Bug 1425257: Introduce a global lock to protect the dependency graph between DrawTargets. r=lsalzman

This patch takes the safest route for securing modifications to the dependency graph for D2D DrawTargets. It's possible a slightly optimal approach is possible, however lock contention should be rare and I believe this is the safest and most upliftable approach.

MozReview-Commit-ID: HGfSdEp2U5N

gfx/2d/2D.h

@@ -1755,6 +1755,9 @@ protected:
   // This guards access to the singleton devices above, as well as the
   // singleton devices in DrawTargetD2D1.
   static StaticMutex mDeviceLock;
+  // This synchronizes access between different D2D drawtargets and their
+  // implied dependency graph.
+  static StaticMutex mDTDependencyLock;
 
   friend class DrawTargetD2D1;
 #endif

gfx/2d/DrawTargetD2D1.cpp

@@ -76,6 +76,7 @@ DrawTargetD2D1::~DrawTargetD2D1()
     mDC->EndDraw();
   }
 
+  StaticMutexAutoLock lock(Factory::mDTDependencyLock);
   // Targets depending on us can break that dependency, since we're obviously not going to
   // be modified in the future.
   for (auto iter = mDependentTargets.begin();
@@ -172,6 +173,7 @@ DrawTargetD2D1::Flush()
     }
   }
  
+  StaticMutexAutoLock lock(Factory::mDTDependencyLock);
   // We no longer depend on any target.
   for (TargetSet::iterator iter = mDependingOnTargets.begin();
        iter != mDependingOnTargets.end(); iter++) {
@@ -1285,6 +1287,8 @@ DrawTargetD2D1::MarkChanged()
       MOZ_ASSERT(!mSnapshot);
     }
   }
+
+  StaticMutexAutoLock lock(Factory::mDTDependencyLock);
   if (mDependentTargets.size()) {
     // Copy mDependentTargets since the Flush()es below will modify it.
     TargetSet tmpTargets = mDependentTargets;
@@ -1464,6 +1468,11 @@ DrawTargetD2D1::FinalizeDrawing(CompositionOp aOp, const Pattern &aPattern)
 void
 DrawTargetD2D1::AddDependencyOnSource(SourceSurfaceD2D1* aSource)
 {
+  StaticMutexAutoLock lock(Factory::mDTDependencyLock);
+
+  // We grab the SnapshotLock as well, this guaranteeds aSource->mDrawTarget
+  // cannot be cleared in between the if statement and the dereference.
+  MutexAutoLock snapshotLock(*aSource->mSnapshotLock);
   if (aSource->mDrawTarget && !mDependingOnTargets.count(aSource->mDrawTarget)) {
     aSource->mDrawTarget->mDependentTargets.insert(this);
     mDependingOnTargets.insert(aSource->mDrawTarget);

gfx/2d/Factory.cpp

@@ -222,6 +222,7 @@ StaticRefPtr<ID2D1Device> Factory::mD2D1Device;
 StaticRefPtr<IDWriteFactory> Factory::mDWriteFactory;
 bool Factory::mDWriteFactoryInitialized = false;
 StaticMutex Factory::mDeviceLock;
+StaticMutex Factory::mDTDependencyLock;
 #endif
 
 DrawEventRecorder *Factory::mRecorder;