Bug 1584568 - add a way to get the background event target; r=KrisWright

This function is needed for people whose needs don't map cleanly to
`NS_DispatchToBackgroundThread`, usually because they're using the event
target to do thread consistency checks.  Once we have this function, we
can start converting singleton threads/thread pools that want to use
functionality like the above.

Differential Revision: https://phabricator.services.mozilla.com/D47454

--HG--
extra : moz-landing-system : lando

xpcom/threads/nsThreadManager.cpp

@@ -462,6 +462,16 @@ nsresult nsThreadManager::DispatchToBackgroundThread(nsIRunnable* aEvent,
   return backgroundTarget->Dispatch(aEvent, aDispatchFlags);
 }
 
+nsresult nsThreadManager::GetBackgroundEventTarget(nsIEventTarget** aEventTarget) {
+  if (!mInitialized) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsCOMPtr<nsIEventTarget> et(mBackgroundEventTarget);
+  et.forget(aEventTarget);
+  return NS_OK;
+}
+
 nsThread* nsThreadManager::GetCurrentThread() {
   // read thread local storage
   void* data = PR_GetThreadPrivate(mCurThreadIndex);

xpcom/threads/nsThreadManager.h

@@ -56,6 +56,8 @@ class nsThreadManager : public nsIThreadManager {
   nsresult DispatchToBackgroundThread(nsIRunnable* aEvent,
                                       uint32_t aDispatchFlags);
 
+  nsresult GetBackgroundEventTarget(nsIEventTarget** aEventTarget);
+
   // Returns the maximal number of threads that have been in existence
   // simultaneously during the execution of the thread manager.
   uint32_t GetHighestNumberOfThreads();

xpcom/threads/nsThreadUtils.cpp

@@ -529,6 +529,10 @@ nsresult NS_DispatchToBackgroundThread(nsIRunnable* aEvent,
   return nsThreadManager::get().DispatchToBackgroundThread(aEvent, aDispatchFlags);
 }
 
+nsresult NS_GetBackgroundEventTarget(nsIEventTarget** aEventTarget) {
+  return nsThreadManager::get().GetBackgroundEventTarget(aEventTarget);
+}
+
 // nsAutoLowPriorityIO
 nsAutoLowPriorityIO::nsAutoLowPriorityIO() {
 #if defined(XP_WIN)

xpcom/threads/nsThreadUtils.h

@@ -1706,6 +1706,11 @@ extern nsresult NS_DispatchToBackgroundThread(already_AddRefed<nsIRunnable> aEve
 extern nsresult NS_DispatchToBackgroundThread(nsIRunnable* aEvent,
                                               uint32_t aDispatchFlags = NS_DISPATCH_NORMAL);
 
+/**
+ * Get the event target that NS_DispatchToBackgroundThread uses.
+ */
+extern nsresult NS_GetBackgroundEventTarget(nsIEventTarget** aEventTarget);
+
 namespace mozilla {
 
 /**