On Tue, Jan 23, 2018 at 8:25 PM, Thomas Munro <thomas.mu...@enterprisedb.com> wrote: >> Hmm, I think that case will be addressed because tuple queues can >> detect if the leader is not attached. It does in code path >> shm_mq_receive->shm_mq_counterparty_gone. In >> shm_mq_counterparty_gone, it can detect if the worker is gone by using >> GetBackgroundWorkerPid. Moreover, I have manually tested this >> particular case before saying your patch is fine. Do you have some >> other case in mind which I am missing? > > Hmm. Yeah. I can't seem to reach a stuck case and was probably just > confused and managed to confuse Robert too. If you make > fork_process() fail randomly (see attached), I see that there are a > couple of easily reachable failure modes (example session at bottom of > message): > > 1. HandleParallelMessages() is reached and raises a "lost connection > to parallel worker" error because shm_mq_receive() returns > SHM_MQ_DETACHED, I think because shm_mq_counterparty_gone() checked > GetBackgroundWorkerPid() just as you said. I guess that's happening > because some other process is (coincidentally) sending > PROCSIG_PARALLEL_MESSAGE at shutdown, causing us to notice that a > process is unexpectedly stopped. > > 2. WaitForParallelWorkersToFinish() is reached and raises a "parallel > worker failed to initialize" error. TupleQueueReaderNext() set done > to true, because shm_mq_receive() returned SHM_MQ_DETACHED. Once > again, that is because shm_mq_counterparty_gone() returned true. This > is the bit Robert and I missed in our off-list discussion. > > As long as we always get our latch set by the postmaster after a fork > failure (ie kill SIGUSR1) and after GetBackgroundWorkerPid() is > guaranteed to return BGWH_STOPPED after that, and as long as we only > ever use latch/CFI loops to wait, and as long as we try to read from a > shm_mq, then I don't see a failure mode that hangs.
What about the parallel_leader_participation=off case? -- Peter Geoghegan
