Fix fuzzy thinking in ReinitializeParallelDSM().

The fact that no workers were successfully launched in the previous
iteration does not excuse us from setting up properly to try again.
This appears to explain crashes I saw in parallel regression testing
due to error_mqh being NULL when it shouldn't be.

Minor other cosmetic fixes too.

src/backend/access/transam/parallel.c

@@ -191,8 +191,8 @@ CreateParallelContextForExternalFunction(char *library_name,
 
 /*
  * Establish the dynamic shared memory segment for a parallel context and
- * copied state and other bookkeeping information that will need by parallel
- * workers into it.
+ * copy state and other bookkeeping information that will be needed by
+ * parallel workers into it.
  */
 void
 InitializeParallelDSM(ParallelContext *pcxt)
@@ -271,7 +271,7 @@ InitializeParallelDSM(ParallelContext *pcxt)
 	 * parallelism than to fail outright.
 	 */
 	segsize = shm_toc_estimate(&pcxt->estimator);
-	if (pcxt->nworkers != 0)
+	if (pcxt->nworkers > 0)
 		pcxt->seg = dsm_create(segsize, DSM_CREATE_NULL_IF_MAXSEGMENTS);
 	if (pcxt->seg != NULL)
 		pcxt->toc = shm_toc_create(PARALLEL_MAGIC,
@@ -397,11 +397,13 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	char	   *error_queue_space;
 	int			i;
 
-	if (pcxt->nworkers_launched == 0)
-		return;
-
-	WaitForParallelWorkersToFinish(pcxt);
-	WaitForParallelWorkersToExit(pcxt);
+	/* Wait for any old workers to exit. */
+	if (pcxt->nworkers_launched > 0)
+	{
+		WaitForParallelWorkersToFinish(pcxt);
+		WaitForParallelWorkersToExit(pcxt);
+		pcxt->nworkers_launched = 0;
+	}
 
 	/* Reset a few bits of fixed parallel state to a clean state. */
 	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED);
@@ -420,9 +422,6 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 		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;
 }
 
 /*
@@ -493,6 +492,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 			 */
 			any_registrations_failed = true;
 			pcxt->worker[i].bgwhandle = NULL;
+			pfree(pcxt->worker[i].error_mqh);
 			pcxt->worker[i].error_mqh = NULL;
 		}
 	}