fix(track): release idempotency key when deduction fails (#1138) - #1838

Fixes #1138.

The bug

/track calls with an idempotency_key permanently 409 on retry after a single failure. The key is SET NX into Redis with a 24-hour TTL before runRedisTrack/runRedisTrackV3 runs, so if the deduction throws (insufficient balance, transient Redis fault, etc.) the key sits there blocking every retry for the next 24 hours — even though no deduction ever committed.

The fix

Add releaseIdempotencyKey() next to checkIdempotencyKey() — same Redis key layout, just DEL. Then wrap the runRedisTrack / runRedisTrackV3 call in runTrackV2 and runTrackV3 in try/catch. On any throw:

1. Release the idempotency claim (so a retry can succeed once the cause is resolved).
2. Re-throw the original error (the caller still sees the real failure — 402, 500, whatever).

This preserves the existing concurrent-request safety — two simultaneous retries still race on SET NX and only one proceeds — while fixing the retry-after-failure path.

I kept this strictly to the body.idempotency_key path. The internal getTrackIdempotencyKey({ctx}) Redis lock used inside executeRedisDeductionV2 is unchanged; that's a per-deduction internal mechanism with its own lifecycle.

Tests

New unit file server/tests/unit/misc/idempotency/checkIdempotencyKey.test.ts — 7/7 green:

• first call sets, second call throws 409 (atomic SET NX)
• release-then-retry succeeds (the Bug: /track idempotency key is stored before deduction executes failed requests permanently block retries #1138 path)
• release is a no-op when Redis isn't ready (matches check's fail-open)
• release swallows Redis errors (best-effort)
• releasing a never-claimed key is harmless
• distinct keys don't cross-talk
• check fails-open when Redis isn't ready

Existing handle-track-queue-fallback.test.ts (3/3) and trackIdempotencyKey.test.ts (1/1) still pass.

Notes for the author of #1138

Your proposed "move storage to after success" change works for the retry path you described, but it opens a window where two concurrent retries both pass the (not-yet-set) check and both perform the deduction. The rollback pattern here keeps concurrent-safety intact. I also left body.skip_event and handleEventIdempotencyKey's commented-out event-insert block alone — those are a separate concern (sync-vs-queue event logging) and didn't seem worth bundling into a focused fix.

Summary by cubic

Fixes a bug where /track requests with an idempotency_key returned 409 on every retry after a failed deduction. We now release the key on errors so retries can proceed, while keeping concurrent safety intact.

• Bug Fixes
  • Added releaseIdempotencyKey() to delete track:<idempotency_key> in Redis when a deduction fails.
  • Wrapped runRedisTrack/runRedisTrackV3 in try/catch inside runTrackV2/runTrackV3; on error, release the key and re-throw.
  • Preserves existing SET NX concurrency guarantees; retries after failure no longer 409.
  • Added unit tests covering release-then-retry, Redis fail-open, and distinct key isolation.

^{Written for commit 5575252. Summary will update on new commits.}