Fix off-by-one possibly leading to skipped XLOG_RUNNING_XACTS records.

anarazel · anarazel · commit e6c44eef55cd · 2017-05-06T16:55:07.000-07:00
Since 6ef2eba ("Skip checkpoints, archiving on idle systems."), GetLastImportantRecPtr() is used to avoid performing superfluous checkpoints, xlog switches, running-xact records when the system is idle. Unfortunately the check concerning running-xact records had a off-by-one error, leading to such records being potentially skipped when only a single record has been inserted since the last running-xact record. An alternative approach would have been to change GetLastImportantRecPtr()'s definition to point to the end of records, but that would make the checkpoint code more complicated. Author: Andres Freund Discussion: https://postgr.es/m/20170505012447.wsrympaxnfis6ojt@alap3.anarazel.de Backpatch: no, code only present in master
diff --git a/src/backend/postmaster/bgwriter.c b/src/backend/postmaster/bgwriter.c
@@ -325,10 +325,13 @@ BackgroundWriterMain(void)
 
 			/*
 			 * Only log if enough time has passed and interesting records have
-			 * been inserted since the last snapshot.
+			 * been inserted since the last snapshot.  Have to compare with <=
+			 * instead of < because GetLastImportantRecPtr() points at the
+			 * start of a record, whereas last_snapshot_lsn points just past
+			 * the end of the record.
 			 */
 			if (now >= timeout &&
-				last_snapshot_lsn < GetLastImportantRecPtr())
+				last_snapshot_lsn <= GetLastImportantRecPtr())
 			{
 				last_snapshot_lsn = LogStandbySnapshot();
 				last_snapshot_ts = now;
diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c
@@ -611,7 +611,8 @@ CheckArchiveTimeout(void)
 	{
 		/*
 		 * Switch segment only when "important" WAL has been logged since the
-		 * last segment switch.
+		 * last segment switch (last_switch_lsn points to end of segment
+		 * switch occurred in).
 		 */
 		if (GetLastImportantRecPtr() > last_switch_lsn)
 		{

Original file line number	Diff line number	Diff line change
`@@ -611,7 +611,8 @@ CheckArchiveTimeout(void)`
`611`	`611`	`{`
`612`	`612`	`/*`
`613`	`613`	`* Switch segment only when "important" WAL has been logged since the`
`614`		`- * last segment switch.`
	`614`	`+ * last segment switch (last_switch_lsn points to end of segment`
	`615`	`+ * switch occurred in).`
`615`	`616`	`*/`
`616`	`617`	`if (GetLastImportantRecPtr() > last_switch_lsn)`
`617`	`618`	`{`