A frozen client can hang the whole server

I can't reproduce this, everything works as expected for me, with both Ctrl+Q and SIGSTOP.

just tried and could not reproduce several time in a row, triangle was stopped but mouse was able to move

Reproduced again with the latest Mir source. Although today it took more effort so I've updated the test case to give you a better chance of experiencing the freeze. Still trying to get a stack trace. gdb is not cooperating.

Predictably, the server is hung when trying to write to the socket with a hung client at the other end. Eventually the socket buffer fills up...

#0 0x00007ffff5c00bfd in poll () at ../sysdeps/unix/syscall-template.S:81
#1 0x00007ffff789d656 in boost::asio::detail::socket_ops::poll_write (s=35,
    state=0 '\000', ec=...)
    at /usr/include/boost/asio/detail/impl/socket_ops.ipp:1819
#2 0x00007ffff789d349 in boost::asio::detail::socket_ops::sync_send (s=35,
    state=82 'R', bufs=0x7fffe6ff9c30, count=1, flags=0, all_empty=false,
    ec=...) at /usr/include/boost/asio/detail/impl/socket_ops.ipp:1174
#3 0x00007ffff789f892 in boost::asio::detail::reactive_socket_service_base::send<boost::asio::detail::consuming_buffers<boost::asio::const_buffer, boost::asio::mutable_buffers_1> > (this=0x6a95b8, impl=..., buffers=..., flags=0, ec=...)
    at /usr/include/boost/asio/detail/reactive_socket_service_base.hpp:184
#4 0x00007ffff789f563 in boost::asio::stream_socket_service<boost::asio::local::stream_protocol>::send<boost::asio::detail::consuming_buffers<boost::asio::const_buffer, boost::asio::mutable_buffers_1> > (this=0x6a9590, impl=...,
    buffers=..., flags=0, ec=...)
    at /usr/include/boost/asio/stream_socket_service.hpp:314
#5 0x00007ffff789eca0 in boost::asio::basic_stream_socket<boost::asio::local::stream_protocol, boost::asio::stream_socket_service<boost::asio::local::stream_protocol> >::write_some<boost::asio::detail::consuming_buffers<boost::asio::const_buffer, boost::asio::mutable_buffers_1> > (this=0x6a83a0, buffers=..., ec=...)
    at /usr/include/boost/asio/basic_stream_socket.hpp:683
#6 0x00007ffff789e1c4 in boost::asio::write<boost::asio::basic_stream_socket<boost::asio::local::stream_protocol, boost::asio::stream_socket_service<boost::asio::local::stream_protocol> >, boost::asio::mutable_buffers_1, boost::asio::detail::transfer_all_t> (s=..., buffers=..., completion_condition=..., ec=...)
    at /usr/include/boost/asio/impl/write.hpp:49
#7 0x00007ffff789dab8 in boost::asio::write<boost::asio::basic_stream_socket<boost::asio::local::stream_protocol, boost::asio::stream_socket_service<boost::asio::local::stream_protocol> >, boost::asio::mutable_buffers_1> (s=...,
    buffers=...) at /usr/include/boost/asio/impl/write.hpp:62
#8 0x00007ffff789bd92 in mir::frontend::detail::SocketMessenger::send (
    this=0x7fffe0000a30, data=0x7fffe6ffab70 "\032\336\b\n\333\b\n\330\b\003",
    length=1121, fd_set=...)
    at /home/dan/bzr/mir/hang/src/server/frontend/socket_messenger.cpp:80
#9 0x00007ffff78a1077 in mir::frontend::detail::EventSender::send_event_sequence (this=0x7fffe00008d0, seq=...)
    at /home/dan/bzr/mir/hang/src/server/frontend/event_sender.cpp:89
#10 0x00007ffff78a0d5e in mir::frontend::detail::EventSender::handle_event (
    this=0x7fffe00008d0, e=...)
    at /home/dan/bzr/mir/hang/src/server/frontend/event_sender.cpp:49
#11 0x00007ffff794e1b6 in mir::scene::SurfaceEventSource::resized_to (
    this=0x7fffe00016d0, size=...)
    at /home/dan/bzr/mir/hang/src/server/scene/surface_event_source.cpp:45
#12 0x00007ffff7917ab3 in mir::scene::SurfaceObservers::<lambda(const std::shared_ptr<mir::scene::SurfaceObserver>&...

Ha. The hanging function call is:

    // TODO: This should be asynchronous, but we are not making sure
    // that a potential call to send_fds is executed _after_ this
    // function has completed (if it would be executed asynchronously.
    // NOTE: we rely on this synchronous behavior as per the comment in
    // mf::SessionMediator::create_surface
    ba::write(*socket, ba::buffer(whole_message.data(), whole_message.size()));

just raising priority as we've cleared out our critical bug list as of late.
good candidate to get priority attention

is this a dup of https://bugs.launchpad.net/mir/+bug/1323985

Fix committed into lp:mir/devel at revision None, scheduled for release in mir, milestone Unknown

Fix committed to lp:mir/0.7 at revision 1907, scheduled for release in Mir 0.7.3.

This bug was fixed in the package mir - 0.7.3+14.10.20140918.1-0ubuntu1

---------------
mir (0.7.3+14.10.20140918.1-0ubuntu1) utopic; urgency=medium

  [ Andreas Pokorny ]
  * New upstream release 0.7.3 (https://launchpad.net/mir/+milestone/0.7.3)
    - Bug fixes
      . Fix constructor syntax errors (LP: #1369389)
      . server: Workaround for unresponsive clients causing the server to hang
        (LP: #1350207)
      . Relax strict dependencies of graphics drivers on an exact version
        of the libmirplatform2 package. It's blocking CI. (LP: #1370866)
      . Update cmake scripts to current version of gcovr.

  [ Ubuntu daily release ]
  * New rebuild forced
 -- Ubuntu daily release <email address hidden> Thu, 18 Sep 2014 14:54:51 +0000