Revert "Fix bug in checking of IDENTIFY_SYSTEM result."

This reverts commit 083d29c.

The commit changed the code so that it causes an errors when
IDENTIFY_SYSTEM returns three columns. But which prevents us
from using the replication-related utilities against the server
with older version. This is not what we want. For that
compatibility, we allow the utilities to receive three columns
as the result of IDENTIFY_SYSTEM eventhough it actually returns
four columns in 9.4 or later.

Pointed out by Andres Freund.

Author: MasaoFujii

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -131,7 +131,7 @@ libpqrcv_identify_system(TimeLineID *primary_tli)
 						"the primary server: %s",
 						PQerrorMessage(streamConn))));
 	}
-	if (PQnfields(res) < 4 || PQntuples(res) != 1)
+	if (PQnfields(res) < 3 || PQntuples(res) != 1)
 	{
 		int			ntuples = PQntuples(res);
 		int			nfields = PQnfields(res);
@@ -140,7 +140,7 @@ libpqrcv_identify_system(TimeLineID *primary_tli)
 		ereport(ERROR,
 				(errmsg("invalid response from primary server"),
 				 errdetail("Could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields.",
-						   ntuples, nfields, 4, 1)));
+						   ntuples, nfields, 3, 1)));
 	}
 	primary_sysid = PQgetvalue(res, 0, 0);
 	*primary_tli = pg_atoi(PQgetvalue(res, 0, 1), 4, 0);

src/bin/pg_basebackup/pg_basebackup.c

@@ -1644,11 +1644,11 @@ BaseBackup(void)
 				progname, "IDENTIFY_SYSTEM", PQerrorMessage(conn));
 		disconnect_and_exit(1);
 	}
-	if (PQntuples(res) != 1 || PQnfields(res) < 4)
+	if (PQntuples(res) != 1 || PQnfields(res) < 3)
 	{
 		fprintf(stderr,
 				_("%s: could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields\n"),
-				progname, PQntuples(res), PQnfields(res), 1, 4);
+				progname, PQntuples(res), PQnfields(res), 1, 3);
 		disconnect_and_exit(1);
 	}
 	sysidentifier = pg_strdup(PQgetvalue(res, 0, 0));

src/bin/pg_basebackup/pg_receivexlog.c

@@ -290,11 +290,11 @@ StreamLog(void)
 				progname, "IDENTIFY_SYSTEM", PQerrorMessage(conn));
 		disconnect_and_exit(1);
 	}
-	if (PQntuples(res) != 1 || PQnfields(res) < 4)
+	if (PQntuples(res) != 1 || PQnfields(res) < 3)
 	{
 		fprintf(stderr,
 				_("%s: could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields\n"),
-				progname, PQntuples(res), PQnfields(res), 1, 4);
+				progname, PQntuples(res), PQnfields(res), 1, 3);
 		disconnect_and_exit(1);
 	}
 	servertli = atoi(PQgetvalue(res, 0, 1));

src/bin/pg_basebackup/receivelog.c

@@ -499,11 +499,11 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 			PQclear(res);
 			return false;
 		}
-		if (PQntuples(res) != 1 || PQnfields(res) < 4)
+		if (PQntuples(res) != 1 || PQnfields(res) < 3)
 		{
 			fprintf(stderr,
 					_("%s: could not identify system: got %d rows and %d fields, expected %d rows and %d or more fields\n"),
-					progname, PQntuples(res), PQnfields(res), 1, 4);
+					progname, PQntuples(res), PQnfields(res), 1, 3);
 			PQclear(res);
 			return false;
 		}