store: Fix: Race condition in cognitive-memory _save_state loses last_reflection timestamp when daily+weekly timers collide

graph/fixes/fix-race-condition-in-cognitive-memory-save-state-loses-last-59268a.md

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+---
+id: 59268a95-4712-48b4-bccb-6171c7885301
+type: fix
+title: "Fix: Race condition in cognitive-memory _save_state loses last_reflection timestamp when daily+weekly timers collide"
+tags: [cognitive-memory, fix, race-condition, systemd, state-management, atomicity]
+importance: 0.8
+confidence: 0.8
+created: "2026-02-20T06:47:07.701221+00:00"
+updated: "2026-02-20T06:47:07.701221+00:00"
+---
+
+# Race Condition in _save_state Corrupts last_reflection Timestamp
+
+## Symptom
+REFLECTION.md showed "Last reflection: never" despite the weekly reflect timer having run successfully.
+
+## Root Cause
+Two concurrent issues:
+
+1. **Timer collision**: The daily timer (decay) and weekly timer (reflect) could fire simultaneously. `OnCalendar=weekly` resolves to Mon midnight, which overlaps with `OnCalendar=daily` on Mondays. Both processes load and save `_state.json` concurrently.
+
+2. **Non-atomic writes with silent failure**: `_save_state()` would overwrite `_state.json` without first merging existing state, so decay's write could clobber the `last_reflection` key written by reflect. Additionally, `_load_state()` silently caught `JSONDecodeError` and returned an empty default dict, permanently losing the key on any write collision.
+
+## Fix
+1. `_save_state()` now merges with existing on-disk state before writing (read-modify-write pattern).
+2. Writes use atomic `tempfile + os.replace()` to prevent partial writes.
+3. Weekly timer changed from `OnCalendar=weekly` to `OnCalendar=Sun *-*-* 02:00:00` to avoid midnight collision with the daily timer.
+
+## Commit
+Commit `7f120f8` in the cognitive-memory repo.