(1) DisposeAsync disposed _gate unconditionally. SemaphoreSlim.Dispose does not
complete pending async waiters, and the in-flight call's Release() then threw
ObjectDisposedException from inside the semaphore (swallowed by the finally), so
a caller parked in WaitAsync was never released and its call never returned. The
gate is now left alive - the pattern SqlPollReader documents - so the waiter is
admitted and meets the session's own guarded ThrowIf(_disposed, this).
(2) LastUsedUtc advanced only on completion, so a slow catalog call (the AdminUI
allows 20 s) was invisible to the TTL reaper for its whole duration and a session
at the edge of its TTL could be disposed mid-call. It is now stamped on entry
too.
Joseph Doherty
cd4b29a368