NBS (--nbuffers=0) causes software clients to crash with std::exception::what: Failed to mmap buffer 13, "Permission denied")

It's because there are bits yet to land, particularly in lp:~kdub/mir/cover-nbs-with-acceptance-tests. Merge it and it'll work.

Confirmed the 'cover-nbs-with-acceptance-tests' fixes NBS for the in-tree demos. However out-of-tree clients still crash under NBS even with that branch:

    $ sudo Xmir :1 -sw

or

   $ sudo env MIR_SOCKET=/tmp/mir_socket gedit

Correction: That branch does fix out-of-tree clients too. I just forgot to direct them to use the newer version of libmirclient.so.9

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.21.0

This bug was fixed in the package mir - 0.21.0+16.04.20160330-0ubuntu1

---------------
mir (0.21.0+16.04.20160330-0ubuntu1) xenial; urgency=medium

  [ Alberto Aguirre ]
  * New upstream release 0.21.0 (https://launchpad.net/mir/+milestone/0.21.0)
    - ABI summary:
      . mirclient ABI unchanged at 9
      . mirserver ABI unchanged at 38
      . mircommon ABI unchanged at 5
      . mirplatform ABI unchanged at 11
      . mirprotobuf ABI unchanged at 3
      . mirplatformgraphics ABI unchaged at 8
      . mirclientplatform ABI bumped to 5
      . mirinputplatform ABI unchanged at 5
    - Enhancements:
      . New display enumeration API
      . Added Android diagnostic tests to assist during porting to
        new devices
      . Added mir_demo_client_camera: a Video4Linux2 client
    - Bugs fixed:
      . Sometimes devices don't suspend - display turns back on
        immediately (LP: #1549701)
      . Mir crashed with exception 'failed to add sync point to command
        buffer' (LP: #1554635)
      . Mouse cursor is unusably slow in Unity 8 with a 1000Hz mouse
        (LP: #1539009)
      . Packaged mir_unit_tests binary is not suitable for general use
        (LP: #1547015)
      . [regression] Mir stops receiving input after a pause/resume
        cycle (LP: #1548989)
      . NBS (--nbuffers=0) causes software clients to crash with
        std::exception::what: Failed to mmap buffer 13, "Permission denied")
        (LP: #1550432)
      . Fullscreen clients freeze when using NBS with multiple monitors
        (LP: #1551536)
      . [ FAILED ] DisplayConfigurationTest.output_position_is_independent_of_
        orientation (LP: #1552065)
      . The server-side use of MIR_SOCKET is confusing (LP: #1290345)
      . [regression] FTBFS with -DMIR_LINK_TIME_OPTIMIZATION=on
        -Duse_debflags=on (LP: #1350343)
      . Mir On X (mesa-x11) keeps receiving mouse movement events even
        when not focused (LP: #1528110)
      . x11 platform: mouse cursor moves strange (LP: #1546324)
      . Cross compiling to wily/vivid doesn't work (LP: #1549152)
      . Rendering stutters when a new client establishes a connection
        (LP: #1549359)
      . 'mir_demo_server --test-client' crashes (SIGSEGV) when client
        dies (LP: #1555620)
      . [testfail] CI failure: TestClientInput.client_input_config_request_
        receives_all_attached_devices (LP: #1555708)
      . [regression] Mir FTBFS when MIR_ENABLE_TESTS=no (LP: #1556080)
      . Mir-on-X11 doesn't exit (until it gets an event) (LP: #1556210)
      . InputPlatformProbe.x11_platform_found_and_used_when_display_connection_
        works breaks with old input drivers present (LP: #1543049)
      . [regression] MIR_CLIENT_PERF_REPORT is missing window/surface
        names (LP: #1546933)
      . Installed binaries fail to run with mir_demo_server --test-client XXXX
        (LP: #1556160)
      . mir_demo_server --test-client [mir_demo_client_scroll|
        mir_demo_client_flicker] fails (LP: #1556205)
      . The contents of debian/mir-demos.examples are out of date and useless
        (LP: #1557446)

  [ CI Train Bot ]
  * No-change rebuild.

 -- Alberto Aguirre <alberto.aguirre@can...