Fix edge case leading to agg transitions skipping ExecAggTransReparent() calls.

The code checking whether an aggregate transition value needs to be
reparented into the current context has always only compared the
transition return value with the previous transition value by datum,
i.e. without regard for NULLness.  This normally works, because when
the transition function returns NULL (via fcinfo->isnull), it'll
return a value that won't be the same as its input value.

But there's no hard requirement that that's the case. And it turns
out, it's possible to hit this case (see discussion or reproducers),
leading to a non-null transition value not being reparented, followed
by a crash caused by that.

Instead of adding another comparison of NULLness, instead have
ExecAggTransReparent() ensure that pergroup->transValue ends up as 0
when the new transition value is NULL. That avoids having to add an
additional branch to the much more common cases of the transition
function returning the old transition value (which is a pointer in
this case), and when the new value is different, but not NULL.

In branches since 69c3936a, also deduplicate the reparenting code
between the expression evaluation based transitions, and the path for
ordered aggregates.

Reported-By: Teodor Sigaev, Nikita Glukhov
Author: Andres Freund
Discussion: https://postgr.es/m/bd34e930-cfec-ea9b-3827-a8bc50891393@sigaev.ru
Backpatch: 9.4-, this issue has existed since at least 7.4

src/backend/executor/execExprInterp.c

@@ -1742,6 +1742,15 @@ ExecInterpExpr(ExprState *state, ExprContext *econtext, bool *isnull)
 			 * anything.  Also, if transfn returned a pointer to a R/W
 			 * expanded object that is already a child of the aggcontext,
 			 * assume we can adopt that value without copying it.
+			 *
+			 * It's safe to compare newVal with pergroup->transValue without
+			 * regard for either being NULL, because ExecAggTransReparent()
+			 * takes care to set transValue to 0 when NULL. Otherwise we could
+			 * end up accidentally not reparenting, when the transValue has
+			 * the same numerical value as newValue, despite being NULL.  This
+			 * is a somewhat hot path, making it undesirable to instead solve
+			 * this with another branch for the common case of the transition
+			 * function returning its (modified) input argument.
 			 */
 			if (DatumGetPointer(newVal) != DatumGetPointer(pergroup->transValue))
 				newVal = ExecAggTransReparent(aggstate, pertrans,
@@ -4186,6 +4195,8 @@ ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,
 					 Datum newValue, bool newValueIsNull,
 					 Datum oldValue, bool oldValueIsNull)
 {
+	Assert(newValue != oldValue);
+
 	if (!newValueIsNull)
 	{
 		MemoryContextSwitchTo(aggstate->curaggcontext->ecxt_per_tuple_memory);
@@ -4199,6 +4210,16 @@ ExecAggTransReparent(AggState *aggstate, AggStatePerTrans pertrans,
 								 pertrans->transtypeByVal,
 								 pertrans->transtypeLen);
 	}
+	else
+	{
+		/*
+		 * Ensure that AggStatePerGroup->transValue ends up being 0, so
+		 * callers can safely compare newValue/oldValue without having to
+		 * check their respective nullness.
+		 */
+		newValue = (Datum) 0;
+	}
+
 	if (!oldValueIsNull)
 	{
 		if (DatumIsReadWriteExpandedObject(oldValue,

src/backend/executor/nodeAgg.c

@@ -221,6 +221,7 @@
 #include "catalog/pg_aggregate.h"
 #include "catalog/pg_proc.h"
 #include "catalog/pg_type.h"
+#include "executor/execExpr.h"
 #include "executor/executor.h"
 #include "executor/nodeAgg.h"
 #include "miscadmin.h"
@@ -626,33 +627,22 @@ advance_transition_function(AggState *aggstate,
 	 * first input, we don't need to do anything.  Also, if transfn returned a
 	 * pointer to a R/W expanded object that is already a child of the
 	 * aggcontext, assume we can adopt that value without copying it.
+	 *
+	 * It's safe to compare newVal with pergroup->transValue without
+	 * regard for either being NULL, because ExecAggTransReparent()
+	 * takes care to set transValue to 0 when NULL. Otherwise we could
+	 * end up accidentally not reparenting, when the transValue has
+	 * the same numerical value as newValue, despite being NULL.  This
+	 * is a somewhat hot path, making it undesirable to instead solve
+	 * this with another branch for the common case of the transition
+	 * function returning its (modified) input argument.
 	 */
 	if (!pertrans->transtypeByVal &&
 		DatumGetPointer(newVal) != DatumGetPointer(pergroupstate->transValue))
-	{
-		if (!fcinfo->isnull)
-		{
-			MemoryContextSwitchTo(aggstate->curaggcontext->ecxt_per_tuple_memory);
-			if (DatumIsReadWriteExpandedObject(newVal,
-											   false,
-											   pertrans->transtypeLen) &&
-				MemoryContextGetParent(DatumGetEOHP(newVal)->eoh_context) == CurrentMemoryContext)
-				 /* do nothing */ ;
-			else
-				newVal = datumCopy(newVal,
-								   pertrans->transtypeByVal,
-								   pertrans->transtypeLen);
-		}
-		if (!pergroupstate->transValueIsNull)
-		{
-			if (DatumIsReadWriteExpandedObject(pergroupstate->transValue,
-											   false,
-											   pertrans->transtypeLen))
-				DeleteExpandedObject(pergroupstate->transValue);
-			else
-				pfree(DatumGetPointer(pergroupstate->transValue));
-		}
-	}
+		newVal = ExecAggTransReparent(aggstate, pertrans,
+									  newVal, fcinfo->isnull,
+									  pergroupstate->transValue,
+									  pergroupstate->transValueIsNull);
 
 	pergroupstate->transValue = newVal;
 	pergroupstate->transValueIsNull = fcinfo->isnull;