Merge lp:~alan-griffiths/mir/fix-1575211 into lp:mir

[Alan Griffiths]

examples: fix racy code that fails to ensure buffers are posted exactly once

examples: fix racy code that fails to ensure buffers are posted exactly once

[Mir CI Bot (mir-ci-bot)]

FAILED: Continuous integration, rev:3481
https://mir-jenkins.ubuntu.com/job/mir-ci/907/
Executed test runs:
    FAILURE: https://mir-jenkins.ubuntu.com/job/build-mir/951/console
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-0-fetch/992
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-1-sourcepkg/release=vivid+overlay/983
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-1-sourcepkg/release=xenial/983
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=vivid+overlay/961
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=clang,platform=mesa,release=vivid+overlay/961/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=xenial/961
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=amd64,compiler=gcc,platform=mesa,release=xenial/961/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=cross-armhf,compiler=gcc,platform=android,release=vivid+overlay/961
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=cross-armhf,compiler=gcc,platform=android,release=vivid+overlay/961/artifact/output/*zip*/output.zip
    SUCCESS: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=android,release=vivid+overlay/961
        deb: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=android,release=vivid+overlay/961/artifact/output/*zip*/output.zip
    FAILURE: https://mir-jenkins.ubuntu.com/job/build-2-binpkg-mir/arch=i386,compiler=gcc,platform=mesa,release=xenial/961/console

Click here to trigger a rebuild:
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review: Needs Fixing (continuous-integration)

[Alberto Aguirre (albaguirre)]

LGTM.

Failure is lp:1570698

review: Approve

[Cemil Azizoglu (cemil-azizoglu)]

lgtm

review: Approve

[Mir CI Bot (mir-ci-bot)]

review: Approve (continuous-integration)

=== modified file 'examples/server_example_window_management_info.cpp'
--- examples/server_example_window_management_info.cpp	2016-04-15 03:41:28 +0000
+++ examples/server_example_window_management_info.cpp	2016-04-27 12:02:30 +0000
@@ -171,29 +171,29 @@
     SwappingPainter(std::shared_ptr<frontend::BufferStream> const& buffer_stream) :
         buffer_stream{buffer_stream}, buffer{nullptr}
     {
-        swap_buffers();
+        swap_buffers(nullptr);
     }
 
-    void swap_buffers()
+    void swap_buffers(graphics::Buffer* buf)
     {
         auto const callback = [this](mir::graphics::Buffer* new_buffer)
             {
                 buffer.store(new_buffer);
             };
 
-        buffer_stream->swap_buffers(buffer, callback);
+        buffer_stream->swap_buffers(buf, callback);
     }
 
     void paint(int intensity) override
     {
-        if (auto const buf = buffer.load())
+        if (graphics::Buffer* buf = buffer.exchange(nullptr))
         {
             auto const format = buffer_stream->pixel_format();
             auto const sz = buf->size().height.as_int() *
                             buf->size().width.as_int() * MIR_BYTES_PER_PIXEL(format);
             std::vector<unsigned char> pixels(sz, intensity);
             buf->write(pixels.data(), sz);
-            swap_buffers();
+            swap_buffers(buf);
         }
     }