[Sqlalchemy-tickets] Issue #4184: `MySQL server has gone away` when raising DisconnectionError in checkout (zzzeek/sqlalchemy)

[Sylvain D. <iss...@bi...>]

New issue 4184: `MySQL server has gone away` when raising DisconnectionError in checkout
https://bitbucket.org/zzzeek/sqlalchemy/issues/4184/mysql-server-has-gone-away-when-raising

Sylvain Duchesne:

The following code should trigger the error:


```
#!python

import logging
logging.basicConfig(level=logging.DEBUG)
import time
from sqlalchemy import create_engine
from sqlalchemy.exc import DisconnectionError
import sqlalchemy
e = create_engine("mysql://test:test@localhost/test", echo=True)

def _check_out(dbapi_connection, connection_record, connection_proxy):
 logging.getLogger().info("checking out")
 checkout_time = connection_record.info.get('checkout_time')
 now = time.time()
 if checkout_time and now - checkout_time > 1:
  logging.getLogger().info("connection expired")
  raise DisconnectionError()
 connection_record.info['checkout_time'] = now

sqlalchemy.event.listen(e, 'checkout', _check_out)

r = e.execute("select 1")
del r
time.sleep(2)
r = e.execute("select 1")
del r
```

Executing the last execute should yield the following output:

```
#!python

>>> del r
>>> time.sleep(2)
>>> r = e.execute("select 1")
INFO:root:checking out
INFO:root:connection expired
INFO:root:checking out
2018-02-08 11:27:52,270 INFO sqlalchemy.engine.base.Engine select 1
INFO:sqlalchemy.engine.base.Engine:select 1
2018-02-08 11:27:52,272 INFO sqlalchemy.engine.base.Engine ()
INFO:sqlalchemy.engine.base.Engine:()
>>> del r
ERROR:sqlalchemy.pool.QueuePool:Exception during reset or similar
Traceback (most recent call last):
  File "F:\rdv_git\rendezvous\venv\rendezvous\lib\site-packages\sqlalchemy\pool.py", line 703, in _finalize_fairy
    fairy._reset(pool)
  File "F:\rdv_git\rendezvous\venv\rendezvous\lib\site-packages\sqlalchemy\pool.py", line 873, in _reset
    pool._dialect.do_rollback(self)
  File "F:\rdv_git\rendezvous\venv\rendezvous\lib\site-packages\sqlalchemy\dialects\mysql\base.py", line 1775, in do_rollback
    dbapi_connection.rollback()
OperationalError: (2006, 'MySQL server has gone away')
>>>
```

I traced the error back to the `weakref` 
https://bitbucket.org/zzzeek/sqlalchemy/src/a54b3bb0a8a3a33c4c68349484ef4f3dc915416e/lib/sqlalchemy/pool.py?at=master&fileviewer=file-view-default#pool.py-540
which has its callback bound with the `dbapi connection` that will be closed.
When the gc passes and the callback gets called, 
https://bitbucket.org/zzzeek/sqlalchemy/src/a54b3bb0a8a3a33c4c68349484ef4f3dc915416e/lib/sqlalchemy/pool.py?at=master&fileviewer=file-view-default#pool.py-703
the `dbapi connection` happens to be closed.

Due to a lack of time (and having already spent quite a bit debugging), I am not able to create a PR. Also, better let the experts tackle the issue (I would verify if the connection is already closed prior to calling `reset`).