Two concrete data-corruption vectors flagged in Opus review of PR #2041,
both fixed atomically so the new repair-safe endpoint is safe for production:
1. Shared tmp filename under concurrent calls
`tmp = target.with_suffix('.json.reconcile.tmp')` produced a fixed path
per session ID. Two simultaneous repair-safe POSTs would interleave bytes
in the same tmp file, then both rename → corrupted JSON. Now matches the
`Session.save()` convention at api/models.py:484 with a pid+tid suffix.
2. TOCTOU between target.exists() check and tmp.replace(target)
`os.replace()` overwrites unconditionally. If a concurrent Session.save()
for the same SID materialized the live sidecar in the microsecond window
between the existence check and the rename, the reconciliation would
silently overwrite a live sidecar with a (lossier) state.db reconstruction.
Switched to `os.link()` + `unlink(tmp)` which is atomic create-or-fail —
on FileExistsError we record `skipped: sidecar_appeared_during_reconcile`
and keep the live sidecar untouched.
Plus a round-trip schema-parity test: materialize a sidecar from state.db,
then load it back through `Session.load()` and assert the messages survive.
Catches future schema drift between `_state_db_row_to_sidecar()` and
`Session.__init__()`. Also adds a guard test confirming the .reconcile.tmp
suffix includes pid+tid (regression guard for hazard #1).
Tests: 23 passing across the recovery suite (was 21; +2 new in this commit).
Co-authored-by: ai-ag2026 <261867348+ai-ag2026@users.noreply.github.com>
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