Segfault on too many accesses to readonly

[sam-s]

I get

*** Error in `/.../bin/python3': free(): invalid pointer: 0x00007fb1336e7760 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x81499)[0x7fb1325ee499]
/usr/lib64/libpython3.6m.so.1.0(_PyEval_EvalFrameDefault+0xa69e)[0x7fb1333a902e]
/usr/lib64/libpython3.6m.so.1.0(PyEval_EvalCodeEx+0x1f2)[0x7fb1333aa822]
/usr/lib64/libpython3.6m.so.1.0(PyEval_EvalCode+0x3b)[0x7fb1333ab4cb]
/usr/lib64/libpython3.6m.so.1.0(+0x1d1eee)[0x7fb133430eee]
/usr/lib64/libpython3.6m.so.1.0(PyRun_FileExFlags+0xa4)[0x7fb1332e5ac2]
/usr/lib64/libpython3.6m.so.1.0(PyRun_SimpleFileExFlags+0x37a)[0x7fb1332e5e72]
/usr/lib64/libpython3.6m.so.1.0(Py_Main+0x88c)[0x7fb133437cfc]
/home/sds/.virtualenvs/algorisk/bin/python3(main+0x119)[0x400b19]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7fb13258f445]
/.../bin/python3[0x400ca1]

when I look at connection.readonly too many times.

The code is:

import psycopg2 as pg
from psycopg2 import extras

conn = pg.connect("...")

conn.set_session(autocommit=False, readonly=False)

with conn.cursor(name="test") as cu:
    cu.execute("select * from twits order by sid limit 11000;")
    for sid,rec in cu:
        if conn.readonly:
            raise ValueError("immutable")
        with conn.cursor() as cu1:
            cu1.execute("INSERT INTO twits (sid, status) VALUES (%s, %s) ON CONFLICT"
                        " (sid) DO UPDATE SET status = excluded.status;",
                        (sid, extras.Json(rec)))
        if cu.rownumber == cu.itersize:
            print(sid)

The table is created by

CREATE TABLE twits (
    sid bigint NOT NULL PRIMARY KEY,
    status jsonb NOT NULL
);

when I remove

        if conn.readonly:
            raise ValueError("immutable")

the problem disappears.
When I remove conn.set_session(autocommit=False, readonly=False), the crash turns into Fatal Python error: deallocating None.

PS. I check readonly because the connection is shared between threads.

[dvarrazzo]

A simpler test to reproduce the crash:

for i in range(10000):
    conn.readonly

fix on the way, thank you for the report :)