Eliminate xmin from hash tag for predicate locks on heap tuples.

If a tuple was frozen while its predicate locks mattered,
read-write dependencies could be missed, resulting in failure to
detect conflicts which could lead to anomalies in committed
serializable transactions.

This field was added to the tag when we still thought that it was
necessary to carry locks forward to a new version of an updated
row.  That was later proven to be unnecessary, which allowed
simplification of the code, but elimination of xmin from the tag
was missed at the time.

Per report and analysis by Heikki Linnakangas.
Backpatch to 9.1.

src/backend/storage/lmgr/predicate.c

@@ -156,9 +156,9 @@
  *		PredicateLockTuple(Relation relation, HeapTuple tuple,
  *						Snapshot snapshot)
  *		PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
- *							   BlockNumber newblkno);
+ *							   BlockNumber newblkno)
  *		PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
- *								 BlockNumber newblkno);
+ *								 BlockNumber newblkno)
  *		TransferPredicateLocksToHeapRelation(Relation relation)
  *		ReleasePredicateLocks(bool isCommit)
  *
@@ -381,7 +381,7 @@ static SHM_QUEUE *FinishedSerializableTransactions;
  * this entry, you can ensure that there's enough scratch space available for
  * inserting one entry in the hash table. This is an otherwise-invalid tag.
  */
-static const PREDICATELOCKTARGETTAG ScratchTargetTag = {0, 0, 0, 0, 0};
+static const PREDICATELOCKTARGETTAG ScratchTargetTag = {0, 0, 0, 0};
 static uint32 ScratchTargetTagHash;
 static int	ScratchPartitionLock;
 
@@ -2492,8 +2492,6 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
 			}
 		}
 	}
-	else
-		targetxmin = InvalidTransactionId;
 
 	/*
 	 * Do quick-but-not-definitive test for a relation lock first.	This will
@@ -2512,8 +2510,7 @@ PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
 									 relation->rd_node.dbNode,
 									 relation->rd_id,
 									 ItemPointerGetBlockNumber(tid),
-									 ItemPointerGetOffsetNumber(tid),
-									 targetxmin);
+									 ItemPointerGetOffsetNumber(tid));
 	PredicateLockAcquire(&tag);
 }
 
@@ -4283,8 +4280,7 @@ CheckForSerializableConflictIn(Relation relation, HeapTuple tuple,
 										 relation->rd_node.dbNode,
 										 relation->rd_id,
 						 ItemPointerGetBlockNumber(&(tuple->t_data->t_ctid)),
-						ItemPointerGetOffsetNumber(&(tuple->t_data->t_ctid)),
-									  HeapTupleHeaderGetXmin(tuple->t_data));
+						ItemPointerGetOffsetNumber(&(tuple->t_data->t_ctid)));
 		CheckTargetForConflictsIn(&targettag);
 	}
 

src/include/storage/predicate_internals.h

@@ -254,10 +254,10 @@ typedef struct SERIALIZABLEXID
  * be the target of predicate locks.
  *
  * Note that the hash function being used doesn't properly respect tag
- * length -- it will go to a four byte boundary past the end of the tag.
- * If you change this struct, make sure any slack space is initialized,
- * so that any random bytes in the middle or at the end are not included
- * in the hash.
+ * length -- if the length of the structure isn't a multiple of four bytes it
+ * will go to a four byte boundary past the end of the tag.  If you change
+ * this struct, make sure any slack space is initialized, so that any random
+ * bytes in the middle or at the end are not included in the hash.
  *
  * TODO SSI: If we always use the same fields for the same type of value, we
  * should rename these.  Holding off until it's clear there are no exceptions.
@@ -272,7 +272,6 @@ typedef struct PREDICATELOCKTARGETTAG
 	uint32		locktag_field2; /* a 32-bit ID field */
 	uint32		locktag_field3; /* a 32-bit ID field */
 	uint32		locktag_field4; /* a 32-bit ID field */
-	uint32		locktag_field5; /* a 32-bit ID field */
 } PREDICATELOCKTARGETTAG;
 
 /*
@@ -283,12 +282,6 @@ typedef struct PREDICATELOCKTARGETTAG
  * added when a predicate lock is requested on an object which doesn't
  * already have one.  An entry is removed when the last lock is removed from
  * its list.
- *
- * Because a particular target might become obsolete, due to update to a new
- * version, before the reading transaction is obsolete, we need some way to
- * prevent errors from reuse of a tuple ID.  Rather than attempting to clean
- * up the targets as the related tuples are pruned or vacuumed, we check the
- * xmin on access.	This should be far less costly.
  */
 typedef struct PREDICATELOCKTARGET
 {
@@ -398,22 +391,19 @@ typedef struct PredicateLockData
 	((locktag).locktag_field1 = (dboid), \
 	 (locktag).locktag_field2 = (reloid), \
 	 (locktag).locktag_field3 = InvalidBlockNumber, \
-	 (locktag).locktag_field4 = InvalidOffsetNumber, \
-	 (locktag).locktag_field5 = InvalidTransactionId)
+	 (locktag).locktag_field4 = InvalidOffsetNumber)
 
 #define SET_PREDICATELOCKTARGETTAG_PAGE(locktag,dboid,reloid,blocknum) \
 	((locktag).locktag_field1 = (dboid), \
 	 (locktag).locktag_field2 = (reloid), \
 	 (locktag).locktag_field3 = (blocknum), \
-	 (locktag).locktag_field4 = InvalidOffsetNumber, \
-	 (locktag).locktag_field5 = InvalidTransactionId)
+	 (locktag).locktag_field4 = InvalidOffsetNumber)
 
-#define SET_PREDICATELOCKTARGETTAG_TUPLE(locktag,dboid,reloid,blocknum,offnum,xmin) \
+#define SET_PREDICATELOCKTARGETTAG_TUPLE(locktag,dboid,reloid,blocknum,offnum) \
 	((locktag).locktag_field1 = (dboid), \
 	 (locktag).locktag_field2 = (reloid), \
 	 (locktag).locktag_field3 = (blocknum), \
-	 (locktag).locktag_field4 = (offnum), \
-	 (locktag).locktag_field5 = (xmin))
+	 (locktag).locktag_field4 = (offnum))
 
 #define GET_PREDICATELOCKTARGETTAG_DB(locktag) \
 	((Oid) (locktag).locktag_field1)
@@ -423,8 +413,6 @@ typedef struct PredicateLockData
 	((BlockNumber) (locktag).locktag_field3)
 #define GET_PREDICATELOCKTARGETTAG_OFFSET(locktag) \
 	((OffsetNumber) (locktag).locktag_field4)
-#define GET_PREDICATELOCKTARGETTAG_XMIN(locktag) \
-	((TransactionId) (locktag).locktag_field5)
 #define GET_PREDICATELOCKTARGETTAG_TYPE(locktag)							 \
 	(((locktag).locktag_field4 != InvalidOffsetNumber) ? PREDLOCKTAG_TUPLE : \
 	 (((locktag).locktag_field3 != InvalidBlockNumber) ? PREDLOCKTAG_PAGE :   \
@@ -462,6 +450,7 @@ typedef struct TwoPhasePredicateXactRecord
 typedef struct TwoPhasePredicateLockRecord
 {
 	PREDICATELOCKTARGETTAG target;
+	uint32		filler;  /* to avoid length change in back-patched fix */
 } TwoPhasePredicateLockRecord;
 
 typedef struct TwoPhasePredicateRecord