Be more user-friendly about unsupported cases for parallel pg_restore.

If we are unable to do a parallel restore because the input file is stdin
or is otherwise unseekable, we should complain and fail immediately, not
after having done some of the restore.  Complaining once per thread isn't
so cool either, and the messages should be worded to make it clear this is
an unsupported case not some weird race-condition bug.  Per complaint from
Lonni Friedman.

Back-patch to 8.4, where parallel restore was introduced.

src/bin/pg_dump/pg_backup_archiver.c

@@ -3265,6 +3265,12 @@ restore_toc_entries_parallel(ArchiveHandle *AH)
 	if (AH->version < K_VERS_1_8)
 		die_horribly(AH, modulename, "parallel restore is not supported with archives made by pre-8.0 pg_dump\n");
 
+	/*
+	 * It's also not gonna work if we can't reopen the input file, so let's
+	 * try that immediately.
+	 */
+	(AH->ReopenPtr) (AH);
+
 	slots = (ParallelSlot *) calloc(sizeof(ParallelSlot), n_slots);
 
 	/* Adjust dependency information */

src/bin/pg_dump/pg_backup_custom.c

@@ -735,10 +735,15 @@ _ReopenArchive(ArchiveHandle *AH)
 
 	if (AH->mode == archModeWrite)
 		die_horribly(AH, modulename, "can only reopen input archives\n");
+
+	/*
+	 * These two cases are user-facing errors since they represent unsupported
+	 * (but not invalid) use-cases.  Word the error messages appropriately.
+	 */
 	if (AH->fSpec == NULL || strcmp(AH->fSpec, "") == 0)
-		die_horribly(AH, modulename, "cannot reopen stdin\n");
+		die_horribly(AH, modulename, "parallel restore from stdin is not supported\n");
 	if (!ctx->hasSeek)
-		die_horribly(AH, modulename, "cannot reopen non-seekable file\n");
+		die_horribly(AH, modulename, "parallel restore from non-seekable file is not supported\n");
 
 	errno = 0;
 	tpos = ftello(AH->FH);