Fix possible failures when a tuplestore switches from in-memory to on-disk

mode while callers hold pointers to in-memory tuples.  I reported this for
the case of nodeWindowAgg's primary scan tuple, but inspection of the code
shows that all of the calls in nodeWindowAgg and nodeCtescan are at risk.
For the moment, fix it with a rather brute-force approach of copying
whenever one of the at-risk callers requests a tuple.  Later we might
think of some sort of reference-count approach to reduce tuple copying.

src/backend/executor/execQual.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.242 2009/03/26 22:26:06 petere Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execQual.c,v 1.243 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1385,7 +1385,7 @@ restart:
 	if (fcache->funcResultStore)
 	{
 		Assert(isDone);				/* it was provided before ... */
-		if (tuplestore_gettupleslot(fcache->funcResultStore, true,
+		if (tuplestore_gettupleslot(fcache->funcResultStore, true, false,
 									fcache->funcResultSlot))
 		{
 			*isDone = ExprMultipleResult;

src/backend/executor/functions.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/functions.c,v 1.132 2009/01/02 20:42:00 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/functions.c,v 1.133 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -736,7 +736,7 @@ fmgr_sql(PG_FUNCTION_ARGS)
 			/* Re-use the junkfilter's output slot to fetch back the tuple */
 			Assert(fcache->junkFilter);
 			slot = fcache->junkFilter->jf_resultSlot;
-			if (!tuplestore_gettupleslot(fcache->tstore, true, slot))
+			if (!tuplestore_gettupleslot(fcache->tstore, true, false, slot))
 				elog(ERROR, "failed to fetch lazy-eval tuple");
 			/* Extract the result as a datum, and copy out from the slot */
 			result = postquel_get_single_result(slot, fcinfo,
@@ -822,7 +822,7 @@ fmgr_sql(PG_FUNCTION_ARGS)
 		{
 			/* Re-use the junkfilter's output slot to fetch back the tuple */
 			slot = fcache->junkFilter->jf_resultSlot;
-			if (tuplestore_gettupleslot(fcache->tstore, true, slot))
+			if (tuplestore_gettupleslot(fcache->tstore, true, false, slot))
 				result = postquel_get_single_result(slot, fcinfo,
 													fcache, oldcontext);
 			else

src/backend/executor/nodeCtescan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeCtescan.c,v 1.3 2009/01/01 17:23:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeCtescan.c,v 1.4 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,10 +71,14 @@ CteScanNext(CteScanState *node)
 
 	/*
 	 * If we can fetch another tuple from the tuplestore, return it.
+	 *
+	 * Note: we have to use copy=true in the tuplestore_gettupleslot call,
+	 * because we are sharing the tuplestore with other nodes that might
+	 * write into the tuplestore before we get called again.
 	 */
 	if (!eof_tuplestore)
 	{
-		if (tuplestore_gettupleslot(tuplestorestate, forward, slot))
+		if (tuplestore_gettupleslot(tuplestorestate, forward, true, slot))
 			return slot;
 		if (forward)
 			eof_tuplestore = true;

src/backend/executor/nodeFunctionscan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeFunctionscan.c,v 1.50 2009/01/01 17:23:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeFunctionscan.c,v 1.51 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -74,6 +74,7 @@ FunctionNext(FunctionScanState *node)
 	slot = node->ss.ss_ScanTupleSlot;
 	(void) tuplestore_gettupleslot(tuplestorestate,
 								   ScanDirectionIsForward(direction),
+								   false,
 								   slot);
 	return slot;
 }

src/backend/executor/nodeMaterial.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeMaterial.c,v 1.65 2009/01/01 17:23:42 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeMaterial.c,v 1.66 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -104,7 +104,7 @@ ExecMaterial(MaterialState *node)
 	slot = node->ss.ps.ps_ResultTupleSlot;
 	if (!eof_tuplestore)
 	{
-		if (tuplestore_gettupleslot(tuplestorestate, forward, slot))
+		if (tuplestore_gettupleslot(tuplestorestate, forward, false, slot))
 			return slot;
 		if (forward)
 			eof_tuplestore = true;

src/backend/executor/nodeWindowAgg.c

@@ -27,7 +27,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeWindowAgg.c,v 1.3 2009/01/01 17:23:42 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeWindowAgg.c,v 1.4 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -480,7 +480,8 @@ eval_windowaggregates(WindowAggState *winstate)
 			spool_tuples(winstate, winstate->aggregatedupto);
 			tuplestore_select_read_pointer(winstate->buffer,
 										   winstate->agg_ptr);
-			if (!tuplestore_gettupleslot(winstate->buffer, true, agg_row_slot))
+			if (!tuplestore_gettupleslot(winstate->buffer, true, true,
+										 agg_row_slot))
 				break;			/* must be end of partition */
 		}
 
@@ -1001,12 +1002,14 @@ restart:
 	/*
 	 * Read the current row from the tuplestore, and save in ScanTupleSlot.
 	 * (We can't rely on the outerplan's output slot because we may have to
-	 * read beyond the current row.)
+	 * read beyond the current row.  Also, we have to actually copy the row
+	 * out of the tuplestore, since window function evaluation might cause
+	 * the tuplestore to dump its state to disk.)
 	 *
 	 * Current row must be in the tuplestore, since we spooled it above.
 	 */
 	tuplestore_select_read_pointer(winstate->buffer, winstate->current_ptr);
-	if (!tuplestore_gettupleslot(winstate->buffer, true,
+	if (!tuplestore_gettupleslot(winstate->buffer, true, true,
 								 winstate->ss.ss_ScanTupleSlot))
 		elog(ERROR, "unexpected end of tuplestore");
 
@@ -1589,14 +1592,14 @@ window_gettupleslot(WindowObject winobj, int64 pos, TupleTableSlot *slot)
 
 	while (winobj->seekpos > pos)
 	{
-		if (!tuplestore_gettupleslot(winstate->buffer, false, slot))
+		if (!tuplestore_gettupleslot(winstate->buffer, false, true, slot))
 			elog(ERROR, "unexpected end of tuplestore");
 		winobj->seekpos--;
 	}
 
 	while (winobj->seekpos < pos)
 	{
-		if (!tuplestore_gettupleslot(winstate->buffer, true, slot))
+		if (!tuplestore_gettupleslot(winstate->buffer, true, true, slot))
 			elog(ERROR, "unexpected end of tuplestore");
 		winobj->seekpos++;
 	}

src/backend/executor/nodeWorktablescan.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.5 2009/01/01 17:23:42 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.6 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -43,6 +43,10 @@ WorkTableScanNext(WorkTableScanState *node)
 	 * worktable plan node, since it cannot appear high enough in the plan
 	 * tree of a scrollable cursor to be exposed to a backward-scan
 	 * requirement.  So it's not worth expending effort to support it.
+	 *
+	 * Note: we are also assuming that this node is the only reader of the
+	 * worktable.  Therefore, we don't need a private read pointer for the
+	 * tuplestore, nor do we need to tell tuplestore_gettupleslot to copy.
 	 */
 	estate = node->ss.ps.state;
 	Assert(ScanDirectionIsForward(estate->es_direction));
@@ -53,7 +57,7 @@ WorkTableScanNext(WorkTableScanState *node)
 	 * Get the next tuple from tuplestore. Return NULL if no more tuples.
 	 */
 	slot = node->ss.ss_ScanTupleSlot;
-	(void) tuplestore_gettupleslot(tuplestorestate, true, slot);
+	(void) tuplestore_gettupleslot(tuplestorestate, true, false, slot);
 	return slot;
 }
 

src/backend/tcop/pquery.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/pquery.c,v 1.129 2009/01/02 20:42:00 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/pquery.c,v 1.130 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1118,7 +1118,8 @@ RunFromStore(Portal portal, ScanDirection direction, long count,
 
 			oldcontext = MemoryContextSwitchTo(portal->holdContext);
 
-			ok = tuplestore_gettupleslot(portal->holdStore, forward, slot);
+			ok = tuplestore_gettupleslot(portal->holdStore, forward, false,
+										 slot);
 
 			MemoryContextSwitchTo(oldcontext);
 

src/backend/utils/sort/tuplestore.c

@@ -47,7 +47,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/sort/tuplestore.c,v 1.46 2009/01/01 17:23:53 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/sort/tuplestore.c,v 1.47 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -871,10 +871,17 @@ tuplestore_gettuple(Tuplestorestate *state, bool forward,
  *
  * If successful, put tuple in slot and return TRUE; else, clear the slot
  * and return FALSE.
+ *
+ * If copy is TRUE, the slot receives a copied tuple (allocated in current
+ * memory context) that will stay valid regardless of future manipulations of
+ * the tuplestore's state.  If copy is FALSE, the slot may just receive a
+ * pointer to a tuple held within the tuplestore.  The latter is more
+ * efficient but the slot contents may be corrupted if additional writes to
+ * the tuplestore occur.  (If using tuplestore_trim, see comments therein.)
  */
 bool
 tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
-						TupleTableSlot *slot)
+						bool copy, TupleTableSlot *slot)
 {
 	MinimalTuple tuple;
 	bool		should_free;
@@ -883,6 +890,11 @@ tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
 
 	if (tuple)
 	{
+		if (copy && !should_free)
+		{
+			tuple = heap_copy_minimal_tuple(tuple);
+			should_free = true;
+		}
 		ExecStoreMinimalTuple(tuple, slot, should_free);
 		return true;
 	}
@@ -1107,7 +1119,10 @@ tuplestore_trim(Tuplestorestate *state)
 	 * since tuplestore_gettuple returns a direct pointer to our
 	 * internal copy of the tuple, it's likely that the caller has
 	 * still got the tuple just before "current" referenced in a slot.
-	 * So we keep one extra tuple before the oldest "current".
+	 * So we keep one extra tuple before the oldest "current".  (Strictly
+	 * speaking, we could require such callers to use the "copy" flag to
+	 * tuplestore_gettupleslot, but for efficiency we allow this one case
+	 * to not use "copy".)
 	 */
 	nremove = oldest - 1;
 	if (nremove <= 0)

src/include/utils/tuplestore.h

@@ -24,7 +24,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/utils/tuplestore.h,v 1.28 2009/01/01 17:24:02 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/utils/tuplestore.h,v 1.29 2009/03/27 18:30:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -71,7 +71,7 @@ extern void tuplestore_trim(Tuplestorestate *state);
 extern bool tuplestore_in_memory(Tuplestorestate *state);
 
 extern bool tuplestore_gettupleslot(Tuplestorestate *state, bool forward,
-						TupleTableSlot *slot);
+						bool copy, TupleTableSlot *slot);
 extern bool tuplestore_advance(Tuplestorestate *state, bool forward);
 
 extern bool tuplestore_ateof(Tuplestorestate *state);