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Commitb2ccb5f

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shm_mq: Fix failure to notice a dead counterparty when nowait is used.
The shm_mq mechanism was intended to optionally notice when the processon the other end of the queue fails to attach to the queue. It doesthis by allowing the user to pass a BackgroundWorkerHandle; if thebackground worker in question is launched and dies without attachingto the queue, then we know it never will. This logic works OK inblocking mode, but when called with nowait = true we fail to noticethat this has happened due to an asymmetry in the logic. Repair.Reported off-list by Rushabh Lathia. Patch by me.
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‎src/backend/storage/ipc/shm_mq.c‎

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Original file line numberDiff line numberDiff line change
@@ -142,6 +142,8 @@ static shm_mq_result shm_mq_send_bytes(shm_mq_handle *mq, Size nbytes,
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constvoid*data,boolnowait,Size*bytes_written);
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staticshm_mq_resultshm_mq_receive_bytes(shm_mq*mq,Sizebytes_needed,
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boolnowait,Size*nbytesp,void**datap);
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staticboolshm_mq_counterparty_gone(volatileshm_mq*mq,
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BackgroundWorkerHandle*handle);
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staticboolshm_mq_wait_internal(volatileshm_mq*mq,PGPROC*volatile*ptr,
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BackgroundWorkerHandle*handle);
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staticuint64shm_mq_get_bytes_read(volatileshm_mq*mq,bool*detached);
@@ -499,6 +501,8 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
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{
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if (nowait)
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{
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if (shm_mq_counterparty_gone(mq,mqh->mqh_handle))
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returnSHM_MQ_DETACHED;
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if (shm_mq_get_sender(mq)==NULL)
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returnSHM_MQ_WOULD_BLOCK;
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}
@@ -794,6 +798,11 @@ shm_mq_send_bytes(shm_mq_handle *mqh, Size nbytes, const void *data,
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*/
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if (nowait)
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{
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if (shm_mq_counterparty_gone(mq,mqh->mqh_handle))
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{
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*bytes_written=sent;
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returnSHM_MQ_DETACHED;
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}
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if (shm_mq_get_receiver(mq)==NULL)
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{
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*bytes_written=sent;
@@ -947,6 +956,45 @@ shm_mq_receive_bytes(shm_mq *mq, Size bytes_needed, bool nowait,
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}
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}
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/*
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* Test whether a counterparty who may not even be alive yet is definitely gone.
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*/
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staticbool
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shm_mq_counterparty_gone(volatileshm_mq*mq,BackgroundWorkerHandle*handle)
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{
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booldetached;
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pid_tpid;
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/* Acquire the lock just long enough to check the pointer. */
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SpinLockAcquire(&mq->mq_mutex);
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detached=mq->mq_detached;
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SpinLockRelease(&mq->mq_mutex);
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/* If the queue has been detached, counterparty is definitely gone. */
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if (detached)
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return true;
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/* If there's a handle, check worker status. */
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if (handle!=NULL)
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{
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BgwHandleStatusstatus;
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/* Check for unexpected worker death. */
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status=GetBackgroundWorkerPid(handle,&pid);
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if (status!=BGWH_STARTED&&status!=BGWH_NOT_YET_STARTED)
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{
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/* Mark it detached, just to make it official. */
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SpinLockAcquire(&mq->mq_mutex);
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mq->mq_detached= true;
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SpinLockRelease(&mq->mq_mutex);
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return true;
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}
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}
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/* Counterparty is not definitively gone. */
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return false;
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}
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/*
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* This is used when a process is waiting for its counterpart to attach to the
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* queue. We exit when the other process attaches as expected, or, if

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