3 files changed, 140 insertions, 1 deletions

diff --git a/xpcom/base/CycleCollectedJSContext.cpp b/xpcom/base/CycleCollectedJSContext.cpp
index e16c15455..033ca562a 100644
--- a/xpcom/base/CycleCollectedJSContext.cpp
+++ b/xpcom/base/CycleCollectedJSContext.cpp
@@ -79,6 +79,7 @@
 #include "nsCycleCollectionParticipant.h"
 #include "nsCycleCollector.h"
 #include "nsDOMJSUtils.h"
+#include "nsDOMMutationObserver.h"
 #include "nsJSUtils.h"
 #include "nsWrapperCache.h"
 
@@ -438,6 +439,8 @@ CycleCollectedJSContext::CycleCollectedJSContext()
   , mJSHolders(256)
   , mDoingStableStates(false)
   , mDisableMicroTaskCheckpoint(false)
+  , mMicroTaskLevel(0)
+  , mMicroTaskRecursionDepth(0)
   , mOutOfMemoryState(OOMState::OK)
   , mLargeAllocationFailureState(OOMState::OK)
 {
@@ -1378,8 +1381,8 @@ CycleCollectedJSContext::AfterProcessTask(uint32_t aRecursionDepth)
 
   // Step 4.1: Execute microtasks.
   if (!mDisableMicroTaskCheckpoint) {
+    PerformMicroTaskCheckPoint();
     if (NS_IsMainThread()) {
-      nsContentUtils::PerformMainThreadMicroTaskCheckpoint();
       Promise::PerformMicroTaskCheckpoint();
     } else {
       Promise::PerformWorkerMicroTaskCheckpoint();
@@ -1659,6 +1662,72 @@ 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::PerformMicroTaskCheckPoint()
+{
+  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;
+  }
+
+  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
 CycleCollectedJSContext::EnvironmentPreparer::invoke(JS::HandleObject scope,
                                                      js::ScriptEnvironmentPreparer::Closure& closure)
diff --git a/xpcom/base/CycleCollectedJSContext.h b/xpcom/base/CycleCollectedJSContext.h
index ac4cf4361..4cd1479ed 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;
@@ -402,6 +414,39 @@ public:
   // Queue an async microtask to the current main or worker thread.
   virtual void DispatchToMicroTask(already_AddRefed<nsIRunnable> aRunnable);
 
+  // Call EnterMicroTask when you're entering JS execution.
+  // Usually the best way to do this is to use nsAutoMicroTask.
+  void EnterMicroTask()
+  {
+    ++mMicroTaskLevel;
+  }
+
+  void LeaveMicroTask()
+  {
+    if (--mMicroTaskLevel == 0) {
+      PerformMicroTaskCheckPoint();
+    }
+  }
+
+  bool IsInMicroTask()
+  {
+    return mMicroTaskLevel != 0;
+  }
+
+  uint32_t MicroTaskLevel()
+  {
+    return mMicroTaskLevel;
+  }
+
+  void SetMicroTaskLevel(uint32_t aLevel)
+  {
+    mMicroTaskLevel = aLevel;
+  }
+
+  void PerformMicroTaskCheckPoint();
+
+  void DispatchMicroTaskRunnable(already_AddRefed<MicroTaskRunnable> aRunnable);
+
   // Storage for watching rejected promises waiting for some client to
   // consume their rejection.
 
@@ -452,6 +497,11 @@ private:
 
   bool mDisableMicroTaskCheckpoint;
 
+  uint32_t mMicroTaskLevel;
+  std::queue<RefPtr<MicroTaskRunnable>> mPendingMicroTaskRunnables;
+
+  uint32_t mMicroTaskRecursionDepth;
+
   OOMState mOutOfMemoryState;
   OOMState mLargeAllocationFailureState;
 
@@ -470,6 +520,25 @@ private:
   EnvironmentPreparer mEnvironmentPreparer;
 };
 
+class MOZ_STACK_CLASS nsAutoMicroTask
+{
+public:
+  nsAutoMicroTask()
+  {
+    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+    if (ccjs) {
+      ccjs->EnterMicroTask();
+    }
+  }
+  ~nsAutoMicroTask()
+  {
+    CycleCollectedJSContext* ccjs = CycleCollectedJSContext::Get();
+    if (ccjs) {
+      ccjs->LeaveMicroTask();
+    }
+  }
+};
+
 void TraceScriptHolder(nsISupports* aHolder, JSTracer* aTracer);
 
 // Returns true if the JS::TraceKind is one the cycle collector cares about.
diff --git a/xpcom/base/moz.build b/xpcom/base/moz.build
index d6a336b40..0fdf47d7d 100644
--- a/xpcom/base/moz.build
+++ b/xpcom/base/moz.build
@@ -152,6 +152,7 @@ FINAL_LIBRARY = 'xul'
 
 LOCAL_INCLUDES += [
     '../build',
+    '/dom/base',
     '/xpcom/ds',
 ]