tcp.startListening can throw socket.error for EADDRINUSE on linux

	nickw reported
Trac ID	trac#7688
Type	defect
Created	2014-10-17 03:30:02Z

http://twistedmatrix.com/trac/browser/trunk/twisted/internet/tcp.py#L974

The code here expects that an exception for a re-used port will be thrown during the socket.bind; but on Linux one can bind the same port twice unless there is an active listening socket bound to the address.

The exception occurring only at the socket.listen when SO_REUSEADDR is set, which is basically always in Twisted.

The code here anticipates that an exception for a re-used port will be thrown during the socket.bind; but (on Linux) one can bind the same port twice unless there is an active listening socket bound to the address.

The exception occurring only at the socket.listen when SO_REUSEADDR is set, which is always in Twisted.

For example, with two interleaved processes:

• process A: bind (success)
• process B: bind (success)
• process A: listen (success)
• process B: listen (socket.error: [Errno 98] Address already in use)

The socket.listen should also be inside the exception handler; it's a bit strange to have a CannotListenError exception that's not thrown in some cases of being unable to listen.

trac-id__7688 7688
type__defect defect
reporter__nickw nickw
priority__normal normal
milestone__ 
branch__ 
branch_author__ 
status__new new
resolution__None None
component__core core
keywords__socket_listen socket listen
time__1413516602442761 1413516602442761
changetime__1413516602442761 1413516602442761
version__None None
owner__None None

Start with twisted/internet/tcp.py around line 974 and inspect the exception handler around socket.bind and socket.listen. Reproduce the interleaved two-process sequence described in the issue, then verify that an address-in-use failure from listen is reported as CannotListenError rather than socket.error.