Update parallel executor support to reuse the same DSM.

Commit b0b0d84 purported to make it
possible to relaunch workers using the same parallel context, but it had
an unpleasant race condition: we might reinitialize after the workers
have sent their last control message but before they have dettached the
DSM, leaving to crashes.  Repair by introducing a new ParallelContext
operation, ReinitializeParallelDSM.

Adjust execParallel.c to use this new support, so that we can rescan a
Gather node by relaunching workers but without needing to recreate the
DSM.

Amit Kapila, with some adjustments by me.  Extracted from latest parallel
sequential scan patch.

Author: robertmhaas

src/backend/access/transam/README.parallel

@@ -222,7 +222,9 @@ pattern looks like this:
 
 	ExitParallelMode();
 
-If desired, after WaitForParallelWorkersToFinish() has been called, another
-call to LaunchParallelWorkers() can be made using the same parallel context.
-Calls to these two functions can be alternated any number of times before
-destroying the parallel context.
+If desired, after WaitForParallelWorkersToFinish() has been called, the
+context can be reset so that workers can be launched anew using the same
+parallel context.  To do this, first call ReinitializeParallelDSM() to
+reinitialize state managed by the parallel context machinery itself; then,
+perform any other necessary resetting of state; after that, you can again
+call LaunchParallelWorkers.

src/backend/access/transam/parallel.c

@@ -110,6 +110,7 @@ static void HandleParallelMessage(ParallelContext *, int, StringInfo msg);
 static void ParallelErrorContext(void *arg);
 static void ParallelExtensionTrampoline(dsm_segment *seg, shm_toc *toc);
 static void ParallelWorkerMain(Datum main_arg);
+static void WaitForParallelWorkersToExit(ParallelContext *pcxt);
 
 /*
  * Establish a new parallel context.  This should be done after entering
@@ -383,6 +384,46 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	MemoryContextSwitchTo(oldcontext);
 }
 
+/*
+ * Reinitialize the dynamic shared memory segment for a parallel context such
+ * that we could launch workers for it again.
+ */
+void
+ReinitializeParallelDSM(ParallelContext *pcxt)
+{
+	FixedParallelState *fps;
+	char	   *error_queue_space;
+	int			i;
+
+	if (pcxt->nworkers_launched == 0)
+		return;
+
+	WaitForParallelWorkersToFinish(pcxt);
+	WaitForParallelWorkersToExit(pcxt);
+
+	/* Reset a few bits of fixed parallel state to a clean state. */
+	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
+	fps->workers_attached = 0;
+	fps->last_xlog_end = 0;
+
+	/* Recreate error queues. */
+	error_queue_space =
+		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE);
+	for (i = 0; i < pcxt->nworkers; ++i)
+	{
+		char	   *start;
+		shm_mq	   *mq;
+
+		start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
+		mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
+		shm_mq_set_receiver(mq, MyProc);
+		pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+	}
+
+	/* Reset number of workers launched. */
+	pcxt->nworkers_launched = 0;
+}
+
 /*
  * Launch parallel workers.
  */
@@ -404,52 +445,6 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	/* We might be running in a short-lived memory context. */
 	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
 
-	/*
-	 * This function can be called for a parallel context for which it has
-	 * already been called previously, but only if all of the old workers
-	 * have already exited.  When this case arises, we need to do some extra
-	 * reinitialization.
-	 */
-	if (pcxt->nworkers_launched > 0)
-	{
-		FixedParallelState *fps;
-		char	   *error_queue_space;
-
-		/* Clean out old worker handles. */
-		for (i = 0; i < pcxt->nworkers; ++i)
-		{
-			if (pcxt->worker[i].error_mqh != NULL)
-				elog(ERROR, "previously launched worker still alive");
-			if (pcxt->worker[i].bgwhandle != NULL)
-			{
-				pfree(pcxt->worker[i].bgwhandle);
-				pcxt->worker[i].bgwhandle = NULL;
-			}
-		}
-
-		/* Reset a few bits of fixed parallel state to a clean state. */
-		fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
-		fps->workers_attached = 0;
-		fps->last_xlog_end = 0;
-
-		/* Recreate error queues. */
-		error_queue_space =
-			shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE);
-		for (i = 0; i < pcxt->nworkers; ++i)
-		{
-			char	   *start;
-			shm_mq	   *mq;
-
-			start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
-			mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
-			shm_mq_set_receiver(mq, MyProc);
-			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
-		}
-
-		/* Reset number of workers launched. */
-		pcxt->nworkers_launched = 0;
-	}
-
 	/* Configure a worker. */
 	snprintf(worker.bgw_name, BGW_MAXLEN, "parallel worker for PID %d",
 			 MyProcPid);
@@ -501,7 +496,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 }
 
 /*
- * Wait for all workers to exit.
+ * Wait for all workers to finish computing.
  *
  * Even if the parallel operation seems to have completed successfully, it's
  * important to call this function afterwards.  We must not miss any errors
@@ -552,6 +547,46 @@ WaitForParallelWorkersToFinish(ParallelContext *pcxt)
 	}
 }
 
+/*
+ * Wait for all workers to exit.
+ *
+ * This function ensures that workers have been completely shutdown.  The
+ * difference between WaitForParallelWorkersToFinish and this function is
+ * that former just ensures that last message sent by worker backend is
+ * received by master backend whereas this ensures the complete shutdown.
+ */
+static void
+WaitForParallelWorkersToExit(ParallelContext *pcxt)
+{
+	int			i;
+
+	/* Wait until the workers actually die. */
+	for (i = 0; i < pcxt->nworkers; ++i)
+	{
+		BgwHandleStatus status;
+
+		if (pcxt->worker == NULL || pcxt->worker[i].bgwhandle == NULL)
+			continue;
+
+		status = WaitForBackgroundWorkerShutdown(pcxt->worker[i].bgwhandle);
+
+		/*
+		 * If the postmaster kicked the bucket, we have no chance of cleaning
+		 * up safely -- we won't be able to tell when our workers are actually
+		 * dead.  This doesn't necessitate a PANIC since they will all abort
+		 * eventually, but we can't safely continue this session.
+		 */
+		if (status == BGWH_POSTMASTER_DIED)
+			ereport(FATAL,
+					(errcode(ERRCODE_ADMIN_SHUTDOWN),
+				 errmsg("postmaster exited during a parallel transaction")));
+
+		/* Release memory. */
+		pfree(pcxt->worker[i].bgwhandle);
+		pcxt->worker[i].bgwhandle = NULL;
+	}
+}
+
 /*
  * Destroy a parallel context.
  *
@@ -578,10 +613,10 @@ DestroyParallelContext(ParallelContext *pcxt)
 	{
 		for (i = 0; i < pcxt->nworkers; ++i)
 		{
-			if (pcxt->worker[i].bgwhandle != NULL)
-				TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);
 			if (pcxt->worker[i].error_mqh != NULL)
 			{
+				TerminateBackgroundWorker(pcxt->worker[i].bgwhandle);
+
 				pfree(pcxt->worker[i].error_mqh);
 				pcxt->worker[i].error_mqh = NULL;
 			}
@@ -609,38 +644,14 @@ DestroyParallelContext(ParallelContext *pcxt)
 		pcxt->private_memory = NULL;
 	}
 
-	/* Wait until the workers actually die. */
-	for (i = 0; i < pcxt->nworkers; ++i)
-	{
-		BgwHandleStatus status;
-
-		if (pcxt->worker == NULL || pcxt->worker[i].bgwhandle == NULL)
-			continue;
-
-		/*
-		 * We can't finish transaction commit or abort until all of the
-		 * workers are dead.  This means, in particular, that we can't respond
-		 * to interrupts at this stage.
-		 */
-		HOLD_INTERRUPTS();
-		status = WaitForBackgroundWorkerShutdown(pcxt->worker[i].bgwhandle);
-		RESUME_INTERRUPTS();
-
-		/*
-		 * If the postmaster kicked the bucket, we have no chance of cleaning
-		 * up safely -- we won't be able to tell when our workers are actually
-		 * dead.  This doesn't necessitate a PANIC since they will all abort
-		 * eventually, but we can't safely continue this session.
-		 */
-		if (status == BGWH_POSTMASTER_DIED)
-			ereport(FATAL,
-					(errcode(ERRCODE_ADMIN_SHUTDOWN),
-				 errmsg("postmaster exited during a parallel transaction")));
-
-		/* Release memory. */
-		pfree(pcxt->worker[i].bgwhandle);
-		pcxt->worker[i].bgwhandle = NULL;
-	}
+	/*
+	 * We can't finish transaction commit or abort until all of the
+	 * workers have exited.  This means, in particular, that we can't respond
+	 * to interrupts at this stage.
+	 */
+	HOLD_INTERRUPTS();
+	WaitForParallelWorkersToExit(pcxt);
+	RESUME_INTERRUPTS();
 
 	/* Free the worker array itself. */
 	if (pcxt->worker != NULL)
@@ -799,9 +810,7 @@ HandleParallelMessage(ParallelContext *pcxt, int i, StringInfo msg)
 
 		case 'X':				/* Terminate, indicating clean exit */
 			{
-				pfree(pcxt->worker[i].bgwhandle);
 				pfree(pcxt->worker[i].error_mqh);
-				pcxt->worker[i].bgwhandle = NULL;
 				pcxt->worker[i].error_mqh = NULL;
 				break;
 			}

src/backend/executor/execParallel.c

@@ -84,7 +84,8 @@ static bool ExecParallelEstimate(PlanState *node,
 					 ExecParallelEstimateContext *e);
 static bool ExecParallelInitializeDSM(PlanState *node,
 					 ExecParallelInitializeDSMContext *d);
-static shm_mq_handle **ExecParallelSetupTupleQueues(ParallelContext *pcxt);
+static shm_mq_handle **ExecParallelSetupTupleQueues(ParallelContext *pcxt,
+							 bool reinitialize);
 static bool ExecParallelRetrieveInstrumentation(PlanState *planstate,
 						  SharedExecutorInstrumentation *instrumentation);
 
@@ -217,7 +218,7 @@ ExecParallelInitializeDSM(PlanState *planstate,
  * to the main backend and start the workers.
  */
 static shm_mq_handle **
-ExecParallelSetupTupleQueues(ParallelContext *pcxt)
+ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
 {
 	shm_mq_handle **responseq;
 	char	   *tqueuespace;
@@ -231,9 +232,16 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt)
 	responseq = (shm_mq_handle **)
 		palloc(pcxt->nworkers * sizeof(shm_mq_handle *));
 
-	/* Allocate space from the DSM for the queues themselves. */
-	tqueuespace = shm_toc_allocate(pcxt->toc,
-								 PARALLEL_TUPLE_QUEUE_SIZE * pcxt->nworkers);
+	/*
+	 * If not reinitializing, allocate space from the DSM for the queues;
+	 * otherwise, find the already allocated space.
+	 */
+	if (!reinitialize)
+		tqueuespace =
+			shm_toc_allocate(pcxt->toc,
+							 PARALLEL_TUPLE_QUEUE_SIZE * pcxt->nworkers);
+	else
+		tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE);
 
 	/* Create the queues, and become the receiver for each. */
 	for (i = 0; i < pcxt->nworkers; ++i)
@@ -248,12 +256,23 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt)
 	}
 
 	/* Add array of queues to shm_toc, so others can find it. */
-	shm_toc_insert(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, tqueuespace);
+	if (!reinitialize)
+		shm_toc_insert(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, tqueuespace);
 
 	/* Return array of handles. */
 	return responseq;
 }
 
+/*
+ * Re-initialize the response queues for backend workers to return tuples
+ * to the main backend and start the workers.
+ */
+shm_mq_handle **
+ExecParallelReinitializeTupleQueues(ParallelContext *pcxt)
+{
+	return ExecParallelSetupTupleQueues(pcxt, true);
+}
+
 /*
  * Sets up the required infrastructure for backend workers to perform
  * execution and return results to the main backend.
@@ -363,7 +382,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 	pei->buffer_usage = bufusage_space;
 
 	/* Set up tuple queues. */
-	pei->tqueue = ExecParallelSetupTupleQueues(pcxt);
+	pei->tqueue = ExecParallelSetupTupleQueues(pcxt, false);
 
 	/*
 	 * If instrumentation options were supplied, allocate space for the