From 2da8d70e8b49afb77c0064a6751771520c54d3a3 Mon Sep 17 00:00:00 2001
From: Chris Jones <jones.chris.g@gmail.com>
Date: Thu, 20 Sep 2012 22:59:19 -0700
Subject: [PATCH] Bug 792663: Don't SyncFrontBufferToBackBuffer() while a
 buffer tracker isn't around to revoke our buffer provider. r=nrc

---
 gfx/layers/ThebesLayerBuffer.h        |  4 ++++
 gfx/layers/basic/BasicThebesLayer.cpp | 11 +++++------
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/gfx/layers/ThebesLayerBuffer.h b/gfx/layers/ThebesLayerBuffer.h
index db0fbafe60a..243b66f752a 100644
--- a/gfx/layers/ThebesLayerBuffer.h
+++ b/gfx/layers/ThebesLayerBuffer.h
@@ -190,6 +190,10 @@ protected:
     mBufferProvider = aProvider;
     if (!mBufferProvider) {
       mBuffer = nullptr;
+    } else {
+      // Only this buffer provider can give us a buffer.  If we
+      // already have one, something has gone wrong.
+      MOZ_ASSERT(!mBuffer);
     }
   }
 
diff --git a/gfx/layers/basic/BasicThebesLayer.cpp b/gfx/layers/basic/BasicThebesLayer.cpp
index c8d070fc79b..52d96dad498 100644
--- a/gfx/layers/basic/BasicThebesLayer.cpp
+++ b/gfx/layers/basic/BasicThebesLayer.cpp
@@ -314,12 +314,6 @@ BasicShadowableThebesLayer::SetBackBufferAndAttrs(const OptionalThebesBuffer& aB
   mROFrontBuffer = aReadOnlyFrontBuffer;
   mFrontUpdatedRegion = aFrontUpdatedRegion;
   mFrontValidRegion = aValidRegion;
-  if (OptionalThebesBuffer::Tnull_t == mROFrontBuffer.type()) {
-    // For null readonly front, we have single buffer mode
-    // so we can do sync right now, because it does not create new buffer and
-    // don't do any graphic operations
-    SyncFrontBufferToBackBuffer();
-  }
 }
 
 void
@@ -329,6 +323,11 @@ BasicShadowableThebesLayer::SyncFrontBufferToBackBuffer()
     return;
   }
 
+  // We temporarily map our back buffer here in order to copy from the
+  // front buffer.  We need a live buffer tracker in order to unmap
+  // that buffer when appropriate.
+  MOZ_ASSERT(mBufferTracker);
+
   gfxASurface* backBuffer = mBuffer.GetBuffer();
   if (!IsSurfaceDescriptorValid(mBackBuffer)) {
     MOZ_ASSERT(!backBuffer);