Fix pg_basebackup for keepalive messages

Teach pg_basebackup in streaming mode to deal with keepalive messages.
Also change the order of checks to complain at the message rather than
block size when a new message is introduced.

In passing, switch to using sizeof() instead of hardcoded sizes for
WAL protocol structs.

src/bin/pg_basebackup/receivelog.c

@@ -33,8 +33,9 @@
 #include <unistd.h>
 
 
-/* Size of the streaming replication protocol header */
-#define STREAMING_HEADER_SIZE (1+8+8+8)
+/* Size of the streaming replication protocol headers */
+#define STREAMING_HEADER_SIZE (1+sizeof(WalDataMessageHeader))
+#define STREAMING_KEEPALIVE_SIZE (1+sizeof(PrimaryKeepaliveMessage))
 
 const XLogRecPtr InvalidXLogRecPtr = {0, 0};
 
@@ -374,18 +375,33 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline, char *sysi
 					progname, PQerrorMessage(conn));
 			return false;
 		}
-		if (r < STREAMING_HEADER_SIZE + 1)
+		if (copybuf[0] == 'k')
 		{
-			fprintf(stderr, _("%s: streaming header too small: %i\n"),
-					progname, r);
-			return false;
+			/*
+			 * keepalive message, sent in 9.2 and newer. We just ignore
+			 * this message completely, but need to forward past it
+			 * in our reading.
+			 */
+			if (r != STREAMING_KEEPALIVE_SIZE)
+			{
+				fprintf(stderr, _("%s: keepalive message is incorrect size: %i\n"),
+						progname, r);
+				return false;
+			}
+			continue;
 		}
-		if (copybuf[0] != 'w')
+		else if (copybuf[0] != 'w')
 		{
 			fprintf(stderr, _("%s: unrecognized streaming header: \"%c\"\n"),
 					progname, copybuf[0]);
 			return false;
 		}
+		if (r < STREAMING_HEADER_SIZE + 1)
+		{
+			fprintf(stderr, _("%s: streaming header too small: %i\n"),
+					progname, r);
+			return false;
+		}
 
 		/* Extract WAL location for this block */
 		memcpy(&blockpos, copybuf + 1, 8);