[FFI] interference of FFI with poll(3)?

[Nick Rudnick]

Hello Edward,

thanks a lot ... this resembles very much to what I am experiencing ... :-)

A nice weekend, Nick

2012/4/29 Edward Z. Yang <ezyang at mit.edu>

> Hello Nick,
>
> The GHC runtime system uses alarm signals for a variety of purposes,
> and part of this means that your code needs to properly handle
> EINTR.  Please see:
> http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/Signals
> for more information.
>
> Cheers,
> Edward
>
> Excerpts from Nick Rudnick's message of Sat Apr 28 07:21:30 -0400 2012:
> > Dear all,
> >
> > after noticing problems with libssh2, and trying to fix this myself, I
> ran
> > into a strange experience which I wish to get an explanation for.
> >
> > After compiling an OpenSSH server and a raw C libssh2 (for comparison)
> with
> > debug messaging, I with support of the libssh2 community was able to
> trace
> > the problem back to a call to poll(3) in
> session.c::_libssh2_wait_socket(),
> >
> >    rc = poll(sockets, 1, has_timeout?ms_to_next: -1);
> >
> > where sockets consists of a single socket, session->socket_fd.
> >
> > This is roughly a polling with timeout for the connection – and, with the
> > Haskell FFI, an
> >
> >   error 4 / EINTR / Interrupted system call
> >
> > is thrown, and I was explained that this probably is caused by another
> > signal of the same code unit. Not finding anything, I at the end extended
> > libssh2 by a function,
> >
> > LIBSSH2_API void libssh2_test(void){
> >   struct sockaddr_in sin;
> >   LIBSSH2_SESSION *session;
> >   const char *fingerprint;
> >   LIBSSH2_CHANNEL *channel;
> >   const unsigned long hostaddr= htonl(0x7F000001);
> >   const char *username= "i";
> >   const char *keyfile1="/home/i/.ssh/id_rsa.pub";
> >   const char *keyfile2="/home/i/.ssh/id_rsa";
> >   const char *password= "D0r1nha23";
> >   int got= 0;
> >   int sock= socket(AF_INET, SOCK_STREAM, 0);
> >   sin.sin_family= AF_INET;
> >   sin.sin_port= htons(22);
> >   sin.sin_addr.s_addr= hostaddr;
> >   if(connect( sock, (struct sockaddr*)(&sin), sizeof(struct sockaddr_in)
> >             ) != 0 ) {
> >     fprintf(stderr, "failed to connect!\n");
> >     return;
> >   }
> >   session= libssh2_session_init();
> >   libssh2_trace(session,~0);
> >   if(libssh2_session_handshake(session, sock)) {
> >     _libssh2_debug(session, LIBSSH2_TRACE_TRANS
> >                   , "Failure establishing SSH session" );
> >     return;
> >   }
> >   fingerprint= libssh2_hostkey_hash(session, LIBSSH2_HOSTKEY_HASH_SHA1);
> >   libssh2_userauth_list(session, username, strlen(username)); // ??
> >   if(libssh2_userauth_publickey_fromfile( session
> >                                         , username
> >                                         , keyfile1
> >                                         , keyfile2
> >                                         , password )) {
> >     _libssh2_debug(session, LIBSSH2_TRACE_TRANS
> >                   , "\tAuthentication by public key failed!" );
> >     return;
> >   } else {
> >     _libssh2_debug( session, LIBSSH2_TRACE_TRANS
> >                   , "\tAuthentication by public key succeeded." );
> >     if(!(channel= libssh2_channel_open_session(session))) {
> >       _libssh2_debug( session, LIBSSH2_TRACE_TRANS
> >                     , "Unable to open a session" );
> >       return;
> >     } else {
> >       libssh2_channel_setenv(channel, "FOO", "bar");
> >       if(libssh2_channel_request_pty(channel, "vanilla")) {
> >         _libssh2_debug( session, LIBSSH2_TRACE_TRANS
> >                       , "Failed requesting pty" );
> >       } else {
> >         if(libssh2_channel_shell(channel)) {
> >           _libssh2_debug( session, LIBSSH2_TRACE_TRANS
> >                         , "Unable to request shell on allocated pty" );
> >         } else {
> >           if(channel){
> >             libssh2_channel_free(channel);
> >             channel= NULL;
> >           }
> >         }
> >       }
> >     }
> >   }
> >   libssh2_session_disconnect( session
> >                             , "Normal Shutdown, Thank you for playing" );
> >   libssh2_session_free(session);
> >   close(sock);
> >   libssh2_exit();
> >   return;
> > }
> >
> > and called it by
> >
> > foreign import ccall unsafe "libssh2_test"
> >   libssh2Test:: IO ()
> >
> > as well as
> >
> > {# context lib="ssh2" prefix="libssh2" #}
> > {# fun test as test { } -> `()' #}
> >
> > With both approaches, I still got the same EINTR error, while coalling
> > libssh2_test() from C works completely flawless.
> >
> > Is it possible that an interfering signal comes from the FFI? If yes, is
> > there a workaround?
> >
> > Grateful for any kind of enlightenment... :-)
> >
> > Thanks a lot in advance, Nick
>
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