Fix leaks with functions in DML

If plpgsql functions are used in DML queries then we were leaking 8KB
for every invocation of that function. This can quickly add up.

The issue was that the "CurTransactionContext" was not getting cleaned
up after every invocation. The reason was that we were inadvertantly
allocating a temporary list in that context. Postgres then thought that
this CurTransactionContext needs to be re-used further and kept it
around. We now use a proper memory context to avoid this.

Fixes #7053

Author: nikkhils

.unreleased/fix_7088

@@ -0,0 +1,2 @@
+Fixes: #7088 Fix leaks with functions in DML
+Thanks: @Kazmirchuk for reporting this

src/cache.c

@@ -229,8 +229,16 @@ release_subtxn_pinned_caches(SubTransactionId subtxnid, bool abort)
 {
 	ListCell *lc;
 
-	/* Need a copy because cache_release will modify pinned_caches */
+	/*
+	 * Need a copy because cache_release will modify pinned_caches.
+	 *
+	 * This needs to be allocated in pinned cache memory context.
+	 * Otherwise leaks ensue if CurTransactionContext (which is the
+	 * CurrentMemoryContext) gets used!
+	 */
+	MemoryContext old = MemoryContextSwitchTo(pinned_caches_mctx);
 	List *pinned_caches_copy = list_copy(pinned_caches);
+	MemoryContextSwitchTo(old);
 
 	/* Only release caches created in subtxn */
 	foreach (lc, pinned_caches_copy)

test/expected/copy_memory_usage.out

@@ -59,3 +59,40 @@ select * from portal_memory_log where (
 ----+-------
 (0 rows)
 
+-- Test plpgsql leaks
+CREATE TABLE test_ht(tm timestamptz, val float8);
+SELECT * FROM create_hypertable('test_ht', 'tm');
+NOTICE:  adding not-null constraint to column "tm"
+ hypertable_id | schema_name | table_name | created 
+---------------+-------------+------------+---------
+             2 | public      | test_ht    | t
+(1 row)
+
+-- Use a plpgsql function to insert into the hypertable
+CREATE OR REPLACE FUNCTION to_double(_in text, INOUT _out double precision)
+LANGUAGE plpgsql IMMUTABLE parallel safe
+AS $$
+BEGIN
+    SELECT CAST(_in AS double precision) INTO _out;
+EXCEPTION WHEN others THEN
+    --do nothing: _out already carries default
+END;
+$$;
+-- TopTransactionContext usage needs to remain the same after every insert
+-- There was a leak earlier in the child CurTransactionContext
+BEGIN;
+INSERT INTO test_ht VALUES ('1980-01-01 00:00:00-00', to_double('23.11', 0));
+SELECT sum(total_bytes) from pg_backend_memory_contexts where parent = 'TopTransactionContext';
+  sum  
+-------
+ 16384
+(1 row)
+
+INSERT INTO test_ht VALUES ('1980-02-01 00:00:00-00', to_double('24.11', 0));
+SELECT sum(total_bytes) from pg_backend_memory_contexts where parent = 'TopTransactionContext';
+  sum  
+-------
+ 16384
+(1 row)
+
+COMMIT;

test/sql/copy_memory_usage.sql

@@ -56,3 +56,25 @@ select * from portal_memory_log where (
         from portal_memory_log
 );
 
+-- Test plpgsql leaks
+CREATE TABLE test_ht(tm timestamptz, val float8);
+SELECT * FROM create_hypertable('test_ht', 'tm');
+-- Use a plpgsql function to insert into the hypertable
+CREATE OR REPLACE FUNCTION to_double(_in text, INOUT _out double precision)
+LANGUAGE plpgsql IMMUTABLE parallel safe
+AS $$
+BEGIN
+    SELECT CAST(_in AS double precision) INTO _out;
+EXCEPTION WHEN others THEN
+    --do nothing: _out already carries default
+END;
+$$;
+
+-- TopTransactionContext usage needs to remain the same after every insert
+-- There was a leak earlier in the child CurTransactionContext
+BEGIN;
+INSERT INTO test_ht VALUES ('1980-01-01 00:00:00-00', to_double('23.11', 0));
+SELECT sum(total_bytes) from pg_backend_memory_contexts where parent = 'TopTransactionContext';
+INSERT INTO test_ht VALUES ('1980-02-01 00:00:00-00', to_double('24.11', 0));
+SELECT sum(total_bytes) from pg_backend_memory_contexts where parent = 'TopTransactionContext';
+COMMIT;