be evicted
        let evicted = cache.prune(1024 * 1024 * 1024).unwrap();
        assert_eq!(evicted, 0);
        assert!(cache.get("sha256:small").unwrap().is_some());
    }

    #[test]
    fn test_layer_cache_prune_evicts_oldest() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Add three layers with different access times
        for i in 0..3 {
            let source = tmp.path().join(format!("s{}", i));
            // Create a file with enough content to matter
            create_test_layer(&source, &[("data.txt", &"x".repeat(100))]);
            cache.put(&format!("sha256:layer{}", i), &source).unwrap();
            // Small delay to ensure different timestamps
            std::thread::sleep(std::time::Duration::from_millis(10));
        }

        // Access layer2 to make it most recently used
        cache.get("sha256:layer2").unwrap();

        // Prune to a very small limit — should evict oldest first
        let evicted = cache.prune(1).unwrap();
        assert!(evicted >= 2);

        // layer2 was most recently accessed, so it should survive longest
        // (though with limit=1 byte, all may be evicted)
    }

    #[test]
    fn test_layer_cache_metadata_persists() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:meta_test";

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("file.txt", "content")]);
        cache.put(digest, &source).unwrap();

        // Read metadata directly
        let meta_path = tmp.path().join("sha256_meta_test.meta.json");
        assert!(meta_path.is_file());

        let content = std::fs::read_to_string(&meta_path).unwrap();
        let meta: LayerMeta = serde_json::from_str(&content).unwrap();

        assert_eq!(meta.digest, digest);
        assert!(meta.size_bytes > 0);
        assert!(meta.cached_at > 0);
        assert_eq!(meta.cached_at, meta.last_accessed);
    }

    #[test]
    fn test_digest_to_dirname() {
        assert_eq!(
            LayerCache::digest_to_dirname("sha256:abc123"),
            "sha256_abc123"
        );
        assert_eq!(
            LayerCache::digest_to_dirname("plain_digest"),
            "plain_digest"
        );
    }

    #[test]
    fn test_copy_dir_recursive() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src");
        let dst = tmp.path().join("dst");

        create_test_layer(
            &src,
            &[
                ("a.txt", "aaa"),
                ("sub/b.txt", "bbb"),
                ("sub/deep/c.txt", "ccc"),
            ],
        );

        copy_dir_recursive(&src, &dst).unwrap();

        assert_eq!(std::fs::read_to_string(dst.join("a.txt")).unwrap(), "aaa");
        assert_eq!(
            std::fs::read_to_string(dst.join("sub/b.txt")).unwrap(),
            "bbb"
        );
        assert_eq!(
            std::fs::read_to_string(dst.join("sub/deep/c.txt")).unwrap(),
            "ccc"
        );
    }

    #[test]
    fn test_copy_dir_recursive_copies_contents_into_existing_destination() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("rootfs");
        let dst = tmp.path().join("existing");

        create_test_layer(&src, &[("bin/sh", "shell"), ("etc/config", "value")]);
        std::fs::create_dir_all(&dst).unwrap();
        std::fs::write(dst.join("marker"), "keep").unwrap();

        copy_dir_recursive(&src, &dst).unwrap();

        assert_eq!(
            std::fs::read_to_string(dst.join("bin/sh")).unwrap(),
            "shell"
        );
        assert_eq!(
            std::fs::read_to_string(dst.join("etc/config")).unwrap(),
            "value"
        );
        assert_eq!(std::fs::read_to_string(dst.join("marker")).unwrap(), "keep");
        assert!(!dst.join("rootfs").exists());
    }

    #[cfg(unix)]
    #[test]
    fn test_copy_dir_recursive_preserves_symlinks() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src");
        let dst = tmp.path().join("dst");
        std::fs::create_dir_all(&src).unwrap();
        std::fs::write(src.join("target.txt"), "target").unwrap();
        std::os::unix::fs::symlink("target.txt", src.join("link.txt")).unwrap();

        copy_dir_recursive(&src, &dst).unwrap();

        let link_meta = std::fs::symlink_metadata(dst.join("link.txt")).unwrap();
        assert!(link_meta.file_type().is_symlink());
        assert_eq!(
            std::fs::read_link(dst.join("link.txt")).unwrap(),
            std::path::PathBuf::from("target.txt")
        );
    }

    #[cfg(target_os = "windows")]
    #[test]
    fn test_copy_dir_recursive_preserves_windows_symlink() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src");
        let dst = tmp.path().join("dst");
        std::fs::create_dir_all(&src).unwrap();
        std::fs::write(src.join("target.txt"), "target").unwrap();
        if let Err(error) = std::os::windows::fs::symlink_file("target.txt", src.join("link.txt")) {
            // Windows requires Developer Mode or SeCreateSymbolicLinkPrivilege.
            // Keep exercising the copy when the host supports symlinks, while
            // allowing unprivileged Windows CI and developer shells to proceed.
            const ERROR_PRIVILEGE_NOT_HELD: i32 = 1314;
            if error.raw_os_error() == Some(ERROR_PRIVILEGE_NOT_HELD) {
                return;
            }
            panic!("failed to create Windows test symlink: {error}");
        }

        copy_dir_recursive(&src, &dst).unwrap();

        let copied_link = dst.join("link.txt");
        assert!(std::fs::symlink_metadata(&copied_link)
            .unwrap()
            .file_type()
            .is_symlink());
        assert_eq!(
            std::fs::read_link(&copied_link).unwrap(),
            Path::new("target.txt")
        );
        assert_eq!(std::fs::read_to_string(copied_link).unwrap(), "target");
    }

    #[test]
    fn test_dir_size() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().join("sized");
        create_test_layer(
            &dir,
            &[
                ("a.txt", "hello"),     // 5 bytes
                ("sub/b.txt", "world"), // 5 bytes
            ],
        );

        let size = dir_size(&dir).unwrap();
        assert_eq!(size, 10);
    }

    #[cfg(unix)]
    #[test]
    fn test_dir_size_does_not_follow_external_directory_symlink() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().join("sized");
        let outside = tmp.path().join("outside");
        std::fs::create_dir_all(&dir).unwrap();
        std::fs::create_dir_all(&outside).unwrap();
        std::fs::write(dir.join("local"), b"local").unwrap();
        std::fs::write(outside.join("host-data"), vec![0_u8; 4096]).unwrap();
        let link = dir.join("external");
        std::os::unix::fs::symlink(&outside, &link).unwrap();

        let size = dir_size(&dir).unwrap();

        assert_eq!(
            size,
            5 + std::fs::symlink_metadata(link).unwrap().len(),
            "cache sizing must count the symlink itself without entering its target"
        );
    }

    #[cfg(unix)]
    #[test]
    fn test_dir_size_does_not_follow_symlink_cycle() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().join("sized");
        std::fs::create_dir_all(&dir).unwrap();
        let link = dir.join("loop");
        std::os::unix::fs::symlink(".", &link).unwrap();

        let size = dir_size(&dir).unwrap();

        assert_eq!(size, std::fs::symlink_metadata(link).unwrap().len());
    }

    #[test]
    fn test_dir_size_empty_directory() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().join("empty");
        std::fs::create_dir_all(&dir).unwrap();

        let size = dir_size(&dir).unwrap();
        assert_eq!(size, 0);
    }

    #[test]
    fn test_dir_size_nonexistent_returns_zero() {
        let tmp = TempDir::new().unwrap();
        let dir = tmp.path().join("nonexistent");

        // Not a directory, so returns 0
        let size = dir_size(&dir).unwrap();
        assert_eq!(size, 0);
    }

    #[test]
    fn test_copy_dir_recursive_empty_directory() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("empty_src");
        let dst = tmp.path().join("empty_dst");
        std::fs::create_dir_all(&src).unwrap();

        copy_dir_recursive(&src, &dst).unwrap();
        assert!(dst.is_dir());
    }

    #[test]
    fn test_copy_dir_recursive_source_not_exists() {
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("nonexistent");
        let dst = tmp.path().join("dst");

        let result = copy_dir_recursive(&src, &dst);
        assert!(result.is_err());
    }

    #[test]
    fn test_layer_cache_get_updates_last_accessed() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:access_test";

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("f.txt", "data")]);
        cache.put(digest, &source).unwrap();

        // Read initial metadata
        let meta_path = tmp.path().join("sha256_access_test.meta.json");
        let content = std::fs::read_to_string(&meta_path).unwrap();
        let meta_before: LayerMeta = serde_json::from_str(&content).unwrap();

        // Small delay to ensure timestamp difference
        std::thread::sleep(std::time::Duration::from_millis(10));

        // Access the cache entry
        cache.get(digest).unwrap();

        // Read updated metadata
        let content = std::fs::read_to_string(&meta_path).unwrap();
        let meta_after: LayerMeta = serde_json::from_str(&content).unwrap();

        assert!(meta_after.last_accessed >= meta_before.last_accessed);
        // cached_at should not change
        assert_eq!(meta_after.cached_at, meta_before.cached_at);
    }

    #[test]
    fn test_layer_cache_get_corrupted_metadata() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:corrupted";
        let safe_name = LayerCache::digest_to_dirname(digest);

        // Create layer directory manually
        let layer_dir = tmp.path().join(&safe_name);
        std::fs::create_dir_all(&layer_dir).unwrap();

        // Write corrupted metadata
        let meta_path = tmp.path().join(format!("{}.meta.json", safe_name));
        std::fs::write(&meta_path, "not valid json!!!").unwrap();

        // get() should still return Some (directory exists, metadata is best-effort)
        // The directory exists and meta file exists, so it returns the path
        let result = cache.get(digest).unwrap();
        assert!(result.is_some());
    }

    #[test]
    fn test_layer_cache_get_directory_without_metadata() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:no_meta";
        let safe_name = LayerCache::digest_to_dirname(digest);

        // Create layer directory but no metadata file
        let layer_dir = tmp.path().join(&safe_name);
        std::fs::create_dir_all(&layer_dir).unwrap();

        // Should return None (metadata missing)
        let result = cache.get(digest).unwrap();
        assert!(result.is_none());
    }

    #[test]
    fn test_layer_cache_get_metadata_without_directory() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let digest = "sha256:no_dir";
        let safe_name = LayerCache::digest_to_dirname(digest);

        // Create metadata file but no layer directory
        let meta_path = tmp.path().join(format!("{}.meta.json", safe_name));
        let meta = LayerMeta {
            digest: digest.to_string(),
            size_bytes: 0,
            cached_at: 0,
            last_accessed: 0,
        };
        std::fs::write(&meta_path, serde_json::to_string(&meta).unwrap()).unwrap();

        // Should return None (directory missing)
        let result = cache.get(digest).unwrap();
        assert!(result.is_none());
    }

    #[test]
    fn test_layer_cache_put_source_not_exists() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let nonexistent = tmp.path().join("does_not_exist");
        let result = cache.put("sha256:bad_source", &nonexistent);
        assert!(result.is_err());
    }

    #[test]
    fn test_layer_cache_rejects_path_traversal_keys() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let source = tmp.path().join("source");
        create_test_layer(&source, &[("file", "content")]);

        for digest in ["../outside", "nested/layer", "nested\\layer", "..", ""] {
            assert!(cache.get(digest).is_err(), "digest={digest:?}");
            assert!(cache.put(digest, &source).is_err(), "digest={digest:?}");
            assert!(cache.invalidate(digest).is_err(), "digest={digest:?}");
        }
    }

    #[cfg(unix)]
    #[test]
    fn test_layer_cache_does_not_follow_symlink_entries() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();
        let outside = tmp.path().join("outside");
        create_test_layer(&outside, &[("host-data", "must survive")]);
        let digest = "sha256:linked";
        let link = tmp.path().join(LayerCache::digest_to_dirname(digest));
        std::os::unix::fs::symlink(&outside, &link).unwrap();

        assert!(cache.get(digest).unwrap().is_none());
        let source = tmp.path().join("source");
        create_test_layer(&source, &[("file", "content")]);
        assert!(cache.put(digest, &source).is_err());

        cache.invalidate(digest).unwrap();
        assert!(outside.join("host-data").is_file());
        assert!(!link.exists());
    }

    #[test]
    fn test_layer_cache_prune_zero_limit() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let source = tmp.path().join("source");
        create_test_layer(&source, &[("f.txt", "data")]);
        cache.put("sha256:entry1", &source).unwrap();
        cache.put("sha256:entry2", &source).unwrap();

        // Prune with 0 bytes limit — should evict everything
        let evicted = cache.prune(0).unwrap();
        assert_eq!(evicted, 2);
        assert_eq!(cache.list_entries().unwrap().len(), 0);
    }

    #[test]
    fn test_layer_cache_list_entries_ignores_non_meta_files() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Add a valid entry
        let source = tmp.path().join("source");
        create_test_layer(&source, &[("f.txt", "data")]);
        cache.put("sha256:valid", &source).unwrap();

        // Add random non-meta files to cache directory
        std::fs::write(tmp.path().join("random.txt"), "noise").unwrap();
        std::fs::write(tmp.path().join("other.json"), "{}").unwrap();

        // Should only return the valid entry
        let entries = cache.list_entries().unwrap();
        assert_eq!(entries.len(), 1);
        assert_eq!(entries[0].digest, "sha256:valid");
    }

    #[test]
    fn test_layer_cache_list_entries_skips_invalid_json() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        // Add a valid entry
        let source = tmp.path().join("source");
        create_test_layer(&source, &[("f.txt", "data")]);
        cache.put("sha256:valid", &source).unwrap();

        // Add a corrupted .meta.json
        std::fs::write(
            tmp.path().join("sha256_corrupted.meta.json"),
            "not json at all",
        )
        .unwrap();

        // Should only return the valid entry, skip corrupted
        let entries = cache.list_entries().unwrap();
        assert_eq!(entries.len(), 1);
        assert_eq!(entries[0].digest, "sha256:valid");
    }

    #[test]
    fn test_layer_cache_put_preserves_file_content() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let source = tmp.path().join("source");
        create_test_layer(
            &source,
            &[
                ("binary.bin", "\x00\x01\x02\x03"),
                ("text.txt", "hello world\n"),
            ],
        );

        let cached = cache.put("sha256:content_check", &source).unwrap();

        assert_eq!(
            std::fs::read(cached.join("binary.bin")).unwrap(),
            b"\x00\x01\x02\x03"
        );
        assert_eq!(
            std::fs::read_to_string(cached.join("text.txt")).unwrap(),
            "hello world\n"
        );
    }

    #[test]
    fn test_layer_cache_multiple_colons_in_digest() {
        let tmp = TempDir::new().unwrap();
        let cache = LayerCache::new(tmp.path()).unwrap();

        let digest = "sha256:abc:def:ghi";
        let source = tmp.path().join("source");
        create_test_layer(&source, &[("f.txt", "data")]);

        cache.put(digest, &source).unwrap();
        let result = cache.get(digest).unwrap();
        assert!(result.is_some());

        cache.invalidate(digest).unwrap();
        assert!(cache.get(digest).unwrap().is_none());
    }

    #[test]
    fn test_copy_file_cow_preserves_content_and_mode() {
        // Works whether the FS supports reflink (FICLONE) or falls back to a byte
        // copy — both must preserve content and the permission bits.
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src.bin");
        let dst = tmp.path().join("dst.bin");
        std::fs::write(&src, b"hello copy-on-write").unwrap();
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            std::fs::set_permissions(&src, std::fs::Permissions::from_mode(0o755)).unwrap();
        }

        copy_file_cow(&src, &dst).unwrap();

        assert_eq!(std::fs::read(&dst).unwrap(), b"hello copy-on-write");
        #[cfg(unix)]
        {
            use std::os::unix::fs::PermissionsExt;
            let mode = std::fs::metadata(&dst).unwrap().permissions().mode() & 0o777;
            assert_eq!(mode, 0o755, "executable bit must survive the copy");
        }
    }

    #[cfg(unix)]
    #[test]
    fn test_copy_dir_recursive_preserves_directory_modes() {
        use std::os::unix::fs::PermissionsExt;

        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src");
        let nested = src.join("nested");
        let dst = tmp.path().join("dst");
        std::fs::create_dir_all(&nested).unwrap();
        std::fs::write(nested.join("file"), b"content").unwrap();
        std::fs::set_permissions(&src, std::fs::Permissions::from_mode(0o755)).unwrap();
        std::fs::set_permissions(&nested, std::fs::Permissions::from_mode(0o710)).unwrap();

        copy_dir_recursive(&src, &dst).unwrap();

        let root_mode = std::fs::metadata(&dst).unwrap().permissions().mode() & 0o777;
        let nested_mode = std::fs::metadata(dst.join("nested"))
            .unwrap()
            .permissions()
            .mode()
            & 0o777;
        assert_eq!(root_mode, 0o755);
        assert_eq!(nested_mode, 0o710);
    }

    #[test]
    fn test_copy_file_cow_overwrites_existing_dst() {
        // FICLONE and the fs::copy fallback both truncate the destination.
        let tmp = TempDir::new().unwrap();
        let src = tmp.path().join("src");
        let dst = tmp.path().join("dst");
        std::fs::write(&src, b"new").unwrap();
        std::fs::write(&dst, b"old-and-longer").unwrap();
        copy_file_cow(&src, &dst).unwrap();
        assert_eq!(std::fs::read(&dst).unwrap(), b"new");
    }
}