## Summary
Fair queue consumers could leak the per-tenant concurrency slots that
gate admission. Slots were freed on some paths and skipped on others,
and once enough leaked slots accumulated for a tenant, every queue that
tenant owned stopped being served until someone cleared the set by hand.
This PR frees slots on every path and, more importantly, makes the
remaining failure modes self-healing.
## Design
The fix applies one rule uniformly: releasing a concurrency slot is
best-effort cleanup and must never block the message's primary state
transition. Blocking completion re-delivers the message, which
duplicates customer work; blocking a retry loses the attempt increment,
so the message can circle forever; blocking a reclaim strands the
message in flight. A leaked slot is the better failure in every one of
those trades because it is the only one that is recoverable. A failed
release is therefore logged and the transition proceeds.
Leaked slots then heal through two mechanisms:
- `reserve` re-admits a message that is already a member of its own
concurrency set, since re-admitting it does not increase concurrency. A
message whose earlier release failed can no longer be blocked by its own
leftover slot.
- A reconcile loop periodically removes any set member with no in-flight
record (interval configurable via `reconcileIntervalMs`, default 60s).
The check-and-remove is atomic, and it is sound because a message is
always registered in flight before its slot is reserved, so a member
with no in-flight record can only be a leak. This also covers leaks this
PR cannot prevent directly, such as a release that resolves the wrong
concurrency group from queue metadata.
Ordering hardening from earlier revisions stays: slots are released
before the in-flight record needed to describe them is discarded, the
release Lua scripts write the message back to the queue before removing
it from in-flight (Lua does not roll back on error), and dangling
in-flight entries with no payload are dropped instead of being rescanned
forever.
Every guard test was verified to fail without its specific fix,
including the duplicate-execution case: completing a message while its
slot release fails used to re-deliver and re-execute it.
Matt Aitken
1114d9d6f9