Merge lp:~mir-team/mir/compositing-always-finishes into lp:mir

[Daniel van Vugt]

Proof of concept.

Tests fail for now. Just demonstrating an idea.

Unmerged revisions

3306. By Daniel van Vugt on 2016-02-11

Proof of concept

=== modified file 'src/server/scene/basic_surface.cpp'
--- src/server/scene/basic_surface.cpp	2016-01-29 08:18:22 +0000
+++ src/server/scene/basic_surface.cpp	2016-02-11 08:42:17 +0000
@@ -786,8 +786,7 @@
         bool shaped,
         mg::Renderable::ID id)
     : underlying_buffer_stream{stream},
-      compositor_buffer{nullptr},
-      compositor_id{compositor_id},
+      compositor_buffer{underlying_buffer_stream->lock_compositor_buffer(compositor_id)},
       alpha_{alpha},
       shaped_{shaped},
       screen_position_(position),
@@ -802,8 +801,13 @@
  
     std::shared_ptr<mg::Buffer> buffer() const override
     {
-        if (!compositor_buffer)
-            compositor_buffer = underlying_buffer_stream->lock_compositor_buffer(compositor_id);
+        /*
+         * It's been pre-acquired, to ensure any frames a compositor chooses
+         * to not use (e.g. offscreen) don't wake up that compositor
+         * repeatedly. Fortunately acquiring a compositor buffer is an O(1)
+         * non-blocking operation with no performance impact. Only putting it
+         * on screen affects performance :)
+         */
         return compositor_buffer;
     }
 
@@ -823,8 +827,7 @@
     { return id_; }
 private:
     std::shared_ptr<mc::BufferStream> const underlying_buffer_stream;
-    std::shared_ptr<mg::Buffer> mutable compositor_buffer;
-    void const*const compositor_id;
+    std::shared_ptr<mg::Buffer> const compositor_buffer;
     float const alpha_;
     bool const shaped_;
     geom::Rectangle const screen_position_;