[Sqlalchemy-tickets] Issue #3376: SingletonThreadPool and threading.local() usage (zzzeek/sqlalchemy)

[André A. <iss...@bi...>]

New issue 3376: SingletonThreadPool and threading.local() usage
https://bitbucket.org/zzzeek/sqlalchemy/issue/3376/singletonthreadpool-and-threadinglocal

André Anjos:

I'm a bit puzzled by the SingletonThreadPool implementation. As per recommendation (see discussion on [this StackOverflow thread](http://stackoverflow.com/questions/1408171/thread-local-storage-in-python)), the call to `threading.local()` must be global, whereas in `sqlalchemy.pool`, it is rather local, at the constructor of the `SingletonThreadPool` object.

As a result, If many sessions are attempted to the database on the same process, the constructor of the SingletonThreadPool object will be triggered many times and a fake perception that the singleton does not exist will kick-in and trigger the re-construction of a connection.

After sometime, we start getting spurious messages:

```
Exception during reset or similar
Traceback (most recent call last):
  File "/idiap/project/beat/beat.env.develop/usr/lib/python2.7/site-packages/sqlalchemy/pool.py", line 568, in _finalize_fairy
    fairy._reset(pool)
  File "/idiap/project/beat/beat.env.develop/usr/lib/python2.7/site-packages/sqlalchemy/pool.py", line 702, in _reset
    pool._dialect.do_rollback(self)
  File "/idiap/project/beat/beat.env.develop/usr/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 412, in do_rollback
    dbapi_connection.rollback()
ProgrammingError: Cannot operate on a closed database.
```

We wonder if this is not the cause. Could somebody verify this please?