[jasonkuhrt]

Written by an AI agent (Claude Opus 5.5) on behalf of @jasonkuhrt.

Closes #900

Motivation

A cache hit unpacked every archived output over the workspace, so each file was removed and created again even when its content hadn't changed, and a watcher on the outputs saw all of them change. In #900, a replay of a 1,000-file generated client made a Vite dev server send about 1,000 HMR updates and restarted an API running under vite build --watch.

Change

extract_output_archive compares each regular-file entry with the file on disk before writing it:

• A file with the entry's size, permissions and bytes is left as it is, modification time included.
• A missing file, or one that differs in type, size or permissions, is unpacked right away with Entry::unpack_in, as Archive::unpack did, so tar's path checks and the metadata it restores stay the same. The destination is canonicalized once, as Archive::unpack does.
• A file of the same size is compared by streaming the entry and the file side by side, which reads the entry's data. One that differs is unpacked in a second pass over the archive, which stops after the last such entry. A hit over unchanged outputs, or a restore into a fresh checkout, reads the archive once.

Only entries whose path is plain relative components are compared; anything else goes to unpack_in as before. A match writes nothing, so tar's check that a write stays inside the workspace still guards every write.

On the fixture in #900, a replay over unchanged outputs replaces none of the 1,000 files and fires no watcher events, against all 1,000 on main. With one output edited and one deleted, it restores those 2. On Linux the same replay fired 3,000 inotify events on main and none with the change.

Release builds on GitHub's macos-15 and ubuntu-24.04 runners, each job building both revisions and alternating their runs, median of 40 (run):

Cache hit, 1,000 outputs	macOS, main	macOS, this PR	Linux, main	Linux, this PR
Outputs unchanged	311.3 ms	35.4 ms	50.5 ms	13.8 ms
One output edited, same size	244.9 ms	27.3 ms	50.9 ms	14.1 ms
Outputs deleted	200.8 ms	215.4 ms	39.2 ms	41.2 ms

A restore into an empty directory is 5–7% slower, from looking up each missing file before unpacking it.

Two new tests fail on main: matching files keep their modification time, and missing, resized and same-size-changed files are restored beside an untouched one. A third checks that a file whose permissions changed is restored.

#866 (atomic writes) and #885 (symbolic links) also change this function. This change only decides which entries get written, so it combines with either.