Bug 1406922 - Make CycleCollectedJSContext to handle microtasks and make MutationObserver to use them

Tag UXP Issue #1344

2 files changed, 83 insertions, 6 deletions

diff --git a/xpcom/base/CycleCollectedJSContext.cpp b/xpcom/base/CycleCollectedJSContext.cpp
index 0a85ae6ac6..033ca562a5 100644
--- a/xpcom/base/CycleCollectedJSContext.cpp
+++ b/xpcom/base/CycleCollectedJSContext.cpp
@@ -440,6 +440,7 @@ CycleCollectedJSContext::CycleCollectedJSContext()
   , mDoingStableStates(false)
   , mDisableMicroTaskCheckpoint(false)
   , mMicroTaskLevel(0)
+  , mMicroTaskRecursionDepth(0)
   , mOutOfMemoryState(OOMState::OK)
   , mLargeAllocationFailureState(OOMState::OK)
 {
@@ -1380,8 +1381,8 @@ CycleCollectedJSContext::AfterProcessTask(uint32_t aRecursionDepth)
 
   // Step 4.1: Execute microtasks.
   if (!mDisableMicroTaskCheckpoint) {
+    PerformMicroTaskCheckPoint();
     if (NS_IsMainThread()) {
-      PerformMainThreadMicroTaskCheckpoint();
       Promise::PerformMicroTaskCheckpoint();
     } else {
       Promise::PerformWorkerMicroTaskCheckpoint();
@@ -1661,12 +1662,70 @@ CycleCollectedJSContext::DispatchToMicroTask(already_AddRefed<nsIRunnable> aRunn
   mPromiseMicroTaskQueue.push(runnable.forget());
 }
 
+class AsyncMutationHandler final : public mozilla::Runnable
+{
+public:
+  NS_IMETHOD Run() override
+  {
+    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+    if (ccjs) {
+      ccjs->PerformMicroTaskCheckPoint();
+    }
+    return NS_OK;
+  }
+};
+
 void
-CycleCollectedJSContext::PerformMainThreadMicroTaskCheckpoint()
+CycleCollectedJSContext::PerformMicroTaskCheckPoint()
 {
-  MOZ_ASSERT(NS_IsMainThread());
+  if (mPendingMicroTaskRunnables.empty()) {
+    // Nothing to do, return early.
+    return;
+  }
+
+  uint32_t currentDepth = RecursionDepth();
+  if (mMicroTaskRecursionDepth >= currentDepth) {
+    // We are already executing microtasks for the current recursion depth.
+    return;
+  }
 
-  nsDOMMutationObserver::HandleMutations();
+  if (NS_IsMainThread() && !nsContentUtils::IsSafeToRunScript()) {
+    // Special case for main thread where DOM mutations may happen when
+    // it is not safe to run scripts.
+    nsContentUtils::AddScriptRunner(new AsyncMutationHandler());
+    return;
+  }
+
+  mozilla::AutoRestore<uint32_t> restore(mMicroTaskRecursionDepth);
+  MOZ_ASSERT(currentDepth > 0);
+  mMicroTaskRecursionDepth = currentDepth;
+
+  AutoSlowOperation aso;
+
+  std::queue<RefPtr<MicroTaskRunnable>> suppressed;
+  while (!mPendingMicroTaskRunnables.empty()) {
+    RefPtr<MicroTaskRunnable> runnable =
+      mPendingMicroTaskRunnables.front().forget();
+    mPendingMicroTaskRunnables.pop();
+    if (runnable->Suppressed()) {
+      suppressed.push(runnable);
+    } else {
+      runnable->Run(aso);
+    }
+  }
+
+  // Put back the suppressed microtasks so that they will be run later.
+  // Note, it is possible that we end up keeping these suppressed tasks around
+  // for some time, but no longer than spinning the event loop nestedly
+  // (sync XHR, alert, etc.)
+  mPendingMicroTaskRunnables.swap(suppressed);
+}
+
+void
+CycleCollectedJSContext::DispatchMicroTaskRunnable(
+  already_AddRefed<MicroTaskRunnable> aRunnable)
+{
+  mPendingMicroTaskRunnables.push(aRunnable);
 }
 
 void
diff --git a/xpcom/base/CycleCollectedJSContext.h b/xpcom/base/CycleCollectedJSContext.h
index 2197eae920..4cd1479ed7 100644
--- a/xpcom/base/CycleCollectedJSContext.h
+++ b/xpcom/base/CycleCollectedJSContext.h
@@ -33,6 +33,7 @@ struct Class;
 } // namespace js
 
 namespace mozilla {
+class AutoSlowOperation;
 
 class JSGCThingParticipant: public nsCycleCollectionParticipant
 {
@@ -134,6 +135,17 @@ struct CycleCollectorResults
   uint32_t mNumSlices;
 };
 
+class MicroTaskRunnable
+{
+public:
+  MicroTaskRunnable() {}
+  NS_INLINE_DECL_REFCOUNTING(MicroTaskRunnable)
+  virtual void Run(AutoSlowOperation& aAso) = 0;
+  virtual bool Suppressed() { return false; }
+protected:
+  virtual ~MicroTaskRunnable() {}
+};
+
 class CycleCollectedJSContext
 {
   friend class JSGCThingParticipant;
@@ -412,7 +424,7 @@ public:
   void LeaveMicroTask()
   {
     if (--mMicroTaskLevel == 0) {
-      PerformMainThreadMicroTaskCheckpoint();
+      PerformMicroTaskCheckPoint();
     }
   }
 
@@ -431,7 +443,9 @@ public:
     mMicroTaskLevel = aLevel;
   }
 
-  void PerformMainThreadMicroTaskCheckpoint();
+  void PerformMicroTaskCheckPoint();
+
+  void DispatchMicroTaskRunnable(already_AddRefed<MicroTaskRunnable> aRunnable);
 
   // Storage for watching rejected promises waiting for some client to
   // consume their rejection.
@@ -484,6 +498,10 @@ private:
   bool mDisableMicroTaskCheckpoint;
 
   uint32_t mMicroTaskLevel;
+  std::queue<RefPtr<MicroTaskRunnable>> mPendingMicroTaskRunnables;
+
+  uint32_t mMicroTaskRecursionDepth;
+
   OOMState mOutOfMemoryState;
   OOMState mLargeAllocationFailureState;