Clean up thread management in parallel pg_dump for Windows.

Since we start the worker threads with _beginthreadex(), we should use
_endthreadex() to terminate them.  We got this right in the normal-exit
code path, but not so much during an error exit from a worker.
In addition, be sure to apply CloseHandle to the thread handle after
each thread exits.

It's not clear that these oversights cause any user-visible problems,
since the pg_dump run is about to terminate anyway.  Still, it's clearly
better to follow Microsoft's API specifications than ignore them.

Also a few cosmetic cleanups in WaitForTerminatingWorkers(), including
being a bit less random about where to cast between uintptr_t and HANDLE,
and being sure to clear the worker identity field for each dead worker
(not that false matches should be possible later, but let's be careful).

Original observation and patch by Armin Schöffmann, cosmetic improvements
by Michael Paquier and me.  (Armin's patch also included closing sockets
in ShutdownWorkersHard(), but that's been dealt with already in commit
df8d2d8c.)  Back-patch to 9.3 where parallel pg_dump was introduced.

Discussion: <zarafa.570306bd.3418.074bf1420d8f2ba2@root.aegaeon.de>

src/bin/pg_dump/parallel.c

@@ -327,7 +327,7 @@ ShutdownWorkersHard(ParallelState *pstate)
 }
 
 /*
- * Wait for the termination of the processes using the OS-specific method.
+ * Wait for all workers to terminate.
  */
 static void
 WaitForTerminatingWorkers(ParallelState *pstate)
@@ -338,39 +338,58 @@ WaitForTerminatingWorkers(ParallelState *pstate)
 		int			j;
 
 #ifndef WIN32
+		/* On non-Windows, use wait() to wait for next worker to end */
 		int			status;
 		pid_t		pid = wait(&status);
 
+		/* Find dead worker's slot, and clear the PID field */
 		for (j = 0; j < pstate->numWorkers; j++)
-			if (pstate->parallelSlot[j].pid == pid)
-				slot = &(pstate->parallelSlot[j]);
-#else
-		uintptr_t	hThread;
-		DWORD		ret;
-		uintptr_t  *lpHandles = pg_malloc(sizeof(HANDLE) * pstate->numWorkers);
+		{
+			slot = &(pstate->parallelSlot[j]);
+			if (slot->pid == pid)
+			{
+				slot->pid = 0;
+				break;
+			}
+		}
+#else							/* WIN32 */
+		/* On Windows, we must use WaitForMultipleObjects() */
+		HANDLE	   *lpHandles = pg_malloc(sizeof(HANDLE) * pstate->numWorkers);
 		int			nrun = 0;
+		DWORD		ret;
+		uintptr_t	hThread;
 
 		for (j = 0; j < pstate->numWorkers; j++)
+		{
 			if (pstate->parallelSlot[j].workerStatus != WRKR_TERMINATED)
 			{
-				lpHandles[nrun] = pstate->parallelSlot[j].hThread;
+				lpHandles[nrun] = (HANDLE) pstate->parallelSlot[j].hThread;
 				nrun++;
 			}
-		ret = WaitForMultipleObjects(nrun, (HANDLE *) lpHandles, false, INFINITE);
+		}
+		ret = WaitForMultipleObjects(nrun, lpHandles, false, INFINITE);
 		Assert(ret != WAIT_FAILED);
-		hThread = lpHandles[ret - WAIT_OBJECT_0];
+		hThread = (uintptr_t) lpHandles[ret - WAIT_OBJECT_0];
+		free(lpHandles);
 
+		/* Find dead worker's slot, and clear the hThread field */
 		for (j = 0; j < pstate->numWorkers; j++)
-			if (pstate->parallelSlot[j].hThread == hThread)
-				slot = &(pstate->parallelSlot[j]);
-
-		free(lpHandles);
-#endif
-		Assert(slot);
+		{
+			slot = &(pstate->parallelSlot[j]);
+			if (slot->hThread == hThread)
+			{
+				/* For cleanliness, close handles for dead threads */
+				CloseHandle((HANDLE) slot->hThread);
+				slot->hThread = (uintptr_t) INVALID_HANDLE_VALUE;
+				break;
+			}
+		}
+#endif   /* WIN32 */
 
+		/* On all platforms, update workerStatus as well */
+		Assert(j < pstate->numWorkers);
 		slot->workerStatus = WRKR_TERMINATED;
 	}
-	Assert(HasEveryWorkerTerminated(pstate));
 }
 
 #ifndef WIN32

src/bin/pg_dump/pg_backup_utils.c

@@ -149,7 +149,7 @@ exit_nicely(int code)
 
 #ifdef WIN32
 	if (parallel_init_done && GetCurrentThreadId() != mainThreadId)
-		ExitThread(code);
+		_endthreadex(code);
 #endif
 
 	exit(code);