[Imaplib2-devel] imaplib2 might hang

[Nicolas S. <nic...@la...>]

  Hi Piers,

I've done debug sessions to understand why offlineimap can hang. It
appears one of our thread might hang waiting for imaplib2 which never
returns a value.

This happens when I reach the (undocumented) maximum number of
connections to my IMAP server. I guess they limit connections based on
the public IP or similar. It looks like IMAP servers "handle" this case
in quite different ways.

I believe that Gmail drops a previously existing connection.

imap.laposte.net will hold the connection without ever returning a
welcome message. This sucks hard but I wonder other servers have similar
behaviour.

Hence, imaplib2 is blocked at line 396 in __init__:

  self.welcome = self._request_push(name='welcome', tag='continuation').get_response(...)

I wonder if imaplib2 should not assume a response within the global
threading.TIMEOUT_MAX range which is "very" long (considered infinite).
I know that socket.setdefaulttimeout() can handle this, too. We
currently allow the users to configure a value for this. However, both
are expected large enough to avoid reaching the timeout while it should
not.

IOW, I think some very special cases at decisive steps might use a short
timeout while waiting for a response. The above line 396 falls into this
case. We know that the time required for a welcome message is definetly
shorter than for downloading/fetching a mail. So, we might like to add a
"max_timeout" argument to get_response() at line 191. By using short
timeouts when applicable, we would avoid hangs or long waits.


Also, I wonder that waiting for the welcome message might not be the
only one case. Some IMAP servers might behave in a similar way for other
limits: e.g. at authentication time to limit connections in a
per-account basis.

Thoughts?

-- 
Nicolas Sebrecht