Comment: BackgroundModeManager does not unregister ProfileAttributesStorage observer

This was found when auditing all observers of ProfileInfoCache to check
their unregistration.

Bug: None
Change-Id: I100a21b200bfd451db68c0ce5d3d33a0d97f7f9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519710
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Auto-Submit: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827224}

chrome/browser/background/background_mode_manager.cc

@@ -268,6 +268,8 @@ BackgroundModeManager::BackgroundModeManager(
 
   // Add self as an observer for the ProfileAttributesStorage so we know when
   // profiles are deleted and their names change.
+  // This observer is never unregistered because the BackgroundModeManager
+  // outlives the profile storage.
   profile_storage_->AddObserver(this);
 
   UMA_HISTOGRAM_BOOLEAN("BackgroundMode.OnStartup.AutoLaunchState",

chrome/browser/browser_process_impl.cc

@@ -425,6 +425,10 @@ void BrowserProcessImpl::StartTearDown() {
     // can be destroyed.
     UserManager::Hide();
 #endif  // !defined(OS_CHROMEOS)
+    // `profile_manager_` must be destroyed before `background_mode_manager_`,
+    // because the background mode manager does not stop observing profile
+    // changes at destruction (notifying the observers would cause a use-after-
+    // free).
     profile_manager_.reset();
   }
 

chrome/browser/browser_process_impl.h

@@ -316,6 +316,8 @@ class BrowserProcessImpl : public BrowserProcess,
   std::unique_ptr<SystemNotificationHelper> system_notification_helper_;
 
 #if BUILDFLAG(ENABLE_BACKGROUND_MODE)
+  // Must be destroyed after the profile manager, because it doesn't remove
+  // itself as a profile attributes storage observer on destruction.
   std::unique_ptr<BackgroundModeManager> background_mode_manager_;
 #endif