Re: Weird behavior with "sensitive" cursors.

[Alvaro Herrera <alvherre@xxxxxxxxxxxxxxxxx>]

Tom Lane escribió:
> Alvaro Herrera <alvherre@xxxxxxxxxxxxxxxxx> writes:
> > Tom Lane escribió:
> >> Well, the first problem is that 8.4 is failing to duplicate the
> >> historical behavior.
> 
> > Oh!  That's easy.
> 
> I don't think that testing rowMarks is the right thing at all here.
> That tells you whether it's a SELECT FOR UPDATE, but actually we
> want any cursor (and only cursors) to have a private snapshot.

The attached patch implements this.  I intend to apply to 8.4 and HEAD
shortly.

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.

Index: src/backend/commands/portalcmds.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/commands/portalcmds.c,v
retrieving revision 1.79
diff -c -p -r1.79 portalcmds.c
*** src/backend/commands/portalcmds.c	11 Jun 2009 14:48:56 -0000	1.79
--- src/backend/commands/portalcmds.c	1 Oct 2009 19:35:15 -0000
*************** PerformCursorOpen(PlannedStmt *stmt, Par
*** 47,52 ****
--- 47,53 ----
  	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");
*************** PerformCursorOpen(PlannedStmt *stmt, Par
*** 119,127 ****
  	}
  
  	/*
  	 * Start execution, inserting parameters if any.
  	 */
! 	PortalStart(portal, params, GetActiveSnapshot());
  
  	Assert(portal->strategy == PORTAL_ONE_SELECT);
  
--- 120,136 ----
  	}
  
  	/*
+ 	 * 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.
+ 	 */
+ 	snapshot = RegisterCopiedSnapshot(GetActiveSnapshot(),
+ 									  portal->resowner);
+ 
+ 	/*
  	 * Start execution, inserting parameters if any.
  	 */
! 	PortalStart(portal, params, snapshot);
  
  	Assert(portal->strategy == PORTAL_ONE_SELECT);
  
Index: src/backend/utils/time/snapmgr.c
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/backend/utils/time/snapmgr.c,v
retrieving revision 1.10
diff -c -p -r1.10 snapmgr.c
*** src/backend/utils/time/snapmgr.c	11 Jun 2009 14:49:06 -0000	1.10
--- src/backend/utils/time/snapmgr.c	1 Oct 2009 17:24:40 -0000
*************** RegisterSnapshotOnOwner(Snapshot snapsho
*** 385,390 ****
--- 385,403 ----
  }
  
  /*
+  * As above, but create a new, independeny copy of the snapshot.
+  */
+ Snapshot
+ RegisterCopiedSnapshot(Snapshot snapshot, ResourceOwner owner)
+ {
+ 	if (snapshot == InvalidSnapshot)
+ 		return InvalidSnapshot;
+ 
+ 	snapshot = CopySnapshot(snapshot);
+ 	return RegisterSnapshotOnOwner(snapshot, owner);
+ }
+ 
+ /*
   * UnregisterSnapshot
   *
   * Decrement the reference count of a snapshot, remove the corresponding
Index: src/include/utils/snapmgr.h
===================================================================
RCS file: /home/alvherre/Code/cvs/pgsql/src/include/utils/snapmgr.h,v
retrieving revision 1.5
diff -c -p -r1.5 snapmgr.h
*** src/include/utils/snapmgr.h	11 Jun 2009 14:49:13 -0000	1.5
--- src/include/utils/snapmgr.h	1 Oct 2009 17:24:40 -0000
*************** extern bool ActiveSnapshotSet(void);
*** 36,41 ****
--- 36,42 ----
  extern Snapshot RegisterSnapshot(Snapshot snapshot);
  extern void UnregisterSnapshot(Snapshot snapshot);
  extern Snapshot RegisterSnapshotOnOwner(Snapshot snapshot, ResourceOwner owner);
+ extern Snapshot RegisterCopiedSnapshot(Snapshot snapshot, ResourceOwner owner);
  extern void UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner);
  
  extern void AtSubCommit_Snapshot(int level);

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