Android USC crash on multimonitor unplug [std::exception::what: error during hwc prepare()]

[Gerry Boland]

I've a slightly customized USC built in Silo0. On top of that, a multimonitor-aware unity8 is running.
http://people.canonical.com/~platform/citrain_dashboard/#?q=ubuntu/landing-000

On Android devices, Nexus 4 & 7, I occasionally get a crash on external monitor unplug. Here is the relevant log:

ERROR: /build/mir-UAyS26/mir-0.13.4+15.04.20150709.2/src/platforms/android/server/real_hwc_wrapper.cpp(123): Throw in function virtual void mir::graphics::android::RealHwcWrapper::set(const std::array<hwc_display_contents_1*, 3u>&) const
Dynamic exception type: N5boost16exception_detail10clone_implINS0_19error_info_injectorISt13runtime_errorEEEE
std::exception::what: error during hwc prepare(). rc = ffffffff

[Gerry Boland (gerboland)]

Desktop is stable, I can't get it to fail.
The Android bug does not always hit, only occasionally. Happens on both N4 & N7, maybe more often on N4.

[Kevin DuBois (kdub)]

seems a race between the mir main EventLoop, and the driver's notification thread, and the compositing thread. Still on the case, hampered a bit today by low batteries and slimports not charging the device

[Kevin DuBois (kdub)]

So, what seems to be happening on these drivers is that the hotplug-unplug event comes in while the driver is doing work in hwc::set() with an external list. Since the driver cannot post the list to the display anymore (as its physically disconnected), hwc::set() returns -1. At that point, mir throws up the compositor thread (which is trying to be shut down anyways at this point), and the exception is caught, which terminates the server. Given that the thread that gets the throw is shutting down anyways, we can ignore the exception.

[Kevin DuBois (kdub)]

Attached my branch that should avert it, doing some more testing on it before proposing.

[PS Jenkins bot (ps-jenkins)]

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.15.0

[Launchpad Janitor (janitor)]

This bug was fixed in the package mir - 0.15.0+15.10.20150818-0ubuntu1

---------------
mir (0.15.0+15.10.20150818-0ubuntu1) wily; urgency=medium

  [ Daniel van Vugt ]
  * New upstream release 0.15.0 (https://launchpad.net/mir/+milestone/0.15.0)
    - ABI summary: Only servers and graphics drivers need rebuilding;
      . Mirclient ABI unchanged at 9
      . Mirserver ABI bumped to 33
      . Mircommon ABI unchanged at 5
      . Mirplatform ABI bumped to 9
    - Enhancements:
      . Add support for Mir-on-X11.
      . Latency reduction optimizations (around ~15ms reduction in total):
        Reduced input event resampling latency by 5ms. Reduced output latency
        (in system compositors) by around 10ms with the introduction of
        "predictive bypass". And we're not finished; future Mir releases
        should reduce latency further.
      . Introduced a python3-based Mir performance framework.
      . Lots of preparation for an architectural overhaul of buffer swapping,
        required in the least to support future optimizations like nested
        bypass.
      . Added a new cursor: crosshair
      . Added support for 15/16-bit client pixel formats ("high colour").
      . Added a new client function to make picking the right pixel format
        for a given EGLConfig super simple: mir_connection_get_egl_pixel_format
      . Added application-not-responding detection
      . Added client API for specifying input region shape.
      . Fixed the remaining threading flaws identified by ThreadSanitizer and
        turned it on permanently for all continuous integration in future.
      . Added support for relative pointer motion events (e.g. for gaming).
    - Bug fixes:
      . Fix focus issues breaking autopilot tests entering text (LP: #1468029)
      . Fix mir tests failure on armhf with GCC5 (LP: #1478213)
      . mir_buffer_stream_swap_buffers_sync can hang constraints (LP: #1479899)
      . Loading libmirclient.so twice leads to a segfault in libmirprotobuf.so
        (LP: #1391976)
      . Visible corruption in SDL apps (LP: #1460149)
      . MultiThreadedCompositor::destroy_compositing_threads hangs/deadlocks on
        shutdown or display reconfiguration (LP: #1471909)
      . ctest/"make test" reports 100% tests pass even when some fail.
        (LP: #1472911)
      . Mir server crashed - GLib-CRITICAL **: g_source_get_context: assertion
       'source->context != NULL || !SOURCE_DESTROYED (source)' failed
        (LP: #1473869)
      . USC crash on multimonitor unplug [std::exception::what: error during
        hwc prepare()] (LP: #1474891)
      . [regression] Input focus delay after switching app back into focus
        (LP: #1480654)
      . GLibMainLoopTest.propagates_exception_from_server_action fails with
        GCC 5 in armhf (LP: #1482274)
      . [enhancement] Mir lacks relative mouse support (LP: #1276322)
      . ShmBuffer ignores pixel_format (LP: #1424909)
      . Fullscreen bypassed clients stutter with double buffers when other
        clients are running (LP: #1447896)
      . [regression] Demo servers crash on start-up if MIR_ENABLE_TESTS=OFF
        (LP: #1439078)
      . [regression] The software curs...