Add a per-host drain mutex (drainLocks map guarded by drainLocksMu) on
the Server struct. DrainPending acquires it with TryLock: if a drain is
already in-flight for this host, the call returns immediately — the
running drain will see every pending row. This prevents the on-hello
goroutine and the 30s tick from both listing the same host's rows and
dispatching them twice.
Update three existing tests that called srv.DrainPending explicitly
after the on-hello goroutine had already been spawned: replace the
now-redundant direct call with a waitForPendingCount poll so they don't
race the goroutine's mutex ownership. Add TestDrainPendingSerializesPerHost
which fires 10 concurrent DrainPending goroutines against a 5-row queue
and asserts exactly 5 job rows result.
steve
556d65d77f