shm_mq_sendv: Fix flushing bug when receiver not yet attached.

With the old logic, when the reciever had not yet attached, we would
never call shm_mq_inc_bytes_written(), even if force_flush = true
was specified. That could result in a situation where data that the
sender believes it has sent is never received.

Along the way, remove a useless function prototype for a nonexistent
function from shm_mq.h.

Commit 4684643 introduced these
problems.

Pavan Deolasee, with a few changes by me.

Discussion: https://postgr.es/m/CABOikdPkwtLLCTnzzmpSMXo3QZa2yXq0J7Q61ssdLFAJYrOVvQ@mail.gmail.com

Author: robertmhaas

src/backend/storage/ipc/shm_mq.c

@@ -518,8 +518,7 @@ shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov, int iovcnt, bool nowait,
 
 	/*
 	 * If the counterparty is known to have attached, we can read mq_receiver
-	 * without acquiring the spinlock and assume it isn't NULL.  Otherwise,
-	 * more caution is needed.
+	 * without acquiring the spinlock.  Otherwise, more caution is needed.
 	 */
 	if (mqh->mqh_counterparty_attached)
 		receiver = mq->mq_receiver;
@@ -528,9 +527,8 @@ shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov, int iovcnt, bool nowait,
 		SpinLockAcquire(&mq->mq_mutex);
 		receiver = mq->mq_receiver;
 		SpinLockRelease(&mq->mq_mutex);
-		if (receiver == NULL)
-			return SHM_MQ_SUCCESS;
-		mqh->mqh_counterparty_attached = true;
+		if (receiver != NULL)
+			mqh->mqh_counterparty_attached = true;
 	}
 
 	/*
@@ -541,7 +539,8 @@ shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov, int iovcnt, bool nowait,
 	if (force_flush || mqh->mqh_send_pending > (mq->mq_ring_size >> 2))
 	{
 		shm_mq_inc_bytes_written(mq, mqh->mqh_send_pending);
-		SetLatch(&receiver->procLatch);
+		if (receiver != NULL)
+			SetLatch(&receiver->procLatch);
 		mqh->mqh_send_pending = 0;
 	}
 

src/include/storage/shm_mq.h

@@ -76,7 +76,6 @@ extern shm_mq_result shm_mq_sendv(shm_mq_handle *mqh, shm_mq_iovec *iov,
 								  int iovcnt, bool nowait, bool force_flush);
 extern shm_mq_result shm_mq_receive(shm_mq_handle *mqh,
 									Size *nbytesp, void **datap, bool nowait);
-extern void shm_mq_flush(shm_mq_handle *mqh);
 
 /* Wait for our counterparty to attach to the queue. */
 extern shm_mq_result shm_mq_wait_for_attach(shm_mq_handle *mqh);