Improve concurrency of foreign key locking

This patch introduces two additional lock modes for tuples: "SELECT FOR
KEY SHARE" and "SELECT FOR NO KEY UPDATE".  These don't block each
other, in contrast with already existing "SELECT FOR SHARE" and "SELECT
FOR UPDATE".  UPDATE commands that do not modify the values stored in
the columns that are part of the key of the tuple now grab a SELECT FOR
NO KEY UPDATE lock on the tuple, allowing them to proceed concurrently
with tuple locks of the FOR KEY SHARE variety.

Foreign key triggers now use FOR KEY SHARE instead of FOR SHARE; this
means the concurrency improvement applies to them, which is the whole
point of this patch.

The added tuple lock semantics require some rejiggering of the multixact
module, so that the locking level that each transaction is holding can
be stored alongside its Xid.  Also, multixacts now need to persist
across server restarts and crashes, because they can now represent not
only tuple locks, but also tuple updates.  This means we need more
careful tracking of lifetime of pg_multixact SLRU files; since they now
persist longer, we require more infrastructure to figure out when they
can be removed.  pg_upgrade also needs to be careful to copy
pg_multixact files over from the old server to the new, or at least part
of multixact.c state, depending on the versions of the old and new
servers.

Tuple time qualification rules (HeapTupleSatisfies routines) need to be
careful not to consider tuples with the "is multi" infomask bit set as
being only locked; they might need to look up MultiXact values (i.e.
possibly do pg_multixact I/O) to find out the Xid that updated a tuple,
whereas they previously were assured to only use information readily
available from the tuple header.  This is considered acceptable, because
the extra I/O would involve cases that would previously cause some
commands to block waiting for concurrent transactions to finish.

Another important change is the fact that locking tuples that have
previously been updated causes the future versions to be marked as
locked, too; this is essential for correctness of foreign key checks.
This causes additional WAL-logging, also (there was previously a single
WAL record for a locked tuple; now there are as many as updated copies
of the tuple there exist.)

With all this in place, contention related to tuples being checked by
foreign key rules should be much reduced.

As a bonus, the old behavior that a subtransaction grabbing a stronger
tuple lock than the parent (sub)transaction held on a given tuple and
later aborting caused the weaker lock to be lost, has been fixed.

Many new spec files were added for isolation tester framework, to ensure
overall behavior is sane.  There's probably room for several more tests.

There were several reviewers of this patch; in particular, Noah Misch
and Andres Freund spent considerable time in it.  Original idea for the
patch came from Simon Riggs, after a problem report by Joel Jacobson.
Most code is from me, with contributions from Marti Raudsepp, Alexander
Shulgin, Noah Misch and Andres Freund.

This patch was discussed in several pgsql-hackers threads; the most
important start at the following message-ids:
	AANLkTimo9XVcEzfiBR-ut3KVNDkjm2Vxh+t8kAmWjPuv@mail.gmail.com
	1290721684-sup-3951@alvh.no-ip.org
	1294953201-sup-2099@alvh.no-ip.org
	1320343602-sup-2290@alvh.no-ip.org
	1339690386-sup-8927@alvh.no-ip.org
	4FE5FF020200002500048A3D@gw.wicourts.gov
	4FEAB90A0200002500048B7D@gw.wicourts.gov

src/include/access/heapam.h

@@ -30,12 +30,23 @@
 
 typedef struct BulkInsertStateData *BulkInsertState;
 
-typedef enum
+/*
+ * Possible lock modes for a tuple.
+ */
+typedef enum LockTupleMode
 {
-	LockTupleShared,
+	/* SELECT FOR KEY SHARE */
+	LockTupleKeyShare,
+	/* SELECT FOR SHARE */
+	LockTupleShare,
+	/* SELECT FOR NO KEY UPDATE, and UPDATEs that don't modify key columns */
+	LockTupleNoKeyExclusive,
+	/* SELECT FOR UPDATE, UPDATEs that modify key columns, and DELETE */
 	LockTupleExclusive
 } LockTupleMode;
 
+#define MaxLockTupleMode	LockTupleExclusive
+
 /*
  * When heap_update, heap_delete, or heap_lock_tuple fail because the target
  * tuple is already outdated, they fill in this struct to provide information
@@ -129,14 +140,16 @@ extern HTSU_Result heap_delete(Relation relation, ItemPointer tid,
 extern HTSU_Result heap_update(Relation relation, ItemPointer otid,
 			HeapTuple newtup,
 			CommandId cid, Snapshot crosscheck, bool wait,
-			HeapUpdateFailureData *hufd);
+			HeapUpdateFailureData *hufd, LockTupleMode *lockmode);
 extern HTSU_Result heap_lock_tuple(Relation relation, HeapTuple tuple,
 				CommandId cid, LockTupleMode mode, bool nowait,
+				bool follow_update,
 				Buffer *buffer, HeapUpdateFailureData *hufd);
 extern void heap_inplace_update(Relation relation, HeapTuple tuple);
-extern bool heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid);
+extern bool heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
+				  TransactionId cutoff_multi);
 extern bool heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
-						Buffer buf);
+						MultiXactId cutoff_multi, Buffer buf);
 
 extern Oid	simple_heap_insert(Relation relation, HeapTuple tup);
 extern void simple_heap_delete(Relation relation, ItemPointer tid);

src/include/access/heapam_xlog.h

@@ -54,6 +54,7 @@
 #define XLOG_HEAP2_CLEANUP_INFO 0x30
 #define XLOG_HEAP2_VISIBLE		0x40
 #define XLOG_HEAP2_MULTI_INSERT 0x50
+#define XLOG_HEAP2_LOCK_UPDATED 0x60
 
 /*
  * All what we need to find changed tuple
@@ -75,6 +76,8 @@ typedef struct xl_heaptid
 typedef struct xl_heap_delete
 {
 	xl_heaptid	target;			/* deleted tuple id */
+	TransactionId xmax;			/* xmax of the deleted tuple */
+	uint8		infobits_set;	/* infomask bits */
 	bool		all_visible_cleared;	/* PD_ALL_VISIBLE was cleared */
 } xl_heap_delete;
 
@@ -141,7 +144,10 @@ typedef struct xl_multi_insert_tuple
 typedef struct xl_heap_update
 {
 	xl_heaptid	target;			/* deleted tuple id */
+	TransactionId old_xmax;		/* xmax of the old tuple */
+	TransactionId new_xmax;		/* xmax of the new tuple */
 	ItemPointerData newtid;		/* new inserted tuple id */
+	uint8		old_infobits_set;	/* infomask bits to set on old tuple */
 	bool		all_visible_cleared;	/* PD_ALL_VISIBLE was cleared */
 	bool		new_all_visible_cleared;		/* same for the page of newtid */
 	/* NEW TUPLE xl_heap_header AND TUPLE DATA FOLLOWS AT END OF STRUCT */
@@ -197,16 +203,32 @@ typedef struct xl_heap_newpage
 
 #define SizeOfHeapNewpage	(offsetof(xl_heap_newpage, blkno) + sizeof(BlockNumber))
 
+/* flags for infobits_set */
+#define XLHL_XMAX_IS_MULTI		0x01
+#define XLHL_XMAX_LOCK_ONLY		0x02
+#define XLHL_XMAX_EXCL_LOCK		0x04
+#define XLHL_XMAX_KEYSHR_LOCK	0x08
+#define XLHL_KEYS_UPDATED		0x10
+
 /* This is what we need to know about lock */
 typedef struct xl_heap_lock
 {
 	xl_heaptid	target;			/* locked tuple id */
 	TransactionId locking_xid;	/* might be a MultiXactId not xid */
-	bool		xid_is_mxact;	/* is it? */
-	bool		shared_lock;	/* shared or exclusive row lock? */
+	int8		infobits_set;	/* infomask and infomask2 bits to set */
 } xl_heap_lock;
 
-#define SizeOfHeapLock	(offsetof(xl_heap_lock, shared_lock) + sizeof(bool))
+#define SizeOfHeapLock	(offsetof(xl_heap_lock, infobits_set) + sizeof(int8))
+
+/* This is what we need to know about locking an updated version of a row */
+typedef struct xl_heap_lock_updated
+{
+	xl_heaptid	target;
+	TransactionId	xmax;
+	uint8		infobits_set;
+} xl_heap_lock_updated;
+
+#define SizeOfHeapLockUpdated	(offsetof(xl_heap_lock_updated, infobits_set) + sizeof(uint8))
 
 /* This is what we need to know about in-place update */
 typedef struct xl_heap_inplace
@@ -223,10 +245,11 @@ typedef struct xl_heap_freeze
 	RelFileNode node;
 	BlockNumber block;
 	TransactionId cutoff_xid;
+	MultiXactId cutoff_multi;
 	/* TUPLE OFFSET NUMBERS FOLLOW AT THE END */
 } xl_heap_freeze;
 
-#define SizeOfHeapFreeze (offsetof(xl_heap_freeze, cutoff_xid) + sizeof(TransactionId))
+#define SizeOfHeapFreeze (offsetof(xl_heap_freeze, cutoff_multi) + sizeof(MultiXactId))
 
 /* This is what we need to know about setting a visibility map bit */
 typedef struct xl_heap_visible
@@ -254,7 +277,7 @@ extern XLogRecPtr log_heap_clean(Relation reln, Buffer buffer,
 			   OffsetNumber *nowunused, int nunused,
 			   TransactionId latestRemovedXid);
 extern XLogRecPtr log_heap_freeze(Relation reln, Buffer buffer,
-				TransactionId cutoff_xid,
+				TransactionId cutoff_xid, MultiXactId cutoff_multi,
 				OffsetNumber *offsets, int offcnt);
 extern XLogRecPtr log_heap_visible(RelFileNode rnode, BlockNumber block,
 				 Buffer vm_buffer, TransactionId cutoff_xid);

src/include/access/htup.h

@@ -80,7 +80,9 @@ typedef HeapTupleData *HeapTuple;
 extern CommandId HeapTupleHeaderGetCmin(HeapTupleHeader tup);
 extern CommandId HeapTupleHeaderGetCmax(HeapTupleHeader tup);
 extern void HeapTupleHeaderAdjustCmax(HeapTupleHeader tup,
-						  CommandId *cmax,
-						  bool *iscombo);
+						  CommandId *cmax, bool *iscombo);
+
+/* Prototype for HeapTupleHeader accessors in heapam.c */
+extern TransactionId HeapTupleGetUpdateXid(HeapTupleHeader tuple);
 
 #endif   /* HTUP_H */

src/include/access/htup_details.h

@@ -162,12 +162,16 @@ struct HeapTupleHeaderData
 #define HEAP_HASVARWIDTH		0x0002	/* has variable-width attribute(s) */
 #define HEAP_HASEXTERNAL		0x0004	/* has external stored attribute(s) */
 #define HEAP_HASOID				0x0008	/* has an object-id field */
-/* bit 0x0010 is available */
+#define HEAP_XMAX_KEYSHR_LOCK	0x0010	/* xmax is a key-shared locker */
 #define HEAP_COMBOCID			0x0020	/* t_cid is a combo cid */
 #define HEAP_XMAX_EXCL_LOCK		0x0040	/* xmax is exclusive locker */
-#define HEAP_XMAX_SHARED_LOCK	0x0080	/* xmax is shared locker */
-/* if either LOCK bit is set, xmax hasn't deleted the tuple, only locked it */
-#define HEAP_IS_LOCKED	(HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_SHARED_LOCK)
+#define HEAP_XMAX_LOCK_ONLY		0x0080	/* xmax, if valid, is only a locker */
+
+										/* xmax is a shared locker */
+#define HEAP_XMAX_SHR_LOCK	(HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)
+
+#define HEAP_LOCK_MASK	(HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
+						 HEAP_XMAX_KEYSHR_LOCK)
 #define HEAP_XMIN_COMMITTED		0x0100	/* t_xmin committed */
 #define HEAP_XMIN_INVALID		0x0200	/* t_xmin invalid/aborted */
 #define HEAP_XMAX_COMMITTED		0x0400	/* t_xmax committed */
@@ -182,17 +186,42 @@ struct HeapTupleHeaderData
 										 * upgrade support */
 #define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)
 
-#define HEAP_XACT_MASK			0xFFE0	/* visibility-related bits */
+#define HEAP_XACT_MASK			0xFFF0	/* visibility-related bits */
+
+/*
+ * A tuple is only locked (i.e. not updated by its Xmax) if it the
+ * HEAP_XMAX_LOCK_ONLY bit is set.
+ *
+ * See also HeapTupleHeaderIsOnlyLocked, which also checks for a possible
+ * aborted updater transaction.
+ */
+#define HEAP_XMAX_IS_LOCKED_ONLY(infomask) \
+	((infomask) & HEAP_XMAX_LOCK_ONLY)
+/*
+ * Use these to test whether a particular lock is applied to a tuple
+ */
+#define HEAP_XMAX_IS_SHR_LOCKED(infomask) \
+    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_SHR_LOCK)
+#define HEAP_XMAX_IS_EXCL_LOCKED(infomask) \
+    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_EXCL_LOCK)
+#define HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) \
+    (((infomask) & HEAP_LOCK_MASK) == HEAP_XMAX_KEYSHR_LOCK)
+
+/* turn these all off when Xmax is to change */
+#define HEAP_XMAX_BITS (HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID | \
+						HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK | HEAP_XMAX_LOCK_ONLY)
 
 /*
  * information stored in t_infomask2:
  */
 #define HEAP_NATTS_MASK			0x07FF	/* 11 bits for number of attributes */
-/* bits 0x3800 are available */
+/* bits 0x1800 are available */
+#define HEAP_KEYS_UPDATED		0x2000	/* tuple was updated and key cols
+										 * modified, or tuple deleted */
 #define HEAP_HOT_UPDATED		0x4000	/* tuple was HOT-updated */
 #define HEAP_ONLY_TUPLE			0x8000	/* this is heap-only tuple */
 
-#define HEAP2_XACT_MASK			0xC000	/* visibility-related bits */
+#define HEAP2_XACT_MASK			0xE000	/* visibility-related bits */
 
 /*
  * HEAP_TUPLE_HAS_MATCH is a temporary flag used during hash joins.  It is
@@ -219,7 +248,24 @@ struct HeapTupleHeaderData
 	(tup)->t_choice.t_heap.t_xmin = (xid) \
 )
 
-#define HeapTupleHeaderGetXmax(tup) \
+/*
+ * HeapTupleHeaderGetRawXmax gets you the raw Xmax field.  To find out the Xid
+ * that updated a tuple, you might need to resolve the MultiXactId if certain
+ * bits are set.  HeapTupleHeaderGetUpdateXid checks those bits and takes care
+ * to resolve the MultiXactId if necessary.  This might involve multixact I/O,
+ * so it should only be used if absolutely necessary.
+ */
+#define HeapTupleHeaderGetUpdateXid(tup) \
+( \
+	(!((tup)->t_infomask & HEAP_XMAX_INVALID) && \
+	 ((tup)->t_infomask & HEAP_XMAX_IS_MULTI) && \
+	 !((tup)->t_infomask & HEAP_XMAX_LOCK_ONLY)) ? \
+		HeapTupleGetUpdateXid(tup) \
+	: \
+		HeapTupleHeaderGetRawXmax(tup) \
+)
+
+#define HeapTupleHeaderGetRawXmax(tup) \
 ( \
 	(tup)->t_choice.t_heap.t_xmax \
 )

src/include/access/multixact.h

@@ -13,8 +13,15 @@
 
 #include "access/xlog.h"
 
+
+/*
+ * The first two MultiXactId values are reserved to store the truncation Xid
+ * and epoch of the first segment, so we start assigning multixact values from
+ * 2.
+ */
 #define InvalidMultiXactId	((MultiXactId) 0)
 #define FirstMultiXactId	((MultiXactId) 1)
+#define MaxMultiXactId		((MultiXactId) 0xFFFFFFFF)
 
 #define MultiXactIdIsValid(multi) ((multi) != InvalidMultiXactId)
 
@@ -22,6 +29,33 @@
 #define NUM_MXACTOFFSET_BUFFERS		8
 #define NUM_MXACTMEMBER_BUFFERS		16
 
+/*
+ * Possible multixact lock modes ("status").  The first four modes are for
+ * tuple locks (FOR KEY SHARE, FOR SHARE, FOR NO KEY UPDATE, FOR UPDATE); the
+ * next two are used for update and delete modes.
+ */
+typedef enum
+{
+	MultiXactStatusForKeyShare = 0x00,
+	MultiXactStatusForShare = 0x01,
+	MultiXactStatusForNoKeyUpdate = 0x02,
+	MultiXactStatusForUpdate = 0x03,
+	/* an update that doesn't touch "key" columns */
+	MultiXactStatusNoKeyUpdate = 0x04,
+	/* other updates, and delete */
+	MultiXactStatusUpdate = 0x05
+} MultiXactStatus;
+
+#define MaxMultiXactStatus MultiXactStatusUpdate
+
+
+typedef struct MultiXactMember
+{
+	TransactionId	xid;
+	MultiXactStatus	status;
+} MultiXactMember;
+
+
 /* ----------------
  *		multixact-related XLOG entries
  * ----------------
@@ -35,21 +69,24 @@ typedef struct xl_multixact_create
 {
 	MultiXactId mid;			/* new MultiXact's ID */
 	MultiXactOffset moff;		/* its starting offset in members file */
-	int32		nxids;			/* number of member XIDs */
-	TransactionId xids[1];		/* VARIABLE LENGTH ARRAY */
+	int32		nmembers;		/* number of member XIDs */
+	MultiXactMember members[FLEXIBLE_ARRAY_MEMBER];
 } xl_multixact_create;
 
-#define MinSizeOfMultiXactCreate offsetof(xl_multixact_create, xids)
+#define SizeOfMultiXactCreate (offsetof(xl_multixact_create, members))
 
 
-extern MultiXactId MultiXactIdCreate(TransactionId xid1, TransactionId xid2);
-extern MultiXactId MultiXactIdExpand(MultiXactId multi, TransactionId xid);
+extern MultiXactId MultiXactIdCreate(TransactionId xid1,
+				  MultiXactStatus status1, TransactionId xid2,
+				  MultiXactStatus status2);
+extern MultiXactId MultiXactIdExpand(MultiXactId multi, TransactionId xid,
+				  MultiXactStatus status);
+extern MultiXactId ReadNextMultiXactId(void);
 extern bool MultiXactIdIsRunning(MultiXactId multi);
-extern bool MultiXactIdIsCurrent(MultiXactId multi);
-extern void MultiXactIdWait(MultiXactId multi);
-extern bool ConditionalMultiXactIdWait(MultiXactId multi);
 extern void MultiXactIdSetOldestMember(void);
-extern int	GetMultiXactIdMembers(MultiXactId multi, TransactionId **xids);
+extern int	GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **xids,
+					  bool allow_old);
+extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
 
 extern void AtEOXact_MultiXact(void);
 extern void AtPrepare_MultiXact(void);
@@ -60,14 +97,21 @@ extern void MultiXactShmemInit(void);
 extern void BootStrapMultiXact(void);
 extern void StartupMultiXact(void);
 extern void ShutdownMultiXact(void);
+extern void SetMultiXactIdLimit(MultiXactId oldest_datminmxid,
+					Oid oldest_datoid);
 extern void MultiXactGetCheckptMulti(bool is_shutdown,
 						 MultiXactId *nextMulti,
-						 MultiXactOffset *nextMultiOffset);
+						 MultiXactOffset *nextMultiOffset,
+						 MultiXactId *oldestMulti,
+						 Oid *oldestMultiDB);
 extern void CheckPointMultiXact(void);
+extern MultiXactId GetOldestMultiXactId(void);
+extern void TruncateMultiXact(MultiXactId cutoff_multi);
 extern void MultiXactSetNextMXact(MultiXactId nextMulti,
 					  MultiXactOffset nextMultiOffset);
 extern void MultiXactAdvanceNextMXact(MultiXactId minMulti,
 						  MultiXactOffset minMultiOffset);
+extern void MultiXactAdvanceOldest(MultiXactId oldestMulti, Oid oldestMultiDB);
 
 extern void multixact_twophase_recover(TransactionId xid, uint16 info,
 						   void *recdata, uint32 len);
@@ -78,5 +122,7 @@ extern void multixact_twophase_postabort(TransactionId xid, uint16 info,
 
 extern void multixact_redo(XLogRecPtr lsn, XLogRecord *record);
 extern void multixact_desc(StringInfo buf, uint8 xl_info, char *rec);
+extern char *mxid_to_string(MultiXactId multi, int nmembers,
+			   MultiXactMember *members);
 
 #endif   /* MULTIXACT_H */

src/include/access/rewriteheap.h

@@ -21,7 +21,7 @@ typedef struct RewriteStateData *RewriteState;
 
 extern RewriteState begin_heap_rewrite(Relation NewHeap,
 				   TransactionId OldestXmin, TransactionId FreezeXid,
-				   bool use_wal);
+				   MultiXactId MultiXactFrzLimit, bool use_wal);
 extern void end_heap_rewrite(RewriteState state);
 extern void rewrite_heap_tuple(RewriteState state, HeapTuple oldTuple,
 				   HeapTuple newTuple);