Fix dangling-pointer problem in before-row update trigger processing.

ExecUpdate checked for whether ExecBRUpdateTriggers had returned a new
tuple value by seeing if the returned tuple was pointer-equal to the old
one.  But the "old one" was in estate->es_junkFilter's result slot, which
would be scribbled on if we had done an EvalPlanQual update in response to
a concurrent update of the target tuple; therefore we were comparing a
dangling pointer to a live one.  Given the right set of circumstances we
could get a false match, resulting in not forcing the tuple to be stored in
the slot we thought it was stored in.  In the case reported by Maxim Boguk
in bug #5798, this led to "cannot extract system attribute from virtual
tuple" failures when trying to do "RETURNING ctid".  I believe there is a
very-low-probability chance of more serious errors, such as generating
incorrect index entries based on the original rather than the
trigger-modified version of the row.

In HEAD, change all of ExecBRInsertTriggers, ExecIRInsertTriggers,
ExecBRUpdateTriggers, and ExecIRUpdateTriggers so that they continue to
have similar APIs.  In the back branches I just changed
ExecBRUpdateTriggers, since there is no bug in the ExecBRInsertTriggers
case.

1 files changed, 5 insertions, 21 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 10a4b2ae876..7e12d6a1b02 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -1830,30 +1830,14 @@ ExecUpdate(TupleTableSlot *slot,
 	if (resultRelInfo->ri_TrigDesc &&
 		resultRelInfo->ri_TrigDesc->n_before_row[TRIGGER_EVENT_UPDATE] > 0)
 	{
-		HeapTuple	newtuple;
-
-		newtuple = ExecBRUpdateTriggers(estate, resultRelInfo,
-										tupleid, tuple);
+		slot = ExecBRUpdateTriggers(estate, resultRelInfo,
+									tupleid, slot);
 
-		if (newtuple == NULL)	/* "do nothing" */
+		if (slot == NULL)		/* "do nothing" */
 			return;
 
-		if (newtuple != tuple)	/* modified by Trigger(s) */
-		{
-			/*
-			 * Put the modified tuple into a slot for convenience of routines
-			 * below.  We assume the tuple was allocated in per-tuple memory
-			 * context, and therefore will go away by itself. The tuple table
-			 * slot should not try to clear it.
-			 */
-			TupleTableSlot *newslot = estate->es_trig_tuple_slot;
-
-			if (newslot->tts_tupleDescriptor != slot->tts_tupleDescriptor)
-				ExecSetSlotDescriptor(newslot, slot->tts_tupleDescriptor);
-			ExecStoreTuple(newtuple, newslot, InvalidBuffer, false);
-			slot = newslot;
-			tuple = newtuple;
-		}
+		/* trigger might have changed tuple */
+		tuple = ExecMaterializeSlot(slot);
 	}
 
 	/*