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Date: Thu, 21 Aug 2014 18:44:02 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: connect() hangs on a listen()ing AF_UNIX socket
Message-ID: <20140821164402.GB21065@calimero.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
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References: <53F61B70 DOT 2020600 AT t-online DOT de>
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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.  ...
>=20
> Due to the following problem, Postfix hangs during startup (and blocks any
> possible "[ITP] postfix ..."):
>=20
> 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.
>=20
> Testcase:
>=20
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/socket.h>
> #include <sys/un.h>
>=20
> int main()
> {
>   sockaddr_un sa =3D {AF_UNIX, "testsocket"};
>   unlink(sa.sun_path);
>=20
>   int sd1 =3D socket(AF_UNIX, SOCK_STREAM, 0);
>   if (sd1 < 0) {
>     perror("socket"); return 1;
>   }
>   if (bind(sd1, (sockaddr*) &sa, sizeof(sa))) {
>     perror("bind"); return 1;
>   }
>   if (listen(sd1, 10) < 0) {
>     perror("listen"); return 1;
>   }
>=20
>   int sd2 =3D socket(AF_UNIX, SOCK_STREAM, 0);
>   if (sd2 < 0) {
>     perror("socket"); return 1;
>   }
>   printf("connecting to %s ...\n", sa.sun_path);
>=20
>   // Cygwin hangs here:
>   if (connect(sd2, (sockaddr*) &sa, sizeof(sa))) {
>     perror("connect"); return 1;
>   }
>=20
>   // Linux & friends arrive here:
>   printf("connected\n");
>   return 0;
> }
>=20
>=20
> 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.
>=20
> 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 :(

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.

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


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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