fix(cache): leave outputs that already match untouched when restoring a cache hit

CHANGELOG.md

@@ -1,5 +1,6 @@
 # Changelog
 
+- **Fixed** A cache hit no longer rewrites output files that already have the cached content and permissions, so a file watcher on a task's outputs, such as a Vite dev server or `vite build --watch`, no longer sees every output change when none did. Missing and changed outputs are still restored ([#901](https://github.com/voidzero-dev/vite-task/pull/901)).
 - **Fixed** On Linux and macOS, programs using automatic file-access tracking no longer abort when they call `execve`, `execv`, `execl`, `execle`, or `posix_spawn` with an empty path; the call fails with `ENOENT`, as it does without tracking. On Linux, `execveat` with an empty path and without `AT_EMPTY_PATH` now also fails with `ENOENT`, instead of trying to run what its directory descriptor refers to ([#896](https://github.com/voidzero-dev/vite-task/pull/896)).
 - **Perf** `vp run` starts faster in workspaces whose tasks share cache `input` and `output` patterns: each distinct pattern is now parsed once instead of once for every task that uses it. In a 138-package workspace, this saves about 2 seconds of CPU on every run ([#887](https://github.com/voidzero-dev/vite-task/pull/887), [#890](https://github.com/voidzero-dev/vite-task/pull/890)).
 - **Perf** When tasks miss the cache, `vp run` no longer waits for the disk after saving each task's result, so the run finishes sooner. If the computer loses power or the operating system crashes during `vp run`, the cache entries that the run saved can be lost, and those tasks run again next time instead of hitting the cache ([#871](https://github.com/voidzero-dev/vite-task/pull/871)).

crates/vt/src/session/cache/archive.rs

@@ -3,6 +3,7 @@
 use std::{
     fs::File,
     io::{self, Read},
+    path::Component,
 };
 
 use vt_path::{AbsolutePath, RelativePathBuf};
@@ -48,7 +49,12 @@ pub fn create_output_archive(
 
 /// Extract a tar.zst archive, restoring files relative to workspace root.
 ///
-/// Parent directories are created automatically. Existing files are overwritten.
+/// A file that already has its entry's content and permissions is left as it
+/// is, modification time included, so restoring outputs that didn't change
+/// writes nothing and file watchers see no change. Every other entry is
+/// unpacked as [`tar::Archive::unpack`] would: parent directories are created
+/// and an existing file is replaced. (`Archive::unpack` also defers directory
+/// entries to the end, and [`create_output_archive`] writes none.)
 ///
 /// # Errors
 ///
@@ -57,14 +63,127 @@ pub fn extract_output_archive(
     workspace_root: &AbsolutePath,
     archive_path: &AbsolutePath,
 ) -> anyhow::Result<()> {
-    let file = File::open(archive_path.as_path())?;
-    let decoder = zstd::Decoder::new(file)?;
-    let mut archive = tar::Archive::new(decoder);
+    // Canonicalized once, as `Archive::unpack` does: on Windows this gives the
+    // `\\?\` prefix that lifts the 260-character path limit.
+    let dst = workspace_root
+        .as_path()
+        .canonicalize()
+        .unwrap_or_else(|_| workspace_root.as_path().to_path_buf());
+
+    // Comparing an entry with a file of the same size reads the entry's data,
+    // so an entry found to differ that way is unpacked in a second pass.
+    let mut read_and_differ = Vec::new();
+    let mut archive = open_output_archive(archive_path)?;
+    for (index, entry) in archive.entries()?.enumerate() {
+        let mut entry = entry?;
+        match compare_with_disk(workspace_root, &mut entry)? {
+            OnDisk::Matches => {}
+            OnDisk::Differs => {
+                entry.unpack_in(&dst)?;
+            }
+            OnDisk::DiffersAfterRead => read_and_differ.push(index),
+        }
+    }
 
-    archive.unpack(workspace_root.as_path())?;
+    let mut pending = read_and_differ.into_iter().peekable();
+    if pending.peek().is_some() {
+        let mut archive = open_output_archive(archive_path)?;
+        for (index, entry) in archive.entries()?.enumerate() {
+            let Some(&next) = pending.peek() else { break };
+            let mut entry = entry?;
+            if index == next {
+                entry.unpack_in(&dst)?;
+                pending.next();
+            }
+        }
+    }
     Ok(())
 }
 
+fn open_output_archive(
+    archive_path: &AbsolutePath,
+) -> io::Result<tar::Archive<zstd::Decoder<'static, io::BufReader<File>>>> {
+    let file = File::open(archive_path.as_path())?;
+    Ok(tar::Archive::new(zstd::Decoder::new(file)?))
+}
+
+/// How the file an archive entry restores compares with the entry.
+enum OnDisk {
+    /// The file has the entry's content and permissions.
+    Matches,
+    /// The entry isn't a regular file, or the file is missing or differs in
+    /// type, size or permissions. The entry's data is unread.
+    Differs,
+    /// The file differs in content, found by reading the entry's data.
+    DiffersAfterRead,
+}
+
+fn compare_with_disk(
+    workspace_root: &AbsolutePath,
+    entry: &mut tar::Entry<'_, impl Read>,
+) -> io::Result<OnDisk> {
+    if !entry.header().entry_type().is_file() {
+        return Ok(OnDisk::Differs);
+    }
+    let Ok(mode) = entry.header().mode() else {
+        return Ok(OnDisk::Differs);
+    };
+    let target = match entry.path() {
+        // Any other path is left to `unpack_in`, which skips or rejects it.
+        Ok(path) if path.components().all(|part| matches!(part, Component::Normal(_))) => {
+            workspace_root.join(path)
+        }
+        _ => return Ok(OnDisk::Differs),
+    };
+    let Ok(metadata) = std::fs::symlink_metadata(target.as_path()) else {
+        return Ok(OnDisk::Differs);
+    };
+    if !metadata.is_file()
+        || metadata.len() != entry.size()
+        || !has_unpacked_permissions(&metadata, mode)
+    {
+        return Ok(OnDisk::Differs);
+    }
+    let Ok(on_disk) = File::open(target.as_path()) else {
+        return Ok(OnDisk::Differs);
+    };
+    Ok(if same_content(entry, on_disk)? { OnDisk::Matches } else { OnDisk::DiffersAfterRead })
+}
+
+/// Whether two readers of the same length yield the same bytes. Only errors
+/// reading `archived` are returned; one reading `on_disk` counts as a
+/// difference.
+fn same_content(mut archived: impl Read, mut on_disk: impl Read) -> io::Result<bool> {
+    let mut archived_buf = [0; 16 * 1024];
+    let mut on_disk_buf = [0; 16 * 1024];
+    loop {
+        let len = archived.read(&mut archived_buf)?;
+        if len == 0 {
+            return Ok(true);
+        }
+        if on_disk.read_exact(&mut on_disk_buf[..len]).is_err()
+            || archived_buf[..len] != on_disk_buf[..len]
+        {
+            return Ok(false);
+        }
+    }
+}
+
+/// Whether a file has the permissions `unpack_in` gives a file from an entry
+/// with `mode`: its permission bits, without setuid, setgid or sticky.
+#[cfg(unix)]
+fn has_unpacked_permissions(metadata: &std::fs::Metadata, mode: u32) -> bool {
+    use std::os::unix::fs::PermissionsExt;
+    metadata.permissions().mode() & 0o7777 == mode & 0o777
+}
+
+/// Whether a file has the permissions `unpack_in` gives a file from an entry
+/// with `mode`: read-only when the mode lacks the owner's write bit.
+#[cfg(windows)]
+fn has_unpacked_permissions(metadata: &std::fs::Metadata, mode: u32) -> bool {
+    metadata.permissions().readonly() == (mode & 0o200 == 0)
+}
+
 /// Read a tar.zst archive from `reader` to the end without writing any files,
 /// to check that it decodes.
 ///
@@ -103,4 +222,98 @@ mod tests {
             assert!(check_output_archive(corrupt).is_err());
         }
     }
+
+    /// Writes each `(name, content)` output into a temporary workspace and
+    /// archives them all.
+    fn archived_outputs(outputs: &[(&str, &str)]) -> (tempfile::TempDir, AbsolutePathBuf) {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let mut paths = Vec::new();
+        for (name, content) in outputs {
+            let path = RelativePathBuf::new(name).unwrap();
+            std::fs::write(dir.join(&path).as_path(), content).unwrap();
+            paths.push(path);
+        }
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &paths, &archive_path).unwrap();
+        (tmp, archive_path)
+    }
+
+    /// A modification time long before the archive was made, which a restore
+    /// that rewrites the file replaces with the archived one.
+    fn mark_untouched(path: &AbsolutePath) -> std::time::SystemTime {
+        let time = std::time::UNIX_EPOCH + std::time::Duration::from_secs(1_000_000_000);
+        File::options().write(true).open(path.as_path()).unwrap().set_modified(time).unwrap();
+        time
+    }
+
+    fn modified(path: &AbsolutePath) -> std::time::SystemTime {
+        std::fs::metadata(path.as_path()).unwrap().modified().unwrap()
+    }
+
+    #[test]
+    fn extract_leaves_matching_files_untouched() {
+        let (tmp, archive_path) = archived_outputs(&[("a.txt", "a\n"), ("b.txt", "b\n")]);
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let a = dir.join("a.txt");
+        let b = dir.join("b.txt");
+        let untouched = [mark_untouched(&a), mark_untouched(&b)];
+
+        extract_output_archive(&dir, &archive_path).unwrap();
+
+        assert_eq!([modified(&a), modified(&b)], untouched);
+        assert_eq!(std::fs::read_to_string(a.as_path()).unwrap(), "a\n");
+        assert_eq!(std::fs::read_to_string(b.as_path()).unwrap(), "b\n");
+    }
+
+    #[test]
+    fn extract_restores_missing_and_changed_files() {
+        let (tmp, archive_path) = archived_outputs(&[
+            ("missing.txt", "missing\n"),
+            ("resized.txt", "resized\n"),
+            ("same-size.txt", "same size\n"),
+            ("unchanged.txt", "unchanged\n"),
+        ]);
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        std::fs::remove_file(dir.join("missing.txt").as_path()).unwrap();
+        std::fs::write(dir.join("resized.txt").as_path(), "longer content\n").unwrap();
+        std::fs::write(dir.join("same-size.txt").as_path(), "SAME SIZE\n").unwrap();
+        let unchanged = dir.join("unchanged.txt");
+        let untouched = mark_untouched(&unchanged);
+
+        extract_output_archive(&dir, &archive_path).unwrap();
+
+        for (name, content) in [
+            ("missing.txt", "missing\n"),
+            ("resized.txt", "resized\n"),
+            ("same-size.txt", "same size\n"),
+            ("unchanged.txt", "unchanged\n"),
+        ] {
+            assert_eq!(std::fs::read_to_string(dir.join(name).as_path()).unwrap(), content);
+        }
+        assert_eq!(modified(&unchanged), untouched);
+    }
+
+    #[test]
+    fn extract_restores_changed_permissions() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let dir = AbsolutePathBuf::new(tmp.path().to_path_buf()).unwrap();
+        let path = RelativePathBuf::new("output.txt").unwrap();
+        let output = dir.join(&path);
+        std::fs::write(output.as_path(), "built\n").unwrap();
+        let writable = std::fs::metadata(output.as_path()).unwrap().permissions();
+        let mut read_only = writable.clone();
+        read_only.set_readonly(true);
+        std::fs::set_permissions(output.as_path(), read_only).unwrap();
+        let archive_path = dir.join("output.tar.zst");
+        create_output_archive(&dir, &[path], &archive_path).unwrap();
+        std::fs::set_permissions(output.as_path(), writable.clone()).unwrap();
+
+        extract_output_archive(&dir, &archive_path).unwrap();
+
+        let restored = std::fs::metadata(output.as_path()).unwrap().permissions();
+        // Windows can't remove the directory while the file is read-only.
+        std::fs::set_permissions(output.as_path(), writable).unwrap();
+        assert!(restored.readonly());
+    }
 }