[PATCH] plpython function causes server panic

Hi hackers,
I found a problem when executing the plpython function:
After the plpython function returns an error, in the same session, if we
continue to execute
plpython function, the server panic will be caused.

*Reproduce*
preparation

SET max_parallel_workers_per_gather=4;
SET parallel_setup_cost=1;
SET min_parallel_table_scan_size ='4kB';

CREATE TABLE t(i int);
INSERT INTO t SELECT generate_series(1, 10000);

CREATE EXTENSION plpython3u;
CREATE OR REPLACE FUNCTION test_func() RETURNS SETOF int AS
$$
plpy.execute("select pg_backend_pid()")

for i in range(0, 5):
    yield (i)

$$ LANGUAGE plpython3u parallel safe;

execute the function twice in the same session

postgres=# SELECT test_func() from t where i>10 and i<100;
ERROR:  error fetching next item from iterator
DETAIL:  Exception: cannot start subtransactions during a parallel
operation
CONTEXT:  Traceback (most recent call last):
PL/Python function "test_func"

postgres=# SELECT test_func();
server closed the connection unexpectedly
       This probably means the server terminated abnormally
       before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.

*Analysis*

   - There is an SPI call in test_func(): plpy.execute().
   - Then the server will start a subtransaction by
   PLy_spi_subtransaction_begin(); BUT! The Python processor does not know
   whether an error happened during PLy_spi_subtransaction_begin().
   - If there is an error that occurs in PLy_spi_subtransaction_begin(),
   the SPI call will be terminated but the python error indicator won't be set
   and the PyObject won't be free.
   - Then the next plpython UDF in the same session will fail due to the
   wrong Python environment.


*Solution*
Use try-catch to catch the error that occurs in
PLy_spi_subtransaction_begin(), and set the python error indicator.

With Regards
Hao Zhang