In Windows pg_dump, ensure idle workers will shut down during error exit.

The Windows coding of ShutdownWorkersHard() thought that setting termEvent
was sufficient to make workers exit after an error.  But that only helps
if a worker is busy and passes through checkAborting().  An idle worker
will just sit, resulting in pg_dump failing to exit until the user gives up
and hits control-C.  We should close the write end of the command pipe
so that idle workers will see socket EOF and exit, as the Unix coding was
already doing.

Back-patch to 9.3 where parallel pg_dump was introduced.

Kyotaro Horiguchi

src/bin/pg_dump/parallel.c

@@ -299,28 +299,26 @@ checkAborting(ArchiveHandle *AH)
 }
 
 /*
- * Shut down any remaining workers, this has an implicit do_wait == true.
+ * Shut down any remaining workers, waiting for them to finish.
- *
- * The fastest way we can make the workers terminate gracefully is when
- * they are listening for new commands and we just tell them to terminate.
  */
 static void
 ShutdownWorkersHard(ParallelState *pstate)
 {
-#ifndef WIN32
 	int			i;
 
 	/*
-	 * Close our write end of the sockets so that the workers know they can
+	 * Close our write end of the sockets so that any workers waiting for
-	 * exit.
+	 * commands know they can exit.
 	 */
 	for (i = 0; i < pstate->numWorkers; i++)
 		closesocket(pstate->parallelSlot[i].pipeWrite);
 
+#ifndef WIN32
+	/* On non-Windows, send SIGTERM to abort commands-in-progress. */
 	for (i = 0; i < pstate->numWorkers; i++)
 		kill(pstate->parallelSlot[i].pid, SIGTERM);
 #else
-	/* The workers monitor this event via checkAborting(). */
+	/* Non-idle workers monitor this event via checkAborting(). */
 	SetEvent(termEvent);
 #endif