Use mul_size when multiplying by the number of parallel workers.

That way, if the result overflows size_t, you'll get an error instead
of undefined behavior, which seems like a plus.  This also has the
effect of casting the number of workers from int to Size, which is
better because it's harder to overflow int than size_t.

Dilip Kumar reported this issue and provided a patch upon which this
patch is based, but his version did use mul_size.

src/backend/access/transam/parallel.c

@@ -241,7 +241,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
 						 PARALLEL_ERROR_QUEUE_SIZE,
 						 "parallel error queue size not buffer-aligned");
 		shm_toc_estimate_chunk(&pcxt->estimator,
-							   PARALLEL_ERROR_QUEUE_SIZE * pcxt->nworkers);
+							   mul_size(PARALLEL_ERROR_QUEUE_SIZE,
+										pcxt->nworkers));
 		shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 		/* Estimate how much we'll need for extension entrypoint info. */
@@ -347,7 +348,8 @@ InitializeParallelDSM(ParallelContext *pcxt)
 		 */
 		error_queue_space =
 			shm_toc_allocate(pcxt->toc,
-							 PARALLEL_ERROR_QUEUE_SIZE * pcxt->nworkers);
+							 mul_size(PARALLEL_ERROR_QUEUE_SIZE,
+									  pcxt->nworkers));
 		for (i = 0; i < pcxt->nworkers; ++i)
 		{
 			char	   *start;

src/backend/executor/execParallel.c

@@ -287,7 +287,8 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
 	if (!reinitialize)
 		tqueuespace =
 			shm_toc_allocate(pcxt->toc,
-							 PARALLEL_TUPLE_QUEUE_SIZE * pcxt->nworkers);
+							 mul_size(PARALLEL_TUPLE_QUEUE_SIZE,
+									  pcxt->nworkers));
 	else
 		tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE);
 
@@ -296,7 +297,8 @@ ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
 	{
 		shm_mq	   *mq;
 
-		mq = shm_mq_create(tqueuespace + i * PARALLEL_TUPLE_QUEUE_SIZE,
+		mq = shm_mq_create(tqueuespace +
+						   ((Size) i) * PARALLEL_TUPLE_QUEUE_SIZE,
 						   (Size) PARALLEL_TUPLE_QUEUE_SIZE);
 
 		shm_mq_set_receiver(mq, MyProc);
@@ -380,12 +382,12 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 	 * looking at pgBufferUsage, so do it unconditionally.
 	 */
 	shm_toc_estimate_chunk(&pcxt->estimator,
-						   sizeof(BufferUsage) * pcxt->nworkers);
+						   mul_size(sizeof(BufferUsage), pcxt->nworkers));
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/* Estimate space for tuple queues. */
 	shm_toc_estimate_chunk(&pcxt->estimator,
-						   PARALLEL_TUPLE_QUEUE_SIZE * pcxt->nworkers);
+						   mul_size(PARALLEL_TUPLE_QUEUE_SIZE, pcxt->nworkers));
 	shm_toc_estimate_keys(&pcxt->estimator, 1);
 
 	/*
@@ -404,7 +406,9 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 			sizeof(int) * e.nnodes;
 		instrumentation_len = MAXALIGN(instrumentation_len);
 		instrument_offset = instrumentation_len;
-		instrumentation_len += sizeof(Instrumentation) * e.nnodes * nworkers;
+		instrumentation_len +=
+			mul_size(sizeof(Instrumentation),
+					 mul_size(e.nnodes, nworkers));
 		shm_toc_estimate_chunk(&pcxt->estimator, instrumentation_len);
 		shm_toc_estimate_keys(&pcxt->estimator, 1);
 	}
@@ -432,7 +436,7 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 
 	/* Allocate space for each worker's BufferUsage; no need to initialize. */
 	bufusage_space = shm_toc_allocate(pcxt->toc,
-									  sizeof(BufferUsage) * pcxt->nworkers);
+							  mul_size(sizeof(BufferUsage), pcxt->nworkers));
 	shm_toc_insert(pcxt->toc, PARALLEL_KEY_BUFFER_USAGE, bufusage_space);
 	pei->buffer_usage = bufusage_space;
 
@@ -511,7 +515,7 @@ ExecParallelRetrieveInstrumentation(PlanState *planstate,
 		InstrAggNode(planstate->instrument, &instrument[n]);
 
 	/* Also store the per-worker detail. */
-	ibytes = instrumentation->num_workers * sizeof(Instrumentation);
+	ibytes = mul_size(instrumentation->num_workers, sizeof(Instrumentation));
 	planstate->worker_instrument =
 		palloc(ibytes + offsetof(WorkerInstrumentation, instrument));
 	planstate->worker_instrument->num_workers = instrumentation->num_workers;