Use ResourceOwners in the snapshot manager, instead of attempting to track them

by hand.  As an added bonus, the new code is smaller and more understandable,
and the ugly loops are gone.

This had been discussed all along but never implemented.  It became clear that
it really needed to be fixed after a bug report by Pavan Deolasee.

src/backend/utils/resowner/README

-$PostgreSQL: pgsql/src/backend/utils/resowner/README,v 1.7 2008/03/21 13:23:28 momjian Exp $
+$PostgreSQL: pgsql/src/backend/utils/resowner/README,v 1.8 2008/11/25 20:28:29 alvherre Exp $
 
 Notes About Resource Owners
 ===========================
@@ -61,11 +61,11 @@ ResourceOwner transfers lock ownership to the parent instead of actually
 releasing the lock, if isCommit is true.
 
 Currently, ResourceOwners contain direct support for recording ownership of
-buffer pins, lmgr locks, and catcache, relcache, plancache, and tupdesc
-references.  Other objects can be associated with a ResourceOwner by recording
-the address of the owning ResourceOwner in such an object.  There is an API
-for other modules to get control during ResourceOwner release, so that they
-can scan their own data structures to find the objects that need to be
+buffer pins, lmgr locks, and catcache, relcache, plancache, tupdesc, and
+snapshot references.  Other objects can be associated with a ResourceOwner by
+recording the address of the owning ResourceOwner in such an object.  There is
+an API for other modules to get control during ResourceOwner release, so that
+they can scan their own data structures to find the objects that need to be
 deleted.
 
 Whenever we are inside a transaction, the global variable

src/backend/utils/resowner/resowner.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/resowner/resowner.c,v 1.29 2008/06/19 00:46:05 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/resowner/resowner.c,v 1.30 2008/11/25 20:28:29 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,6 +26,7 @@
 #include "utils/memutils.h"
 #include "utils/rel.h"
 #include "utils/resowner.h"
+#include "utils/snapmgr.h"
 
 
 /*
@@ -66,6 +67,11 @@ typedef struct ResourceOwnerData
 	int			ntupdescs;		/* number of owned tupdesc references */
 	TupleDesc  *tupdescs;		/* dynamically allocated array */
 	int			maxtupdescs;	/* currently allocated array size */
+
+	/* We have built-in support for remembering snapshot references */
+	int			nsnapshots;		/* number of owned snapshot references */
+	Snapshot   *snapshots;		/* dynamically allocated array */
+	int			maxsnapshots;	/* currently allocated array size */
 } ResourceOwnerData;
 
 
@@ -98,6 +104,7 @@ static void ResourceOwnerReleaseInternal(ResourceOwner owner,
 static void PrintRelCacheLeakWarning(Relation rel);
 static void PrintPlanCacheLeakWarning(CachedPlan *plan);
 static void PrintTupleDescLeakWarning(TupleDesc tupdesc);
+static void PrintSnapshotLeakWarning(Snapshot snapshot);
 
 
 /*****************************************************************************
@@ -301,6 +308,13 @@ ResourceOwnerReleaseInternal(ResourceOwner owner,
 				PrintTupleDescLeakWarning(owner->tupdescs[owner->ntupdescs - 1]);
 			DecrTupleDescRefCount(owner->tupdescs[owner->ntupdescs - 1]);
 		}
+		/* Ditto for snapshot references */
+		while (owner->nsnapshots > 0)
+		{
+			if (isCommit)
+				PrintSnapshotLeakWarning(owner->snapshots[owner->nsnapshots -1]);
+			UnregisterSnapshot(owner->snapshots[owner->nsnapshots -1]);
+		}
 
 		/* Clean up index scans too */
 		ReleaseResources_hash();
@@ -332,6 +346,7 @@ ResourceOwnerDelete(ResourceOwner owner)
 	Assert(owner->nrelrefs == 0);
 	Assert(owner->nplanrefs == 0);
 	Assert(owner->ntupdescs == 0);
+	Assert(owner->nsnapshots == 0);
 
 	/*
 	 * Delete children.  The recursive call will delink the child from me, so
@@ -360,6 +375,8 @@ ResourceOwnerDelete(ResourceOwner owner)
 		pfree(owner->planrefs);
 	if (owner->tupdescs)
 		pfree(owner->tupdescs);
+	if (owner->snapshots)
+		pfree(owner->snapshots);
 
 	pfree(owner);
 }
@@ -936,3 +953,85 @@ PrintTupleDescLeakWarning(TupleDesc tupdesc)
 		 "TupleDesc reference leak: TupleDesc %p (%u,%d) still referenced",
 		 tupdesc, tupdesc->tdtypeid, tupdesc->tdtypmod);
 }
+
+/*
+ * Make sure there is room for at least one more entry in a ResourceOwner's
+ * snapshot reference array.
+ *
+ * This is separate from actually inserting an entry because if we run out
+ * of memory, it's critical to do so *before* acquiring the resource.
+ */
+void
+ResourceOwnerEnlargeSnapshots(ResourceOwner owner)
+{
+	int			newmax;
+
+	if (owner->nsnapshots < owner->maxsnapshots)
+		return;					/* nothing to do */
+
+	if (owner->snapshots == NULL)
+	{
+		newmax = 16;
+		owner->snapshots = (Snapshot *)
+			MemoryContextAlloc(TopMemoryContext, newmax * sizeof(Snapshot));
+		owner->maxsnapshots = newmax;
+	}
+	else
+	{
+		newmax = owner->maxsnapshots * 2;
+		owner->snapshots = (Snapshot *)
+			repalloc(owner->snapshots, newmax * sizeof(Snapshot));
+		owner->maxsnapshots = newmax;
+	}
+}
+
+/*
+ * Remember that a snapshot reference is owned by a ResourceOwner
+ *
+ * Caller must have previously done ResourceOwnerEnlargeSnapshots()
+ */
+void
+ResourceOwnerRememberSnapshot(ResourceOwner owner, Snapshot snapshot)
+{
+	Assert(owner->nsnapshots < owner->maxsnapshots);
+	owner->snapshots[owner->nsnapshots] = snapshot;
+	owner->nsnapshots++;
+}
+
+/*
+ * Forget that a snapshot reference is owned by a ResourceOwner
+ */
+void
+ResourceOwnerForgetSnapshot(ResourceOwner owner, Snapshot snapshot)
+{
+	Snapshot   *snapshots = owner->snapshots;
+	int			ns1 = owner->nsnapshots -1;
+	int			i;
+
+	for (i = ns1; i >= 0; i--)
+	{
+		if (snapshots[i] == snapshot)
+		{
+			while (i < ns1)
+			{
+				snapshots[i] = snapshots[i + 1];
+				i++;
+			}
+			owner->nsnapshots = ns1;
+			return;
+		}
+	}
+	elog(ERROR, "snapshot reference %p is not owned by resource owner %s",
+		 snapshot, owner->name);
+}
+
+/*
+ * Debugging subroutine
+ */
+static void
+PrintSnapshotLeakWarning(Snapshot snapshot)
+{
+	elog(WARNING,
+		 "Snapshot reference leak: Snapshot %p still referenced",
+		 snapshot);
+}

src/include/utils/resowner.h

@@ -12,7 +12,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/resowner.h,v 1.15 2008/01/01 19:45:59 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/resowner.h,v 1.16 2008/11/25 20:28:29 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -22,6 +22,7 @@
 #include "storage/buf.h"
 #include "utils/catcache.h"
 #include "utils/plancache.h"
+#include "utils/snapshot.h"
 
 
 /*
@@ -121,4 +122,11 @@ extern void ResourceOwnerRememberTupleDesc(ResourceOwner owner,
 extern void ResourceOwnerForgetTupleDesc(ResourceOwner owner,
 							 TupleDesc tupdesc);
 
+/* support for snapshot refcount management */
+extern void ResourceOwnerEnlargeSnapshots(ResourceOwner owner);
+extern void ResourceOwnerRememberSnapshot(ResourceOwner owner,
+							  Snapshot snapshot);
+extern void ResourceOwnerForgetSnapshot(ResourceOwner owner,
+							Snapshot snapshot);
+
 #endif   /* RESOWNER_H */