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Mailbox atomicity — flock-based prepend and msg-id idempotency

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@@ -7,17 +7,17 @@ category: architecture
type: topic
status: active
bcsc_class: public-disclosure-safe
last_edited: 2026-05-18
last_edited: 2026-05-25
editor: pointsav-engineering
cites: []
paired_with: mailbox-atomicity.es.md
---

Sessions communicate by prepending messages to flat-file mailboxes at `.agent/inbox.md` and `.agent/outbox.md`. Messages carry a YAML envelope (from, to, re, created, status, optional msg-id) followed by a free-text body. Newest messages live at the top so a Command Session reading an outbox via `head` sees the most recent activity first.
Sessions communicate by prepending messages to flat-file mailboxes at `.agent/inbox.md` and `.agent/outbox.md`. Messages carry a YAML envelope (from, to, re, created, status, optional msg-id) followed by a free-text body. Newest messages live at the top so the hub session reading an outbox sees the most recent activity first.

The atomicity problem: two sessions prepending to the same mailbox without coordination produce the classic read-modify-write race. Session A reads the current file, prepends its message, and writes back. Session B does the same simultaneously. Whichever write lands last wins; the other message is lost. This is reproducible with parallel AI-coding sub-agents; cross-user concurrency makes it inevitable.

`bin/mailbox-prepend.sh` solves this with `flock`. It takes the target mailbox path, derives a lock path under `.agent/locks/` for archive-scoped mailboxes (or `/tmp/foundry-mailbox-<sha>.lock` for ad-hoc cases), and acquires an exclusive lock with a 30-second timeout before performing the read-modify-write. Two simultaneous calls serialise rather than collide.
The `mailbox-prepend` helper script solves this with `flock`. It takes the target mailbox path, derives a lock path for archive-scoped mailboxes, and acquires an exclusive lock with a 30-second timeout before performing the read-modify-write. Two simultaneous calls serialise rather than collide.

A second safety: msg-id idempotency. If the message body contains a `msg-id:` field, the script scans the first 200 lines of the target for an existing entry with the same id and skips the prepend if found. This means a script that gets retried — a timer firing twice, an operator re-running, a hook misfiring — will not double-send. The 200-line window is generous enough to catch same-day duplicates without making the scan expensive on large archives.
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