NestedServer.display_configuration_reset_when_application_exits segfaults

It looks like the culprit is SurfaceStack::add_observer, where the mutex guarding state changes to the vector of surfaces is dropped when calling the observer. This is problematic as another thread could call SurfaceStack::remove_surface.

Just holding the lock while calling Observer::surface_exists but that can lead to deadlocks. Indeed CursorController installs an observer, in which it's surface_exists handler calls back into the scene.

Taking a copy of the list of surfaces and then releasing a lock can lead to calling Observer::surface_removed before Observer::surface_exists which could lead to the observer having stale and/or invalid Surface pointers around leading to other subtle runtime failures.

Maybe a secondary lock that serializes add_observer and surface_removed is enough.

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.19.0

The fix landed after the branch, so we should represent the two separate fixes.

This bug was fixed in the package mir - 0.18.0+16.04.20151216.1-0ubuntu1

---------------
mir (0.18.0+16.04.20151216.1-0ubuntu1) xenial; urgency=medium

  [ Kevin DuBois ]
  * New upstream release 0.18.0 (https://launchpad.net/mir/+milestone/0.18.0)
    - ABI summary: Only servers need rebuilding;
      . Mirclient ABI unchanged at 9
      . Mirserver ABI bumped to 36
      . Mircommon ABI unchanged at 5
      . Mirplatform ABI unchanged at 11
      . Mirprotobuf ABI unchanged at 3
      . Mirplatformgraphics ABI bumped to 7
      . Mirclientplatform ABI unchanged at 3
      . Mirinputplatform ABI added. Current version is 4
    - Enhancements:
      . Use libinput by default, and remove the android input stack
      . Add x11 input probing
      . Add alternative buffer swapping mechanism internally, available with
        --nbuffers 0
      . Automatic searching and selection of input platforms
      . Better support for themed cursors
      . Add demo client that uses multiple buffer streams in one surface
      . Improve fingerpaint demo to use touch pressure
      . Allow for configuring cursor acceleration, scroll speed and left or
        right handed mice
      . Allow for setting a base display configuration via client api
      . Various nested server multimonitor fixes and stability improvements
      . Remove DepthId from the SurfaceStack
    - Bug fixes:
      . Unit test failures in Display.* on Android (LP: #1519276)
      . Build failure due to missing dependency of client rpc code on mir
        protobuf (LP: #1518372)
      . Test failure in
        NestedServer.display_configuration_reset_when_application_exits
        (LP: #1517990)
      . CI test failures in various NesterServer tests (LP: #1517781)
      . FTBFS with -DMIR_PLATFORM=android (LP: #1517532)
      . Nesting Mir servers with assorted display configs causes lockup
        (LP: #1516670)
      . [testsfail] RaiseSurfaces.motion_events_dont_prevent_raise
        (LP: #1515931)
      . CI test failures in GLMark2Test (LP: #1515660)
      . Shells that inject user input events need to agree with the system
        compositor on the clock to use (LP: #1515515)
      . mircookie-dev is missing nettle-dev dependency (LP: #1514391)
      . Segmentation fault on server shutdown with mesa-kms (LP: #1513901)
      . mircookie requires nettle but libmircookie-dev doesn't depend on it
        (LP: #1513792)
      . libmircookie1 package does not list libnettle as dependency
        (LP: #1513225)
      . display configuration not reset when application exits (LP: #1511798)
      . unplugging external monitor causes nested server to throttle client
        (LP: #1511723)
      . 1/2 screen on external monitor (LP: #1511538)
      . unity-system-compositor crash, no interaction on windowed mode
        (LP: #1511095)
      . [regression] arm64/powerpc cross compile doesn't build any more
        (LP: #1510778)
      . mir_connection_get_egl_pixel_format() crashes if libEGL is loaded
        RTLD_LAZY (LP: #1510218)
      . [multimonitor] nested server surface positioning incorrect
        (LP: #1506846)
      . unity-system-compositor fails to build against lp:mir r3027
   ...

Fix Released in Mir 0.18.0. Although it's good to mention this bug got different fixes between the 0.18 and 0.19 branches, it's probably still best to only mention it in the changelog for one of them.