Ensure result of an aggregate's finalfunc is made read-only.

The finalfunc might return a read-write expanded object.  If we
de-duplicate multiple call sites for the aggregate, any function(s)
receiving the aggregate result earlier could alter or destroy the
value that reaches the ones called later.  This is a brown-paper-bag
bug in commit 42b746d4c, because we actually considered the need
for read-only-ness but failed to realize that it applied to the case
with a finalfunc as well as the case without.

Per report from Justin Pryzby.  New error in HEAD,
no need for back-patch.

Discussion: https://postgr.es/m/ZDm5TuKsh3tzoEjz@telsasoft.com

src/backend/executor/nodeAgg.c

@@ -1040,9 +1040,10 @@ process_ordered_aggregate_multi(AggState *aggstate,
  * (But note that in some cases, such as when there is no finalfn, the
  * result might be a pointer to or into the agg's transition value.)
  *
- * The finalfn uses the state as set in the transno. This also might be
+ * The finalfn uses the state as set in the transno.  This also might be
  * being used by another aggregate function, so it's important that we do
- * nothing destructive here.
+ * nothing destructive here.  Moreover, the aggregate's final value might
+ * get used in multiple places, so we mustn't return a R/W expanded datum.
  */
 static void
 finalize_aggregate(AggState *aggstate,
@@ -1116,8 +1117,13 @@ finalize_aggregate(AggState *aggstate,
 		}
 		else
 		{
-			*resultVal = FunctionCallInvoke(fcinfo);
+			Datum		result;
+
+			result = FunctionCallInvoke(fcinfo);
 			*resultIsNull = fcinfo->isnull;
+			*resultVal = MakeExpandedObjectReadOnly(result,
+													fcinfo->isnull,
+													peragg->resulttypeLen);
 		}
 		aggstate->curperagg = NULL;
 	}
@@ -1165,6 +1171,7 @@ finalize_partialaggregate(AggState *aggstate,
 		else
 		{
 			FunctionCallInfo fcinfo = pertrans->serialfn_fcinfo;
+			Datum		result;
 
 			fcinfo->args[0].value =
 				MakeExpandedObjectReadOnly(pergroupstate->transValue,
@@ -1173,8 +1180,11 @@ finalize_partialaggregate(AggState *aggstate,
 			fcinfo->args[0].isnull = pergroupstate->transValueIsNull;
 			fcinfo->isnull = false;
 
-			*resultVal = FunctionCallInvoke(fcinfo);
+			result = FunctionCallInvoke(fcinfo);
 			*resultIsNull = fcinfo->isnull;
+			*resultVal = MakeExpandedObjectReadOnly(result,
+													fcinfo->isnull,
+													peragg->resulttypeLen);
 		}
 	}
 	else

src/backend/executor/nodeWindowAgg.c

@@ -623,10 +623,15 @@ finalize_windowaggregate(WindowAggState *winstate,
 		}
 		else
 		{
+			Datum		res;
+
 			winstate->curaggcontext = peraggstate->aggcontext;
-			*result = FunctionCallInvoke(fcinfo);
+			res = FunctionCallInvoke(fcinfo);
 			winstate->curaggcontext = NULL;
 			*isnull = fcinfo->isnull;
+			*result = MakeExpandedObjectReadOnly(res,
+												 fcinfo->isnull,
+												 peraggstate->resulttypeLen);
 		}
 	}
 	else

src/test/regress/expected/aggregates.out

@@ -2758,6 +2758,43 @@ SELECT balk(hundred) FROM tenk1;
      
 (1 row)
 
+ROLLBACK;
+-- test multiple usage of an aggregate whose finalfn returns a R/W datum
+BEGIN;
+CREATE FUNCTION rwagg_sfunc(x anyarray, y anyarray) RETURNS anyarray
+LANGUAGE plpgsql IMMUTABLE AS $$
+BEGIN
+    RETURN array_fill(y[1], ARRAY[4]);
+END;
+$$;
+CREATE FUNCTION rwagg_finalfunc(x anyarray) RETURNS anyarray
+LANGUAGE plpgsql STRICT IMMUTABLE AS $$
+DECLARE
+    res x%TYPE;
+BEGIN
+    -- assignment is essential for this test, it expands the array to R/W
+    res := array_fill(x[1], ARRAY[4]);
+    RETURN res;
+END;
+$$;
+CREATE AGGREGATE rwagg(anyarray) (
+    STYPE = anyarray,
+    SFUNC = rwagg_sfunc,
+    FINALFUNC = rwagg_finalfunc
+);
+CREATE FUNCTION eatarray(x real[]) RETURNS real[]
+LANGUAGE plpgsql STRICT IMMUTABLE AS $$
+BEGIN
+    x[1] := x[1] + 1;
+    RETURN x;
+END;
+$$;
+SELECT eatarray(rwagg(ARRAY[1.0::real])), eatarray(rwagg(ARRAY[1.0::real]));
+ eatarray  | eatarray  
+-----------+-----------
+ {2,1,1,1} | {2,1,1,1}
+(1 row)
+
 ROLLBACK;
 -- test coverage for aggregate combine/serial/deserial functions
 BEGIN;

src/test/regress/sql/aggregates.sql

@@ -1207,6 +1207,45 @@ SELECT balk(hundred) FROM tenk1;
 
 ROLLBACK;
 
+-- test multiple usage of an aggregate whose finalfn returns a R/W datum
+BEGIN;
+
+CREATE FUNCTION rwagg_sfunc(x anyarray, y anyarray) RETURNS anyarray
+LANGUAGE plpgsql IMMUTABLE AS $$
+BEGIN
+    RETURN array_fill(y[1], ARRAY[4]);
+END;
+$$;
+
+CREATE FUNCTION rwagg_finalfunc(x anyarray) RETURNS anyarray
+LANGUAGE plpgsql STRICT IMMUTABLE AS $$
+DECLARE
+    res x%TYPE;
+BEGIN
+    -- assignment is essential for this test, it expands the array to R/W
+    res := array_fill(x[1], ARRAY[4]);
+    RETURN res;
+END;
+$$;
+
+CREATE AGGREGATE rwagg(anyarray) (
+    STYPE = anyarray,
+    SFUNC = rwagg_sfunc,
+    FINALFUNC = rwagg_finalfunc
+);
+
+CREATE FUNCTION eatarray(x real[]) RETURNS real[]
+LANGUAGE plpgsql STRICT IMMUTABLE AS $$
+BEGIN
+    x[1] := x[1] + 1;
+    RETURN x;
+END;
+$$;
+
+SELECT eatarray(rwagg(ARRAY[1.0::real])), eatarray(rwagg(ARRAY[1.0::real]));
+
+ROLLBACK;
+
 -- test coverage for aggregate combine/serial/deserial functions
 BEGIN;