Rework the way multixact truncations work.

The fact that multixact truncations are not WAL logged has caused a fair
share of problems. Amongst others it requires to do computations during
recovery while the database is not in a consistent state, delaying
truncations till checkpoints, and handling members being truncated, but
offset not.

We tried to put bandaids on lots of these issues over the last years,
but it seems time to change course. Thus this patch introduces WAL
logging for multixact truncations.

This allows:
1) to perform the truncation directly during VACUUM, instead of delaying it
   to the checkpoint.
2) to avoid looking at the offsets SLRU for truncation during recovery,
   we can just use the master's values.
3) simplify a fair amount of logic to keep in memory limits straight,
   this has gotten much easier

During the course of fixing this a bunch of additional bugs had to be
fixed:
1) Data was not purged from memory the member's SLRU before deleting
   segments. This happened to be hard or impossible to hit due to the
   interlock between checkpoints and truncation.
2) find_multixact_start() relied on SimpleLruDoesPhysicalPageExist - but
   that doesn't work for offsets that haven't yet been flushed to
   disk. Add code to flush the SLRUs to fix. Not pretty, but it feels
   slightly safer to only make decisions based on actual on-disk state.
3) find_multixact_start() could be called concurrently with a truncation
   and thus fail. Via SetOffsetVacuumLimit() that could lead to a round
   of emergency vacuuming. The problem remains in
   pg_get_multixact_members(), but that's quite harmless.

For now this is going to only get applied to 9.5+, leaving the issues in
the older branches in place. It is quite possible that we need to
backpatch at a later point though.

For the case this gets backpatched we need to handle that an updated
standby may be replaying WAL from a not-yet upgraded primary. We have to
recognize that situation and use "old style" truncation (i.e. looking at
the SLRUs) during WAL replay. In contrast to before, this now happens in
the startup process, when replaying a checkpoint record, instead of the
checkpointer. Doing truncation in the restartpoint is incorrect, they
can happen much later than the original checkpoint, thereby leading to
wraparound.  To avoid "multixact_redo: unknown op code 48" errors
standbys would have to be upgraded before primaries.

A later patch will bump the WAL page magic, and remove the legacy
truncation codepaths. Legacy truncation support is just included to make
a possible future backpatch easier.

Discussion: 20150621192409.GA4797@alap3.anarazel.de
Reviewed-By: Robert Haas, Alvaro Herrera, Thomas Munro
Backpatch: 9.5 for now

1 files changed, 17 insertions, 2 deletions

diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index f1448fe0635..9f9df9cda04 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -71,6 +71,7 @@ typedef struct MultiXactMember
 #define XLOG_MULTIXACT_ZERO_OFF_PAGE	0x00
 #define XLOG_MULTIXACT_ZERO_MEM_PAGE	0x10
 #define XLOG_MULTIXACT_CREATE_ID		0x20
+#define XLOG_MULTIXACT_TRUNCATE_ID		0x30
 
 typedef struct xl_multixact_create
 {
@@ -82,6 +83,21 @@ typedef struct xl_multixact_create
 
 #define SizeOfMultiXactCreate (offsetof(xl_multixact_create, members))
 
+typedef struct xl_multixact_truncate
+{
+	Oid			oldestMultiDB;
+
+	/* to-be-truncated range of multixact offsets */
+	MultiXactId startTruncOff;	/* just for completeness' sake */
+	MultiXactId endTruncOff;
+
+	/* to-be-truncated range of multixact members */
+	MultiXactOffset startTruncMemb;
+	MultiXactOffset endTruncMemb;
+} xl_multixact_truncate;
+
+#define SizeOfMultiXactTruncate (sizeof(xl_multixact_truncate))
+
 
 extern MultiXactId MultiXactIdCreate(TransactionId xid1,
 				  MultiXactStatus status1, TransactionId xid2,
@@ -119,13 +135,12 @@ extern void MultiXactGetCheckptMulti(bool is_shutdown,
 						 Oid *oldestMultiDB);
 extern void CheckPointMultiXact(void);
 extern MultiXactId GetOldestMultiXactId(void);
-extern void TruncateMultiXact(void);
+extern void TruncateMultiXact(MultiXactId oldestMulti, Oid oldestMultiDB, bool in_recovery);
 extern void MultiXactSetNextMXact(MultiXactId nextMulti,
 					  MultiXactOffset nextMultiOffset);
 extern void MultiXactAdvanceNextMXact(MultiXactId minMulti,
 						  MultiXactOffset minMultiOffset);
 extern void MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB);
-extern void MultiXactSetSafeTruncate(MultiXactId safeTruncateMulti);
 extern int	MultiXactMemberFreezeThreshold(void);
 
 extern void multixact_twophase_recover(TransactionId xid, uint16 info,