BUG #16630: TupleDesc reference leak: TupleDesc 0x7fad89c9c928 (16386,-1) still referenced

The following bug has been logged on the website:

Bug reference:      16630
Logged by:          Philipp Menke
Email address:      pg@pmenke.de
PostgreSQL version: 13rc1
Operating system:   Linux
Description:

Hi there,

i was playing around with transaction control statements to make changes of
a long-running processing procedure visible and stumbled over the following
warning / error:

```
WARNING:  TupleDesc reference leak: TupleDesc 0x7f4207cbcf20 (16386,-1)
still referenced
WARNING:  TupleDesc reference leak: TupleDesc 0x7f4207cbcf20 (16386,-1)
still referenced
ERROR:  tupdesc reference 0x7f4207cbcf20 is not owned by resource owner
TopTransaction
```

As i don't think this is expected behavior when executing some "plain"
pl/pgsql, i figured i should report it.
I boiled the causing code down to the following example, which reproduces
the error:

```
CREATE TABLE test_output(tuple INT);
DECLARE test_cursor CURSOR WITH HOLD FOR SELECT * FROM generate_series(1,
1000);
DO LANGUAGE plpgsql $$
DECLARE
    v_counter INT;
    v_tuple test_output;
BEGIN
    CREATE TEMPORARY TABLE IF NOT EXISTS tmp_data (tuple INT);
    WHILE TRUE LOOP
        v_counter := 0;
        TRUNCATE tmp_data;
        EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
        WHILE v_tuple IS NOT NULL AND v_counter < 100 LOOP
            IF v_tuple IS NOT NULL THEN
                INSERT INTO tmp_data VALUES (v_tuple.*);
                v_counter := v_counter + 1;
            END IF;
            IF v_counter < 100 THEN
                EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
            END IF;
        END LOOP;
        IF v_counter > 0 THEN
            -- in my normal program i would have processed tmp_data here
            COMMIT;
        ELSE
            RETURN;
        END IF;
    END LOOP;
END;
$$;
```

I've tested this with PostgreSQL 12.4 as well as with 13-rc1 through the
following procedure:

In one terminal run: `docker run -e POSTGRES_HOST_AUTH_METHOD=trust
postgres:13-rc1`
In another terminal run: `docker exec -ttiu postgres $(docker ps -q)
/usr/bin/psql`
In this second terminal execute the example code:
```
psql (13rc1 (Debian 13~rc1-1.pgdg100+1))
Type "help" for help.

postgres=# CREATE TABLE test_output(tuple INT);
CREATE TABLE
postgres=# DECLARE test_cursor CURSOR WITH HOLD FOR SELECT * FROM
generate_series(1, 1000);
DECLARE CURSOR
postgres=# DO LANGUAGE plpgsql $$
postgres$# DECLARE
postgres$#     v_counter INT;
postgres$#     v_tuple test_output;
postgres$# BEGIN
postgres$#     CREATE TEMPORARY TABLE IF NOT EXISTS tmp_data (tuple INT);
postgres$#     WHILE TRUE LOOP
postgres$#         v_counter := 0;
postgres$#         TRUNCATE tmp_data;
postgres$#         EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
postgres$#         WHILE v_tuple IS NOT NULL AND v_counter < 100 LOOP
postgres$#             IF v_tuple IS NOT NULL THEN
postgres$#                 INSERT INTO tmp_data VALUES (v_tuple.*);
postgres$#                 v_counter := v_counter + 1;
postgres$#             END IF;
postgres$#             IF v_counter < 100 THEN
postgres$#                 EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
postgres$#             END IF;
postgres$#         END LOOP;
postgres$#         IF v_counter > 0 THEN
postgres$#             -- in my normal program i would have processed
tmp_data here
postgres$#             COMMIT;
postgres$#         ELSE
postgres$#             RETURN;
postgres$#         END IF;
postgres$#     END LOOP;
postgres$# END;
postgres$# $$;
WARNING:  TupleDesc reference leak: TupleDesc 0x7f0b2a60af20 (16386,-1)
still referenced
WARNING:  TupleDesc reference leak: TupleDesc 0x7f0b2a60af20 (16386,-1)
still referenced
ERROR:  tupdesc reference 0x7f0b2a60af20 is not owned by resource owner
TopTransaction
```

The server log (in the first terminal) will yield:
```
2020-09-23 08:11:06.619 UTC [1] LOG:  database system is ready to accept
connections
2020-09-23 08:11:16.135 UTC [75] WARNING:  TupleDesc reference leak:
TupleDesc 0x7f0b2a60af20 (16386,-1) still referenced
2020-09-23 08:11:16.135 UTC [75] CONTEXT:  PL/pgSQL function
inline_code_block line 22 at COMMIT
2020-09-23 08:11:16.136 UTC [75] WARNING:  TupleDesc reference leak:
TupleDesc 0x7f0b2a60af20 (16386,-1) still referenced
2020-09-23 08:11:16.136 UTC [75] CONTEXT:  PL/pgSQL function
inline_code_block line 22 at COMMIT
2020-09-23 08:11:16.178 UTC [75] ERROR:  tupdesc reference 0x7f0b2a60af20 is
not owned by resource owner TopTransaction
2020-09-23 08:11:16.178 UTC [75] STATEMENT:  DO LANGUAGE plpgsql $$
        DECLARE
            v_counter INT;
            v_tuple test_output;
        BEGIN
            CREATE TEMPORARY TABLE IF NOT EXISTS tmp_data (tuple INT);
            WHILE TRUE LOOP
                v_counter := 0;
                TRUNCATE tmp_data;
                EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
                WHILE v_tuple IS NOT NULL AND v_counter < 100 LOOP
                    IF v_tuple IS NOT NULL THEN
                        INSERT INTO tmp_data VALUES (v_tuple.*);
                        v_counter := v_counter + 1;
                    END IF;
                    IF v_counter < 100 THEN
                        EXECUTE 'FETCH FROM test_cursor' INTO v_tuple;
                    END IF;
                END LOOP;
                IF v_counter > 0 THEN
                    -- in my normal program i would have processed tmp_data
here
                    COMMIT;
                ELSE
                    RETURN;
                END IF;
            END LOOP;
        END;
        $$;
```

As i was just fooling around with the feature, this bug is not critical to
me personally. But i might deserve a look.

Thank you for your otherwise awesome product and kind regards,
Philipp