Report WAL flush, not insert, position in replication IDENTIFY_SYSTEM

hlinnaka · hlinnaka · commit 2af568c6b502 · 2015-02-06T11:32:37.000+02:00
When beginning streaming replication, the client usually issues the
IDENTIFY_SYSTEM command, which used to return the current WAL insert
position. That's not suitable for the intended purpose of that field,
however. pg_receivexlog uses it to start replication from the reported
point, but if it hasn't been flushed to disk yet, it will fail. Change
IDENTIFY_SYSTEM to report the flush position instead.

Backpatch to 9.1 and above. 9.0 doesn't report any WAL position.
diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
@@ -1338,7 +1338,7 @@ The commands accepted in walsender mode are:
       </term>
       <listitem>
       <para>
-       Current xlog write location. Useful to get a known location in the
+       Current xlog flush location. Useful to get a known location in the
        transaction log where streaming can start.
       </para>
       </listitem>
diff --git a/src/backend/replication/walsender.c b/src/backend/replication/walsender.c
@@ -298,7 +298,7 @@ IdentifySystem(void)
 			 GetSystemIdentifier());
 	snprintf(tli, sizeof(tli), "%u", ThisTimeLineID);
 
-	logptr = am_cascading_walsender ? GetStandbyFlushRecPtr(NULL) : GetInsertRecPtr();
+	logptr = am_cascading_walsender ? GetStandbyFlushRecPtr(NULL) : GetFlushRecPtr();
 
 	snprintf(xpos, sizeof(xpos), "%X/%X",
 			 logptr.xlogid, logptr.xrecoff);
