Bug 1015278 - Replace the std::string with a char[20] in FrameMetrics…

… to make it cross-process shmem friendly. r=botond
carlosdp · May 24, 2014 · 870ae6f · 870ae6f
1 parent ac5d74b
commit 870ae6f
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Showing 2 changed files with 16 additions and 6 deletions.
diff --git a/gfx/ipc/GfxMessageUtils.h b/gfx/ipc/GfxMessageUtils.h
@@ -763,7 +763,8 @@ struct ParamTraits<mozilla::layers::FrameMetrics>
     WriteParam(aMsg, aParam.mHasScrollgrab);
     WriteParam(aMsg, aParam.mUpdateScrollOffset);
     WriteParam(aMsg, aParam.mScrollGeneration);
-    WriteParam(aMsg, aParam.mContentDescription);
+    aMsg->WriteBytes(aParam.mContentDescription,
+                     sizeof(aParam.mContentDescription));
     WriteParam(aMsg, aParam.mTransformScale);
   }
 
@@ -789,7 +790,9 @@ struct ParamTraits<mozilla::layers::FrameMetrics>
             ReadParam(aMsg, aIter, &aResult->mHasScrollgrab) &&
             ReadParam(aMsg, aIter, &aResult->mUpdateScrollOffset) &&
             ReadParam(aMsg, aIter, &aResult->mScrollGeneration) &&
-            ReadParam(aMsg, aIter, &aResult->mContentDescription) &&
+            aMsg->ReadBytes(aIter,
+                            reinterpret_cast<const char**>(&aResult->mContentDescription),
+                            sizeof(aResult->mContentDescription)) &&
             ReadParam(aMsg, aIter, &aResult->mTransformScale));
   }
 };

diff --git a/gfx/layers/FrameMetrics.h b/gfx/layers/FrameMetrics.h
@@ -56,6 +56,10 @@ 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>;
@@ -84,6 +88,7 @@ struct FrameMetrics {
     , mZoom(1)
     , mUpdateScrollOffset(false)
     , mScrollGeneration(0)
+    , mContentDescription()
     , mRootCompositionSize(0, 0)
     , mDisplayPortMargins(0, 0, 0, 0)
     , mUseDisplayPortMargins(false)
@@ -380,14 +385,16 @@ struct FrameMetrics {
     return mScrollGeneration;
   }
 
-  const std::string& GetContentDescription() const
+  std::string GetContentDescription() const
   {
-    return mContentDescription;
+    return std::string(mContentDescription);
   }
 
   void SetContentDescription(const std::string& aContentDescription)
   {
-    mContentDescription = aContentDescription;
+    strncpy(mContentDescription, aContentDescription.c_str(), sizeof(mContentDescription));
+    // forcibly null-terminate in case aContentDescription is too long
+    mContentDescription[sizeof(mContentDescription) - 1] = '\0';
   }
 
   ViewID GetScrollId() const
@@ -478,7 +485,7 @@ struct FrameMetrics {
 
   // A description of the content element corresponding to this frame.
   // This is empty unless the apz.printtree pref is turned on.
-  std::string mContentDescription;
+  char mContentDescription[20];
 
   // The size of the root scrollable's composition bounds, but in local CSS pixels.
   CSSSize mRootCompositionSize;