Audit #6. `KvServiceImpl` wrote the row, waited for it to commit, then
asked the emitter to resolve matching triggers and insert outbox rows —
a second transaction on a second connection. If that second step failed,
the row was permanently in the store with its trigger having never
fired: invisible to the caller (the write "succeeded"), unrecoverable by
any retry, and only ever logged. The code said as much:
// Audit finding (Medium): this is non-transactional with the data
// write — the row above has already committed by the time emit()
// runs, so an emit failure means triggers silently don't fire.
Introduce `atomic_write::KvWriter` — the mutating half of the service (the
write AND the fan-out it produces), so the two can share a transaction:
* `PostgresKvWriter` opens a tx, writes via `kv_repo::*_on(&mut *tx, …)`,
runs the fan-out on the SAME connection via `emit_on(&mut *tx, …)`, and
commits. An emit failure now rolls the write back and surfaces to the
script, which is the honest outcome — the caller learns the write did
not happen instead of silently getting a store that disagrees with its
triggers.
* `BestEffortKvWriter` keeps the old write-then-log-on-emit-failure
semantics for the in-memory unit tests (no Postgres).
Both sit behind one trait, so the service body has a single code path and
the choice is a constructor detail (`with_atomic_writes(pool)` in the host).
Pool-deadlock rule, documented on the module: everything inside the tx runs
on the tx's connection. A writer that held a tx and then reached for a
second pooled connection could starve — the pool is sized to the execution
concurrency cap, so N executions each wanting 2 connections deadlock. The
fan-out takes `&mut *tx` and never touches a repo.
`tests/atomic_write.rs` pins it by injecting an outbox failure (a Postgres
BEFORE-INSERT trigger scoped to the test's own app_id, so parallel tests are
unaffected): the set errors and the key is NOT in the store; likewise a
delete rolls back rather than dropping a key nothing downstream hears about.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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