Review finding from @kartik-mem0 on this PR.
O_CREAT|O_EXCL then write leaves a window where the salt file exists and is
empty. Hooks are short-lived processes firing on every tool call and people run
several agent windows, so a concurrent reader lands in that window, reads
nothing, and falls back to a digest of the salt file's own path, memoized for
its whole run. That path is guessable, so the race silently replaced the privacy
control with something an attacker can compute, and hashed the same repository
two ways depending on timing.
The value is now written to a private temp file, fsynced, and published with
os.link, which is atomic and fails if another process already published one.
Link rather than replace, so losing the race adopts their salt instead of
clobbering it. The temp file is removed either way.
The derived fallback is gone rather than fixed. _scoped_digest returns "" when
there is no salt and record() omits the property, because an unsalted digest
over a git remote or a home-directory path is close to plaintext, and shipping
one under a name that says hash is worse than sending nothing.
Three tests: the racing reader never sees the name half-written, a second writer
adopts the first's salt and leaves no temp file, and an unwritable data
directory drops the property instead of emitting a weak one. The old test
asserted the fallback behaviour and is replaced.
265 passed, 8 skipped.
Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb
Saket Aryan
7710a4e180