Ensure BackgroundWorker struct contents are well-defined.

Coverity complained because bgw.bgw_extra wasn't being filled in by
ApplyLauncherRegister().  The most future-proof fix is to memset the
whole BackgroundWorker struct to zeroes.  While at it, let's apply the
same coding rule to other places that set up BackgroundWorker structs;
four out of five had the same or related issues.

src/backend/access/transam/parallel.c

@@ -435,6 +435,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
 
 	/* Configure a worker. */
+	memset(&worker, 0, sizeof(worker));
 	snprintf(worker.bgw_name, BGW_MAXLEN, "parallel worker for PID %d",
 			 MyProcPid);
 	worker.bgw_flags =
@@ -446,7 +447,6 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	sprintf(worker.bgw_function_name, "ParallelWorkerMain");
 	worker.bgw_main_arg = UInt32GetDatum(dsm_segment_handle(pcxt->seg));
 	worker.bgw_notify_pid = MyProcPid;
-	memset(&worker.bgw_extra, 0, BGW_EXTRALEN);
 
 	/*
 	 * Start workers.

src/backend/replication/logical/launcher.c

@@ -291,6 +291,7 @@ logicalrep_worker_launch(Oid dbid, Oid subid, const char *subname, Oid userid,
 	LWLockRelease(LogicalRepWorkerLock);
 
 	/* Register the new dynamic worker. */
+	memset(&bgw, 0, sizeof(bgw));
 	bgw.bgw_flags =	BGWORKER_SHMEM_ACCESS |
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;
@@ -560,6 +561,10 @@ ApplyLauncherShmemSize(void)
 	return size;
 }
 
+/*
+ * ApplyLauncherRegister
+ *		Register a background worker running the logical replication launcher.
+ */
 void
 ApplyLauncherRegister(void)
 {
@@ -568,6 +573,7 @@ ApplyLauncherRegister(void)
 	if (max_logical_replication_workers == 0)
 		return;
 
+	memset(&bgw, 0, sizeof(bgw));
 	bgw.bgw_flags =	BGWORKER_SHMEM_ACCESS |
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;

src/test/modules/test_shm_mq/setup.c

@@ -213,6 +213,7 @@ setup_background_workers(int nworkers, dsm_segment *seg)
 				  PointerGetDatum(wstate));
 
 	/* Configure a worker. */
+	memset(&worker, 0, sizeof(worker));
 	worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
 	worker.bgw_start_time = BgWorkerStart_ConsistentState;
 	worker.bgw_restart_time = BGW_NEVER_RESTART;

src/test/modules/worker_spi/worker_spi.c

@@ -343,6 +343,7 @@ _PG_init(void)
 							NULL);
 
 	/* set up common data for all our workers */
+	memset(&worker, 0, sizeof(worker));
 	worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
@@ -375,6 +376,7 @@ worker_spi_launch(PG_FUNCTION_ARGS)
 	BgwHandleStatus status;
 	pid_t		pid;
 
+	memset(&worker, 0, sizeof(worker));
 	worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
 		BGWORKER_BACKEND_DATABASE_CONNECTION;
 	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;