Improve some check logic in pg_receivewal

The following things are improved:
- Fetch the system identifier from the source server before any
WAL streaming loop.  This triggers extra checks to make sure that
pg_receivewal is still connected to a server with the same system ID
with a correct timeline.
- Switch umask() (for file creation mode mask) and RetrieveWalSegSize()
(to fetch the size of WAL segments) a bit later before the initial
stream attempt.  If the connection was done with a database,
pg_receivewal would fail but those commands were still executed, which
was a waste.  The slot creation and drop are now done before retrieving
the segment size.

Author: Bharath Rupireddy
Reviewed-by: Ronan Dunklau, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACX00YYeyBfoi55Cy=NrP-FcfMgiYYx1qRUEib3yjCVoaA@mail.gmail.com

src/bin/pg_basebackup/pg_receivewal.c

@@ -372,6 +372,7 @@ StreamLog(void)
 	XLogRecPtr	serverpos;
 	TimeLineID	servertli;
 	StreamCtl	stream;
+	char	   *sysidentifier;
 
 	MemSet(&stream, 0, sizeof(stream));
 
@@ -399,7 +400,7 @@ StreamLog(void)
 	 * at the same time, necessary if not valid data can be found in the
 	 * existing output directory.
 	 */
-	if (!RunIdentifySystem(conn, NULL, &servertli, &serverpos, NULL))
+	if (!RunIdentifySystem(conn, &sysidentifier, &servertli, &serverpos, NULL))
 		exit(1);
 
 	/*
@@ -435,6 +436,7 @@ StreamLog(void)
 												stream.do_sync);
 	stream.partial_suffix = ".partial";
 	stream.replication_slot = replication_slot;
+	stream.sysidentifier = sysidentifier;
 
 	ReceiveXlogStream(conn, &stream);
 
@@ -449,6 +451,7 @@ StreamLog(void)
 
 	FreeWalDirectoryMethod();
 	pg_free(stream.walmethod);
+	pg_free(stream.sysidentifier);
 }
 
 /*
@@ -687,20 +690,6 @@ main(int argc, char **argv)
 	if (!RunIdentifySystem(conn, NULL, NULL, NULL, &db_name))
 		exit(1);
 
-	/*
-	 * Set umask so that directories/files are created with the same
-	 * permissions as directories/files in the source data directory.
-	 *
-	 * pg_mode_mask is set to owner-only by default and then updated in
-	 * GetConnection() where we get the mode from the server-side with
-	 * RetrieveDataDirCreatePerm() and then call SetDataDirectoryCreatePerm().
-	 */
-	umask(pg_mode_mask);
-
-	/* determine remote server's xlog segment size */
-	if (!RetrieveWalSegSize(conn))
-		exit(1);
-
 	/*
 	 * Check that there is a database associated with connection, none should
 	 * be defined in this context.
@@ -712,6 +701,16 @@ main(int argc, char **argv)
 		exit(1);
 	}
 
+	/*
+	 * Set umask so that directories/files are created with the same
+	 * permissions as directories/files in the source data directory.
+	 *
+	 * pg_mode_mask is set to owner-only by default and then updated in
+	 * GetConnection() where we get the mode from the server-side with
+	 * RetrieveDataDirCreatePerm() and then call SetDataDirectoryCreatePerm().
+	 */
+	umask(pg_mode_mask);
+
 	/*
 	 * Drop a replication slot.
 	 */
@@ -737,6 +736,10 @@ main(int argc, char **argv)
 		exit(0);
 	}
 
+	/* determine remote server's xlog segment size */
+	if (!RetrieveWalSegSize(conn))
+		exit(1);
+
 	/*
 	 * Don't close the connection here so that subsequent StreamLog() can
 	 * reuse it.