Review of this PR was right that maxsize=8 bounds the wrong thing. The
LifoQueue caps how many connections are RETURNED; _checkout_read_conn opened
unconditionally on a miss, so N readers arriving on a cold pool all missed, all
opened, and peaked at N. The surplus was closed on release, so nothing
accumulated forever -- but EMFILE is a peak-instant condition and the burst
that empties the pool is exactly the burst that exhausts the fd table, so the
original wedge was still reachable. Measured on the previous commit: 64
concurrent readers held 64 live connections at once.
A connection now holds a permit for its whole lifetime -- acquired in
_get_read_conn() before the open, released in _close_read_conn() after the
close -- so open+checked-out is bounded together. A pool hit costs no permit
because the connection it hands back already holds one, which leaves
_get_read_conn() as the only place that can open. The acquire is non-blocking:
past the ceiling readers fall back to the locked writer connection rather than
queueing, since blocking would convert descriptor exhaustion into a stall,
which is the same outage with a different stack trace. Same burst now peaks at
8. BoundedSemaphore rather than Semaphore so an unpaired release raises instead
of silently widening the ceiling.
Two latent leaks in the same function, found while doing this:
- a CJK extension load that failed after a successful open returned None
without closing the connection, leaking a descriptor the tracking registry
still counted -- the same leak shape one level down;
- any non-sqlite3.Error between open and return stranded a permit
permanently, which would ratchet the ceiling down to zero and silently
demote every later read to the writer lock.
On the test: the existing one joins every worker before counting, so it
measures the pool at rest and structurally cannot observe peak -- which is why
this got through. The new one uses a barrier so all 64 workers hold their
connections until every worker has checked out, making the count taken at that
moment the actual simultaneous peak. Verified it fails against the previous
commit (64 checked out, 65 live) and passes at 8/9. Also covers the
writer-connection fallback, permit recovery after a failed open, and that
close() releases exactly the permits it drained.
Yishova