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

2 files changed, 76 insertions, 0 deletions

diff --git a/src/test/regress/expected/aggregates.out b/src/test/regress/expected/aggregates.out
index f0517f95b6..d8271da4d1 100644
--- a/src/test/regress/expected/aggregates.out
+++ b/src/test/regress/expected/aggregates.out
@@ -2759,6 +2759,43 @@ 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;
 SET parallel_setup_cost = 0;
diff --git a/src/test/regress/sql/aggregates.sql b/src/test/regress/sql/aggregates.sql
index 1783d19bd5..75c78be640 100644
--- a/src/test/regress/sql/aggregates.sql
+++ b/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;