Bug 1062411 - Make FrameMetrics::mContentDescription an nsCString so there is no limit to its length. r=BenWa

gfx/layers/FrameMetrics.h

@@ -60,10 +60,6 @@ namespace layers {
  * time of a layer-tree transaction.  These metrics are especially
  * useful for shadow layers, because the metrics values are updated
  * atomically with new pixels.
- *
- * Note that the FrameMetrics struct is sometimes stored in shared
- * memory and shared across processes, so it should be a "Plain Old
- * Data (POD)" type with no members that use dynamic memory.
  */
 struct FrameMetrics {
   friend struct IPC::ParamTraits<mozilla::layers::FrameMetrics>;
@@ -101,7 +97,6 @@ public:
     , mViewport(0, 0, 0, 0)
     , mBackgroundColor(0, 0, 0, 0)
   {
-    mContentDescription[0] = '\0';
   }
 
   // Default copy ctor and operator= are fine
@@ -128,8 +123,7 @@ public:
            mScrollOffset == aOther.mScrollOffset &&
            mHasScrollgrab == aOther.mHasScrollgrab &&
            mUpdateScrollOffset == aOther.mUpdateScrollOffset &&
-           mBackgroundColor == aOther.mBackgroundColor &&
-           !strcmp(mContentDescription, aOther.mContentDescription);
+           mBackgroundColor == aOther.mBackgroundColor;
   }
   bool operator!=(const FrameMetrics& aOther) const
   {
@@ -247,6 +241,16 @@ public:
     mScrollGeneration = aOther.mScrollGeneration;
   }
 
+  // Make a copy of this FrameMetrics object which does not have any pointers
+  // to heap-allocated memory (i.e. is Plain Old Data, or 'POD'), and is
+  // therefore safe to be placed into shared memory.
+  FrameMetrics MakePODObject() const
+  {
+    FrameMetrics copy = *this;
+    copy.mContentDescription.Truncate();
+    return copy;
+  }
+
   // ---------------------------------------------------------------------------
   // The following metrics are all in widget space/device pixels.
   //
@@ -484,16 +488,14 @@ public:
     mBackgroundColor = aBackgroundColor;
   }
 
-  nsCString GetContentDescription() const
+  const nsCString& GetContentDescription() const
   {
-    return nsCString(mContentDescription);
+    return mContentDescription;
   }
 
   void SetContentDescription(const nsCString& aContentDescription)
   {
-    strncpy(mContentDescription, aContentDescription.get(),
-            sizeof(mContentDescription));
-    mContentDescription[sizeof(mContentDescription) - 1] = 0;
+    mContentDescription = aContentDescription;
   }
 
 private:
@@ -569,9 +571,9 @@ private:
   gfxRGBA mBackgroundColor;
 
   // A description of the content element corresponding to this frame.
-  // This is empty unless this is a scrollable ContainerLayer and the
+  // This is empty unless this is a scrollable layer and the
   // apz.printtree pref is turned on.
-  char mContentDescription[20];
+  nsCString mContentDescription;
 };
 
 /**

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2868,7 +2868,7 @@ void AsyncPanZoomController::UpdateSharedCompositorFrameMetrics()
 
   if (frame && mSharedLock && gfxPrefs::UseProgressiveTilePainting()) {
     mSharedLock->Lock();
-    *frame = mFrameMetrics;
+    *frame = mFrameMetrics.MakePODObject();
     mSharedLock->Unlock();
   }
 }