Fix pfree issue in presorted DISTINCT aggregate code

david-rowley · david-rowley · commit ec5a010ab25b · 2023-02-13T23:38:21.000+13:00
The logic in this area was recently changed in 7da5159, however, in that commit, I neglected to consider that the conditions in which we should pfree the old Datum needed to be updated after that change. This could result in trying to pfree a NULL value, as was demonstrated by Alexander Lakhin. Reported-by: Alexander Lakhin Discussion: https://postgr.es/m/4103db46-d888-6d1d-e88d-87c21ed99472@gmail.com
diff --git a/src/backend/executor/execExprInterp.c b/src/backend/executor/execExprInterp.c
@@ -4254,7 +4254,8 @@ ExecEvalPreOrderedDistinctSingle(AggState *aggstate, AggStatePerTrans pertrans)
 													pertrans->aggCollation,
 													pertrans->lastdatum, value))))
 	{
-		if (pertrans->haslast && !pertrans->inputtypeByVal)
+		if (pertrans->haslast && !pertrans->inputtypeByVal &&
+			!pertrans->lastisnull)
 			pfree(DatumGetPointer(pertrans->lastdatum));
 
 		pertrans->haslast = true;

Original file line number	Diff line number	Diff line change
`@@ -4254,7 +4254,8 @@ ExecEvalPreOrderedDistinctSingle(AggState *aggstate, AggStatePerTrans pertrans)`
`4254`	`4254`	`pertrans->aggCollation,`
`4255`	`4255`	`pertrans->lastdatum, value))))`
`4256`	`4256`	`{`
`4257`		`- if (pertrans->haslast && !pertrans->inputtypeByVal)`
	`4257`	`+ if (pertrans->haslast && !pertrans->inputtypeByVal &&`
	`4258`	`+ !pertrans->lastisnull)`
`4258`	`4259`	`pfree(DatumGetPointer(pertrans->lastdatum));`
`4259`	`4260`
`4260`	`4261`	`pertrans->haslast = true;`