Bug 1634253 - P2. Dissociate running a BackgroundHangMonitor from main thread. r=froydnj

This will allow to have other threads to use one such as the compositor thread.

Differential Revision: https://phabricator.services.mozilla.com/D73820

xpcom/threads/nsThread.cpp

@@ -588,6 +588,7 @@ nsThread::nsThread(NotNull<SynchronizedEventQueue*> aQueue,
       mShutdownRequired(false),
       mPriority(PRIORITY_NORMAL),
       mIsMainThread(aMainThread == MAIN_THREAD),
+      mUseHangMonitor(aMainThread == MAIN_THREAD),
       mIsAPoolThreadFree(nullptr),
       mCanInvokeJS(false),
 #ifdef EARLY_BETA_OR_EARLIER
@@ -606,6 +607,7 @@ nsThread::nsThread()
       mShutdownRequired(false),
       mPriority(PRIORITY_NORMAL),
       mIsMainThread(false),
+      mUseHangMonitor(false),
       mCanInvokeJS(false),
 #ifdef EARLY_BETA_OR_EARLIER
       mLastWakeupCheckTime(TimeStamp::Now()),
@@ -1077,8 +1079,13 @@ nsThread::ProcessNextEvent(bool aMayWait, bool* aResult) {
   }
 
   Maybe<dom::AutoNoJSAPI> noJSAPI;
+
+  if (mUseHangMonitor && reallyWait) {
+    BackgroundHangMonitor().NotifyWait();
+  }
+
   if (mIsMainThread) {
-    DoMainThreadSpecificProcessing(reallyWait);
+    DoMainThreadSpecificProcessing();
   }
 
   ++mNestedEventLoopDepth;
@@ -1155,7 +1162,7 @@ nsThread::ProcessNextEvent(bool aMayWait, bool* aResult) {
 
       mozilla::TimeStamp now = mozilla::TimeStamp::Now();
 
-      if (mIsMainThread) {
+      if (mUseHangMonitor) {
         BackgroundHangMonitor().NotifyActivity();
       }
 
@@ -1352,15 +1359,11 @@ void nsThread::SetScriptObserver(
   mScriptObserver = aScriptObserver;
 }
 
-void nsThread::DoMainThreadSpecificProcessing(bool aReallyWait) const {
+void nsThread::DoMainThreadSpecificProcessing() const {
   MOZ_ASSERT(mIsMainThread);
 
   ipc::CancelCPOWs();
 
-  if (aReallyWait) {
-    BackgroundHangMonitor().NotifyWait();
-  }
-
   // Fire a memory pressure notification, if one is pending.
   if (!ShuttingDown()) {
     MemoryPressureState mpPending = NS_GetPendingMemoryPressure();

xpcom/threads/nsThread.h

@@ -257,8 +257,13 @@ class nsThread : public nsIThreadInternal,
   // notifications, etc.
   nsLocalExecutionRecord EnterLocalExecution();
 
+  void SetUseHangMonitor(bool aValue) {
+    MOZ_ASSERT(IsOnCurrentThread());
+    mUseHangMonitor = aValue;
+  }
+
  private:
-  void DoMainThreadSpecificProcessing(bool aReallyWait) const;
+  void DoMainThreadSpecificProcessing() const;
 
  protected:
   friend class nsThreadShutdownEvent;
@@ -329,6 +334,7 @@ class nsThread : public nsIThreadInternal,
   int8_t mPriority;
 
   const bool mIsMainThread;
+  bool mUseHangMonitor;
   mozilla::Atomic<bool, mozilla::Relaxed>* mIsAPoolThreadFree;
 
   // Set to true if this thread creates a JSRuntime.