From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: Hangs on connect to UNIX socket being listened on in the same process (was: Cygwin hanging in pselect)
Date: Mon, 09 Jan 2017 17:16:00 -0000 [thread overview]
Message-ID: <20170109171635.GB26337@calimero.vinschen.de> (raw)
In-Reply-To: <CAOTD34b-h4WyYiUtA8sizN6riPe1YsDWdaypDNbODC71xv13UQ@mail.gmail.com>
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On Jan 9 16:46, Erik Bray wrote:
> Hi Corinna,
>
> Thanks for the response.
>
> On Mon, Jan 9, 2017 at 3:13 PM, Corinna Vinschen wrote:
> > Right. It has to do with how connect/accept works on AF_LOCAL sockets.
> > The handshake doesn't work well for situations like yours, where the
> > same thread tries to connect and accept on the same socket.
>
> Actually I'm not entirely sure now that that's the issue, even
> considering that this has come up before. Or at the very least,
> there's an additional issue. I realized that when I tried separate
> client/server processes, in the server I had put an accept() call at
> the end so it would block there. With the server waiting to accept a
> connection it succeeded. However, when I replaced the accept() with a
> long sleep(), the client's connect() never returns.
That's because connect infinitely waits for the accept to reply the
second half of the handshake.
> IIUC the handshake can't succeed until and unless the server accepts a
> connection from the client.
This is exactly the underlying problem. And interesting enough, even
though the handshake is in Cygwin since 2001, we never had a problem
with this until Christian started porting postfix in 2014!
> I almost wonder if the server side in this case
> shouldn't start up a thread to accept the af_local handshake, but you
> would know better.
No, I don't. We discussed this issue briefly back in 2014, but as
you can see we don't have a solution for this border case yet.
Starting a thread may or may not work, but there are a couple of
use-cases to keep in mind (which I can't reproduce off the top of my head).
The old postfix cygwin-apps thread from 2014 might give you some idea.
> > This has been found a problem in porting postfix already and at the time
> > we added a patch to circumvent the problem. Before calling connect, add
> > this:
> >
> > setsockopt (sock_server, SOL_SOCKET, SO_PEERCRED, NULL, 0);
> > setsockopt (sock_client, SOL_SOCKET, SO_PEERCRED, NULL, 0);
> >
> > This is, of course, a hack. The problem here is that server and client
> > of a socket are independent of each other, and there's typically no
> > way to know which process created the server side unless you already
> > are connected. Chicken/egg.
>
> I tried it and it worked, both in the single process and separate
> process examples. I see now--this sets
> fhandler_socket::no_getpeerid=true, so it doesn't have to do the
> handshake at all.
Right. A better solution for the problem would be nice. Ultimately
we want to check if the other side of the socket is actually a Cygwin
process which knows the secret, not a stray native Windows process
which accidentally hopped on the bandwagon, and we want to exchange
the credentials so a subsequent SO_PEERCRED call returns correct values.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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next prev parent reply other threads:[~2017-01-09 17:16 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-09 13:29 Erik Bray
2017-01-09 14:13 ` Corinna Vinschen
2017-01-09 15:46 ` Erik Bray
2017-01-09 17:16 ` Corinna Vinschen [this message]
2017-01-12 10:59 ` Erik Bray
2017-01-12 22:13 ` Corinna Vinschen
2017-01-13 0:54 ` Michael Enright
2017-01-13 8:42 ` Corinna Vinschen
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