Mac: Clean up ui::Compositor recycling

content/browser/renderer_host/browser_compositor_view_mac.mm

Index: content/browser/renderer_host/browser_compositor_view_mac.mm
diff --git a/content/browser/renderer_host/browser_compositor_view_mac.mm b/content/browser/renderer_host/browser_compositor_view_mac.mm
index 58d2a2ff368bda50738bfc8682458caf71c4e603..5b32fcf9a8a48e88f47ef097a4184afd1d88bc9e 100644
--- a/content/browser/renderer_host/browser_compositor_view_mac.mm
+++ b/content/browser/renderer_host/browser_compositor_view_mac.mm
@@ -12,6 +12,7 @@
 #include "base/trace_event/trace_event.h"
 #include "content/browser/compositor/image_transport_factory.h"
 #include "content/browser/renderer_host/resize_lock.h"
+#include "content/public/browser/browser_thread.h"
 #include "content/public/browser/context_factory.h"
 #include "ui/accelerated_widget_mac/accelerated_widget_mac.h"
 #include "ui/accelerated_widget_mac/window_resize_helper_mac.h"
@@ -27,12 +28,21 @@
 
 // The number of placeholder objects allocated. If this reaches zero, then
 // the RecyclableCompositorMac being held on to for recycling,
-// |g_spare_recyclable_compositor|, will be freed.
+// |g_spare_recyclable_compositors|, will be freed.
 uint32_t g_browser_compositor_count = 0;
 
 // A spare RecyclableCompositorMac kept around for recycling.
-base::LazyInstance<std::unique_ptr<RecyclableCompositorMac>>
-    g_spare_recyclable_compositor;
+base::LazyInstance<std::deque<std::unique_ptr<RecyclableCompositorMac>>>
+    g_spare_recyclable_compositors;
+
+void ReleaseSpareCompositors() {
+  // Allow at most one spare recyclable compositor.
+  while (g_spare_recyclable_compositors.Get().size() > 1)
+    g_spare_recyclable_compositors.Get().pop_front();
+
+  if (!g_browser_compositor_count)
+    g_spare_recyclable_compositors.Get().clear();
+}
 
 }  // namespace
 
@@ -120,8 +130,12 @@ void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
 // static
 std::unique_ptr<RecyclableCompositorMac> RecyclableCompositorMac::Create() {
   DCHECK(ui::WindowResizeHelperMac::Get()->task_runner());
-  if (g_spare_recyclable_compositor.Get())
-    return std::move(g_spare_recyclable_compositor.Get());
+  if (!g_spare_recyclable_compositors.Get().empty()) {
+    std::unique_ptr<RecyclableCompositorMac> result;
+    result = std::move(g_spare_recyclable_compositors.Get().front());
+    g_spare_recyclable_compositors.Get().pop_front();

[enne (OOO), 2016/07/20 23:53:48]

Did you want to delete the remaining spare compositors here, if they exist? Or
the case where there is more than one is only when there is an outstanding
cleanup task?

[ccameron, 2016/07/20 23:59:33]

There will only be >0 when there is an outstanding cleanup task -- we want to
avoid doing the cleanup here, cause if we're here, we probably just created a
window/tab that is going to want to paint (and is going to be doing that in a
nested loop -- we'd rather not hit the dtor inside that nested loop).

+    return result;
+  }
   return std::unique_ptr<RecyclableCompositorMac>(new RecyclableCompositorMac);
 }
 
@@ -137,12 +151,13 @@ void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
   CHECK(!g_has_shut_down);
 
   // Make this RecyclableCompositorMac recyclable for future instances.
-  g_spare_recyclable_compositor.Get().swap(compositor);
+  g_spare_recyclable_compositors.Get().push_back(std::move(compositor));
 
-  // If there are no placeholders allocated, destroy the recyclable
-  // RecyclableCompositorMac that we just populated.
-  if (!g_browser_compositor_count)
-    g_spare_recyclable_compositor.Get().reset();
+  // Post a task to free up the spare ui::Compositors when needed. Post this
+  // to the browser main thread so that we won't free any compositors while
+  // in a nested loop waiting to put up a new frame.
+  BrowserThread::PostTask(
+      BrowserThread::UI, FROM_HERE, base::Bind(&ReleaseSpareCompositors));
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -180,7 +195,7 @@ void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
   // If there are no compositors allocated, destroy the recyclable
   // RecyclableCompositorMac.
   if (!g_browser_compositor_count)
-    g_spare_recyclable_compositor.Get().reset();
+    g_spare_recyclable_compositors.Get().clear();
 }
 
 ui::AcceleratedWidgetMac* BrowserCompositorMac::GetAcceleratedWidgetMac() {
@@ -367,7 +382,7 @@ void OnCompositingShuttingDown(ui::Compositor* compositor) override {}
 // static
 void BrowserCompositorMac::DisableRecyclingForShutdown() {
   g_has_shut_down = true;
-  g_spare_recyclable_compositor.Get().reset();
+  g_spare_recyclable_compositors.Get().clear();
 }
 
 void BrowserCompositorMac::SetNeedsBeginFrames(bool needs_begin_frames) {