Bug 1400435 - Use a more precise check in the nsCSSValue destructor. r=xidorn

MozReview-Commit-ID: KFdgtxyOZ01

layout/style/ServoBindingList.h

@@ -615,6 +615,9 @@ SERVO_BINDING_FUNC(Servo_TraverseSubtree,
 // Assert that the tree has no pending or unconsumed restyles.
 SERVO_BINDING_FUNC(Servo_AssertTreeIsClean, void, RawGeckoElementBorrowed root)
 
+// Returns true if the current thread is a Servo parallel worker thread.
+SERVO_BINDING_FUNC(Servo_IsWorkerThread, bool, )
+
 // Checks whether the rule tree has crossed its threshold for unused rule nodes,
 // and if so, frees them.
 SERVO_BINDING_FUNC(Servo_MaybeGCRuleTree, void, RawServoStyleSetBorrowed set)

layout/style/ServoStyleSet.cpp

@@ -40,6 +40,14 @@ using namespace mozilla::dom;
 
 ServoStyleSet* ServoStyleSet::sInServoTraversal = nullptr;
 
+#ifdef DEBUG
+bool
+ServoStyleSet::IsCurrentThreadInServoTraversal()
+{
+  return sInServoTraversal && (NS_IsMainThread() || Servo_IsWorkerThread());
+}
+#endif
+
 namespace mozilla {
 // On construction, sets sInServoTraversal to the given ServoStyleSet.
 // On destruction, clears sInServoTraversal and calls RunPostTraversalTasks.

layout/style/ServoStyleSet.h

@@ -97,6 +97,13 @@ public:
     return sInServoTraversal;
   }
 
+#ifdef DEBUG
+  // Used for debug assertions. We make this debug-only to prevent callers from
+  // accidentally using it instead of IsInServoTraversal, which is cheaper. We
+  // can change this if a use-case arises.
+  static bool IsCurrentThreadInServoTraversal();
+#endif
+
   static ServoStyleSet* Current()
   {
     return sInServoTraversal;

layout/style/nsCSSValue.cpp

@@ -417,10 +417,11 @@ nscoord nsCSSValue::GetPixelLength() const
 // traversal, since the refcounts aren't thread-safe.
 // Note that the caller might be an OMTA thread, which is allowed to operate off
 // main thread because it owns all of the corresponding nsCSSValues and any that
-// they might be sharing members with.
-#define DO_RELEASE(member) {                                                     \
-  MOZ_ASSERT(NS_IsInCompositorThread() || !ServoStyleSet::IsInServoTraversal()); \
-  mValue.member->Release();                                                      \
+// they might be sharing members with. Since this can happen concurrently with
+// the servo traversal, we have to use a more-precise (but slower) test.
+#define DO_RELEASE(member) {                                     \
+  MOZ_ASSERT(!ServoStyleSet::IsCurrentThreadInServoTraversal()); \
+  mValue.member->Release();                                      \
 }
 
 void nsCSSValue::DoReset()