connect() hangs on a listen()ing AF_UNIX socket

Christian Franke
Thu Aug 21 19:14:00 GMT 2014

Corinna Vinschen wrote:
> On Aug 21 18:16, Christian Franke wrote:
>> Corinna Vinschen wrote (in thread "[ITP] libsuexec 1.0"):
>>> Postfix for Cygwin would be *so* nice.  Sigh.  ...
>> Due to the following problem, Postfix hangs during startup (and blocks any
>> possible "[ITP] postfix ..."):
>> If a AF_UNIX socket is in listen()ing state, a client connect() should
>> succeed immediately. On Cygwin, connect() waits until the server site
>> accept()s the connection.
>> Testcase:
>> ...
>> This is likely because fhandler_socket::af_local_connect() waits for some
>> secret. Sending it in af_local_accept() is too late in this case.
>> Unfortunately the event handling of postfix relies on the correct behavior
>> and there is possibly no easy workaround.
> Off the top of my head I don't see one inside the Cygwin DLL :(

Complex but may work: A fhandler_socket::listen() on a 
AF_UNIX/SOCK_STREAM socket starts a thread which accept()s connections, 
performs the handshake and puts the new socket descs in a queue. 
fhandler_socket::accept4() then no longer calls accept() but waits for 
the next entry in the queue.

> The problem is that the package exchange at the start of an
> accept/connect is required to be able to exchange credentials.  This in
> turn is required for getpeereid and the SO_PEERCRED socket option which
> is utilized at least by sshd.

Easier and may work for Postfix: Add a Cygwin specific socket option 
like SO_DONT_NEED_PEERCRED which is set immediately after Postfix calls 
socket(AF_UNIX, SOCK_STREAM). If set, no handshake occurs on 
connect()/accept(). getpeerid()/SO_PEERCRED should fail then.

> Would it help to patch postfix for Cygwin to use a local-only AF_INET
> socket at this point instead?

Postfix heavily relies on unix domain sockets (see so this 
would possibly be complex. Such a patch would likely not be accepted 


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