From 1a706a942e6f42b35083a251744795995810269c Mon Sep 17 00:00:00 2001
From: Jens Stutte <jstutte@mozilla.com>
Date: Mon, 12 Dec 2022 15:21:26 +0000
Subject: [PATCH] Bug 1709184 - Part 1: Process pending MT events before
 advancing the shutdown phase. r=xpcom-reviewers,nika,kmag

Differential Revision: https://phabricator.services.mozilla.com/D160176
---
 xpcom/base/AppShutdown.cpp | 21 +++++++++++++++++++++
 xpcom/build/XPCOMInit.cpp  |  7 ++-----
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/xpcom/base/AppShutdown.cpp b/xpcom/base/AppShutdown.cpp
index 680739b73c12..5844551095cb 100644
--- a/xpcom/base/AppShutdown.cpp
+++ b/xpcom/base/AppShutdown.cpp
@@ -340,6 +340,23 @@ void AppShutdown::AdvanceShutdownPhaseInternal(
   if (sCurrentShutdownPhase >= aPhase) {
     return;
   }
+
+  nsCOMPtr<nsIThread> thread = do_GetCurrentThread();
+  if (sCurrentShutdownPhase >= ShutdownPhase::AppShutdownConfirmed) {
+    // Give runnables dispatched so far as part of the ongoing phase a chance
+    // to run before actually advancing the phase. We can do this only after
+    // we passed the point of no return and thus can expect a linear flow
+    // through our shutdown phases. This way the processing is also covered
+    // by the terminator's timer.
+    // Note that this happens only for main thread runnables, such that the
+    // correct way of ensuring shutdown processing remains to have an async
+    // shutdown blocker.
+    if (thread) {
+      NS_ProcessPendingEvents(thread);
+    }
+  }
+
+  // From now on any IsInOrBeyond checks will find the new phase set.
   sCurrentShutdownPhase = aPhase;
 
   // TODO: Bug 1768581
@@ -371,6 +388,10 @@ void AppShutdown::AdvanceShutdownPhaseInternal(
 #endif
         obsService->NotifyObservers(aNotificationSubject, aTopic,
                                     aNotificationData);
+        // Empty our MT event queue again after the notification has finished
+        if (thread) {
+          NS_ProcessPendingEvents(thread);
+        }
       }
     }
   }
diff --git a/xpcom/build/XPCOMInit.cpp b/xpcom/build/XPCOMInit.cpp
index 7ef9a98abd91..ae2ff7027c22 100644
--- a/xpcom/build/XPCOMInit.cpp
+++ b/xpcom/build/XPCOMInit.cpp
@@ -585,7 +585,6 @@ nsresult ShutdownXPCOM(nsIServiceManager* aServMgr) {
 
     // This must happen after the shutdown of media and widgets, which
     // are triggered by the NS_XPCOM_SHUTDOWN_OBSERVER_ID notification.
-    NS_ProcessPendingEvents(thread);
     gfxPlatform::ShutdownLayersIPC();
 
     mozilla::AppShutdown::AdvanceShutdownPhase(
@@ -595,11 +594,11 @@ nsresult ShutdownXPCOM(nsIServiceManager* aServMgr) {
     // dispatches to non main-thread threads.
     ThreadEventTarget::XPCOMShutdownThreadsNotificationFinished();
 #endif
-    NS_ProcessPendingEvents(thread);
 
     // Shutdown the timer thread and all timers that might still be alive
     nsTimerImpl::Shutdown();
 
+    // Have an extra round of processing after the timers went away.
     NS_ProcessPendingEvents(thread);
 
     // Shutdown all remaining threads.  This method does not return until
@@ -617,11 +616,9 @@ nsresult ShutdownXPCOM(nsIServiceManager* aServMgr) {
     // XPCOMShutdownFinal is the default phase for ClearOnShutdown.
     // This AdvanceShutdownPhase will thus free most ClearOnShutdown()'ed
     // smart pointers. Some destructors may fire extra main thread runnables
-    // that will be processed below.
+    // that will be processed inside AdvanceShutdownPhase.
     AppShutdown::AdvanceShutdownPhase(ShutdownPhase::XPCOMShutdownFinal);
 
-    NS_ProcessPendingEvents(thread);
-
     // Shutdown the main thread, processing our last round of events, and then
     // mark that we've finished main thread event processing.
     nsThreadManager::get().ShutdownMainThread();