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

Since 6ef2eba3 ("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

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;

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)
 		{