Fix snapshot management, take two.

Partially revert the previous patch I installed and replace it with a more
general fix: any time a snapshot is pushed as Active, we need to ensure that it
will not be modified in the future.  This means that if the same snapshot is
used as CurrentSnapshot, it needs to be copied separately.  This affects
serializable transactions only, because CurrentSnapshot has already been copied
by RegisterSnapshot and so PushActiveSnapshot does not think it needs another
copy.  However, CommandCounterIncrement would modify CurrentSnapshot, whereas
ActiveSnapshots must not have their command counters incremented.

I say "partially" because the regression test I added for the previous bug
has been kept.

(This restores 8.3 behavior, because before snapmgr.c existed, any snapshot set
as Active was copied.)

Per bug report from Stuart Bishop in
6bc73d4c0910042358k3d1adff3qa36f8df75198ecea@mail.gmail.com

src/backend/commands/portalcmds.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.80 2009/10/02 17:57:29 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/portalcmds.c,v 1.81 2009/10/07 16:27:18 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -47,7 +47,6 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 	DeclareCursorStmt *cstmt = (DeclareCursorStmt *) stmt->utilityStmt;
 	Portal		portal;
 	MemoryContext oldContext;
-	Snapshot	snapshot;
 
 	if (cstmt == NULL || !IsA(cstmt, DeclareCursorStmt))
 		elog(ERROR, "PerformCursorOpen called for non-cursor query");
@@ -119,18 +118,10 @@ PerformCursorOpen(PlannedStmt *stmt, ParamListInfo params,
 			portal->cursorOptions |= CURSOR_OPT_NO_SCROLL;
 	}
 
-	/*
-	 * Set up snapshot for portal.  Note that we need a fresh, independent copy
-	 * of the snapshot because we don't want it to be modified by future
-	 * CommandCounterIncrement calls.  We do not register it, because
-	 * portalmem.c will take care of that internally.
-	 */
-	snapshot = CopySnapshot(GetActiveSnapshot());
-
 	/*
 	 * Start execution, inserting parameters if any.
 	 */
-	PortalStart(portal, params, snapshot);
+	PortalStart(portal, params, GetActiveSnapshot());
 
 	Assert(portal->strategy == PORTAL_ONE_SELECT);
 

src/backend/utils/time/snapmgr.c

@@ -19,7 +19,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.11 2009/10/02 17:57:30 alvherre Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.12 2009/10/07 16:27:18 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -104,6 +104,7 @@ bool		FirstSnapshotSet = false;
 static bool registered_serializable = false;
 
 
+static Snapshot CopySnapshot(Snapshot snapshot);
 static void FreeSnapshot(Snapshot snapshot);
 static void SnapshotResetXmin(void);
 
@@ -191,7 +192,7 @@ SnapshotSetCommandId(CommandId curcid)
  * The copy is palloc'd in TopTransactionContext and has initial refcounts set
  * to 0.  The returned snapshot has the copied flag set.
  */
-Snapshot
+static Snapshot
 CopySnapshot(Snapshot snapshot)
 {
 	Snapshot	newsnap;
@@ -254,8 +255,9 @@ FreeSnapshot(Snapshot snapshot)
  * PushActiveSnapshot
  *		Set the given snapshot as the current active snapshot
  *
- * If this is the first use of this snapshot, create a new long-lived copy with
- * active refcount=1.  Otherwise, only increment the refcount.
+ * If the passed snapshot is a statically-allocated one, or it is possibly
+ * subject to a future command counter update, create a new long-lived copy
+ * with active refcount=1.  Otherwise, only increment the refcount.
  */
 void
 PushActiveSnapshot(Snapshot snap)
@@ -265,8 +267,16 @@ PushActiveSnapshot(Snapshot snap)
 	Assert(snap != InvalidSnapshot);
 
 	newactive = MemoryContextAlloc(TopTransactionContext, sizeof(ActiveSnapshotElt));
-	/* Static snapshot?  Create a persistent copy */
-	newactive->as_snap = snap->copied ? snap : CopySnapshot(snap);
+
+	/*
+	 * Checking SecondarySnapshot is probably useless here, but it seems better
+	 * to be sure.
+	 */
+	if (snap == CurrentSnapshot || snap == SecondarySnapshot || !snap->copied)
+		newactive->as_snap = CopySnapshot(snap);
+	else
+		newactive->as_snap = snap;
+
 	newactive->as_next = ActiveSnapshot;
 	newactive->as_level = GetCurrentTransactionNestLevel();
 

src/include/utils/snapmgr.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.6 2009/10/02 17:57:30 alvherre Exp $
+ * $PostgreSQL: pgsql/src/include/utils/snapmgr.h,v 1.7 2009/10/07 16:27:18 alvherre Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,7 +26,6 @@ extern TransactionId RecentGlobalXmin;
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);
 extern void SnapshotSetCommandId(CommandId curcid);
-extern Snapshot CopySnapshot(Snapshot snapshot);
 
 extern void PushActiveSnapshot(Snapshot snapshot);
 extern void PushUpdatedSnapshot(Snapshot snapshot);

src/test/regress/expected/triggers.out

@@ -537,6 +537,37 @@ NOTICE:  row 1 not changed
 NOTICE:  row 2 not changed
 DROP TABLE trigger_test;
 DROP FUNCTION mytrigger();
+-- Test snapshot management in serializable transactions involving triggers
+-- per bug report in 6bc73d4c0910042358k3d1adff3qa36f8df75198ecea@mail.gmail.com
+CREATE FUNCTION serializable_update_trig() RETURNS trigger LANGUAGE plpgsql AS
+$$
+declare
+	rec record;
+begin
+	new.description = 'updated in trigger';
+	return new;
+end;
+$$;
+CREATE TABLE serializable_update_tab (
+	id int,
+	filler  text,
+	description text
+);
+CREATE TRIGGER serializable_update_trig BEFORE UPDATE ON serializable_update_tab
+	FOR EACH ROW EXECUTE PROCEDURE serializable_update_trig();
+INSERT INTO serializable_update_tab SELECT a, repeat('xyzxz', 100), 'new'
+	FROM generate_series(1, 50) a;
+BEGIN;
+SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+UPDATE serializable_update_tab SET description = 'no no', id = 1 WHERE id = 1;
+COMMIT;
+SELECT description FROM serializable_update_tab WHERE id = 1;
+    description     
+--------------------
+ updated in trigger
+(1 row)
+
+DROP TABLE serializable_update_tab;
 -- minimal update trigger
 CREATE TABLE min_updates_test (
 	f1	text,

src/test/regress/sql/triggers.sql

@@ -416,6 +416,36 @@ DROP TABLE trigger_test;
 
 DROP FUNCTION mytrigger();
 
+-- Test snapshot management in serializable transactions involving triggers
+-- per bug report in 6bc73d4c0910042358k3d1adff3qa36f8df75198ecea@mail.gmail.com
+CREATE FUNCTION serializable_update_trig() RETURNS trigger LANGUAGE plpgsql AS
+$$
+declare
+	rec record;
+begin
+	new.description = 'updated in trigger';
+	return new;
+end;
+$$;
+
+CREATE TABLE serializable_update_tab (
+	id int,
+	filler  text,
+	description text
+);
+
+CREATE TRIGGER serializable_update_trig BEFORE UPDATE ON serializable_update_tab
+	FOR EACH ROW EXECUTE PROCEDURE serializable_update_trig();
+
+INSERT INTO serializable_update_tab SELECT a, repeat('xyzxz', 100), 'new'
+	FROM generate_series(1, 50) a;
+
+BEGIN;
+SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
+UPDATE serializable_update_tab SET description = 'no no', id = 1 WHERE id = 1;
+COMMIT;
+SELECT description FROM serializable_update_tab WHERE id = 1;
+DROP TABLE serializable_update_tab;
 
 -- minimal update trigger