Renumber SnapshotNow and the other special snapshot codes so that

((Snapshot) NULL) can no longer be confused with a valid snapshot,
as per my recent suggestion.  Define a macro InvalidSnapshot for 0.
Use InvalidSnapshot instead of SnapshotAny as the do-nothing special
case for heap_update and heap_delete crosschecks; this seems a little
cleaner even though the behavior is really the same.

src/backend/access/heap/heapam.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.173 2004/08/29 05:06:40 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.174 2004/09/11 18:28:32 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -1262,7 +1262,7 @@ simple_heap_insert(Relation relation, HeapTuple tup)
  *	tid - TID of tuple to be deleted
  *	ctid - output parameter, used only for failure case (see below)
  *	cid - delete command ID to use in verifying tuple visibility
- *	crosscheck - if not SnapshotAny, also check tuple against this
+ *	crosscheck - if not InvalidSnapshot, also check tuple against this
  *	wait - true if should wait for any conflicting update to commit/abort
  *
  * Normal, successful return value is HeapTupleMayBeUpdated, which
@@ -1274,7 +1274,8 @@ simple_heap_insert(Relation relation, HeapTuple tup)
  */
 int
 heap_delete(Relation relation, ItemPointer tid,
-		 ItemPointer ctid, CommandId cid, Snapshot crosscheck, bool wait)
+			ItemPointer ctid, CommandId cid,
+			Snapshot crosscheck, bool wait)
 {
 	ItemId		lp;
 	HeapTupleData tp;
@@ -1339,7 +1340,7 @@ l1:
 			result = HeapTupleUpdated;
 	}
 
-	if (crosscheck != SnapshotAny && result == HeapTupleMayBeUpdated)
+	if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
 	{
 		/* Perform additional check for serializable RI updates */
 		if (!HeapTupleSatisfiesSnapshot(tp.t_data, crosscheck))
@@ -1443,7 +1444,7 @@ simple_heap_delete(Relation relation, ItemPointer tid)
 
 	result = heap_delete(relation, tid,
 						 &ctid,
-						 GetCurrentCommandId(), SnapshotAny,
+						 GetCurrentCommandId(), InvalidSnapshot,
 						 true /* wait for commit */ );
 	switch (result)
 	{
@@ -1477,7 +1478,7 @@ simple_heap_delete(Relation relation, ItemPointer tid)
  *	newtup - newly constructed tuple data to store
  *	ctid - output parameter, used only for failure case (see below)
  *	cid - update command ID to use in verifying old tuple visibility
- *	crosscheck - if not SnapshotAny, also check old tuple against this
+ *	crosscheck - if not InvalidSnapshot, also check old tuple against this
  *	wait - true if should wait for any conflicting update to commit/abort
  *
  * Normal, successful return value is HeapTupleMayBeUpdated, which
@@ -1491,7 +1492,8 @@ simple_heap_delete(Relation relation, ItemPointer tid)
  */
 int
 heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
-		 ItemPointer ctid, CommandId cid, Snapshot crosscheck, bool wait)
+			ItemPointer ctid, CommandId cid,
+			Snapshot crosscheck, bool wait)
 {
 	ItemId		lp;
 	HeapTupleData oldtup;
@@ -1566,7 +1568,7 @@ l2:
 			result = HeapTupleUpdated;
 	}
 
-	if (crosscheck != SnapshotAny && result == HeapTupleMayBeUpdated)
+	if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
 	{
 		/* Perform additional check for serializable RI updates */
 		if (!HeapTupleSatisfiesSnapshot(oldtup.t_data, crosscheck))
@@ -1804,7 +1806,7 @@ simple_heap_update(Relation relation, ItemPointer otid, HeapTuple tup)
 
 	result = heap_update(relation, otid, tup,
 						 &ctid,
-						 GetCurrentCommandId(), SnapshotAny,
+						 GetCurrentCommandId(), InvalidSnapshot,
 						 true /* wait for commit */ );
 	switch (result)
 	{

src/backend/commands/async.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.116 2004/09/06 23:32:54 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/async.c,v 1.117 2004/09/11 18:28:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -544,7 +544,7 @@ AtCommit_Notify(void)
 				 */
 				result = heap_update(lRel, &lTuple->t_self, rTuple,
 									 &ctid,
-									 GetCurrentCommandId(), SnapshotAny,
+									 GetCurrentCommandId(), InvalidSnapshot,
 									 false /* no wait for commit */ );
 				switch (result)
 				{

src/backend/executor/execMain.c

@@ -26,7 +26,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.236 2004/08/29 05:06:42 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execMain.c,v 1.237 2004/09/11 18:28:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -176,7 +176,7 @@ ExecutorStart(QueryDesc *queryDesc, bool useCurrentSnapshot, bool explainOnly)
 	{
 		/* normal query --- use query snapshot, no crosscheck */
 		estate->es_snapshot = CopyQuerySnapshot();
-		estate->es_crosscheck_snapshot = SnapshotAny;
+		estate->es_crosscheck_snapshot = InvalidSnapshot;
 	}
 
 	/*

src/backend/executor/execUtils.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execUtils.c,v 1.114 2004/08/29 05:06:42 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execUtils.c,v 1.115 2004/09/11 18:28:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -179,7 +179,7 @@ CreateExecutorState(void)
 	 */
 	estate->es_direction = ForwardScanDirection;
 	estate->es_snapshot = SnapshotNow;
-	estate->es_crosscheck_snapshot = SnapshotAny;		/* means no crosscheck */
+	estate->es_crosscheck_snapshot = InvalidSnapshot;	/* no crosscheck */
 	estate->es_range_table = NIL;
 
 	estate->es_result_relations = NULL;

src/include/utils/tqual.h

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1996-2004, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/tqual.h,v 1.50 2004/08/29 04:13:11 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/tqual.h,v 1.51 2004/09/11 18:28:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -19,6 +19,20 @@
 #include "access/xact.h"
 
 
+/*
+ * "Regular" snapshots are pointers to a SnapshotData structure.
+ *
+ * We also have some "special" snapshot values that have fixed meanings
+ * and don't need any backing SnapshotData.  These are encoded by small
+ * integer values, which of course is a gross violation of ANSI C, but
+ * it works fine on all known platforms.
+ *
+ * SnapshotDirty is an even more special case: its semantics are fixed,
+ * but there is a backing SnapshotData struct for it.  That struct is
+ * actually used as *output* data from tqual.c, not input into it.
+ * (But hey, SnapshotDirty ought to have a dirty implementation, no? ;-))
+ */
+
 typedef struct SnapshotData
 {
 	TransactionId xmin;			/* XID < xmin are visible to me */
@@ -32,10 +46,12 @@ typedef struct SnapshotData
 
 typedef SnapshotData *Snapshot;
 
-#define SnapshotNow					((Snapshot) 0x0)
-#define SnapshotSelf				((Snapshot) 0x1)
-#define SnapshotAny					((Snapshot) 0x2)
-#define SnapshotToast				((Snapshot) 0x3)
+/* Special snapshot values: */
+#define InvalidSnapshot				((Snapshot) 0x0) /* same as NULL */
+#define SnapshotNow					((Snapshot) 0x1)
+#define SnapshotSelf				((Snapshot) 0x2)
+#define SnapshotAny					((Snapshot) 0x3)
+#define SnapshotToast				((Snapshot) 0x4)
 
 extern DLLIMPORT Snapshot SnapshotDirty;
 extern DLLIMPORT Snapshot QuerySnapshot;