[Extensions] Observe unloading extensions in the TabHelper

chrome/browser/extensions/tab_helper.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/extensions/tab_helper.h"
 
 #include "base/logging.h"
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "build/build_config.h"
@@ skipping 66 matching lines @@
       profile_(Profile::FromBrowserContext(web_contents->GetBrowserContext())),
       extension_app_(NULL),
       pending_web_app_action_(NONE),
       last_committed_nav_entry_unique_id_(0),
       update_shortcut_on_load_complete_(false),
       script_executor_(
           new ScriptExecutor(web_contents, &script_execution_observers_)),
       location_bar_controller_(new LocationBarController(web_contents)),
       extension_action_runner_(new ExtensionActionRunner(web_contents)),
       webstore_inline_installer_factory_(new WebstoreInlineInstallerFactory()),
+      registry_observer_(this),
       image_loader_ptr_factory_(this),
       weak_ptr_factory_(this) {
   // The ActiveTabPermissionManager requires a session ID; ensure this
   // WebContents has one.
   SessionTabHelper::CreateForWebContents(web_contents);
   // The Unretained() is safe because ForEachFrame is synchronous.
   web_contents->ForEachFrame(
       base::Bind(&TabHelper::SetTabId, base::Unretained(this)));
   active_tab_permission_granter_.reset(new ActiveTabPermissionGranter(
       web_contents,
@@ skipping 65 matching lines @@
   script_execution_observers_.RemoveObserver(observer);
 }
 
 void TabHelper::SetExtensionApp(const Extension* extension) {
   DCHECK(!extension || AppLaunchInfo::GetFullLaunchURL(extension).is_valid());
   if (extension_app_ == extension)
     return;
 
   extension_app_ = extension;
 
+  if (extension_app_) {

[lazyboy, 2016/06/03 01:43:56]

lgtm given this is not true:
Is it possible that two extensions get associated with the same TabHelper?
SetExtensionApp(extension1)
SetExtensionApp(extension2)
Uninstall -> extension 2
Uninstall -> extension 1, the DCHECK in line 545 below will fail.

[Devlin, 2016/06/06 16:50:22]

That flow might be able to happen (if e.g. you navigate from one hosted app to
another), but that shouldn't trigger the DCHECK:
SetExtensionApp(extension1) -> register observer, extension_app_ = extension1
SetExtensionApp(extension2) -> extension_app_ = extension2
Uninstall(extension2) -> extension_app_ = nullptr, unregister observer
Uninstall(extension1) -> since unregistered, never see notification

However, a similar flow would cause a DCHECK:
SetExtensionApp(extension1)
SetExtensionApp(nullptr)
Uninstall(extension1)

I've add logic to SetExtensionApp() to remove the observer if setting to nullptr
to address it.

[lazyboy, 2016/06/06 18:10:49]

Acknowledged.

+    registry_observer_.Add(
+        ExtensionRegistry::Get(web_contents()->GetBrowserContext()));
+  }
+
   UpdateExtensionAppIcon(extension_app_);
 
   content::NotificationService::current()->Notify(
       chrome::NOTIFICATION_TAB_CONTENTS_APPLICATION_EXTENSION_CHANGED,
       content::Source<TabHelper>(this),
       content::NotificationService::NoDetails());
 }
 
 void TabHelper::SetExtensionAppById(const std::string& extension_app_id) {
   const Extension* extension = GetExtension(extension_app_id);
@@ skipping 343 matching lines @@
       install_id,
       success,
       success ? std::string() : error,
       result));
 }
 
 WebContents* TabHelper::GetAssociatedWebContents() const {
   return web_contents();
 }
 
+void TabHelper::OnExtensionUnloaded(
+    content::BrowserContext* browser_context,
+    const Extension* extension,
+    UnloadedExtensionInfo::Reason reason) {
+  DCHECK(extension_app_);
+  if (extension == extension_app_) {
+    SetExtensionApp(nullptr);
+    registry_observer_.RemoveAll();
+  }
+}
+
 void TabHelper::GetApplicationInfo(WebAppAction action) {
   NavigationEntry* entry =
       web_contents()->GetController().GetLastCommittedEntry();
   if (!entry)
     return;
 
   pending_web_app_action_ = action;
   last_committed_nav_entry_unique_id_ = entry->GetUniqueID();
 
   Send(new ChromeViewMsg_GetWebApplicationInfo(routing_id()));
@@ skipping 18 matching lines @@
   }
 }
 
 void TabHelper::SetTabId(content::RenderFrameHost* render_frame_host) {
   render_frame_host->Send(
       new ExtensionMsg_SetTabId(render_frame_host->GetRoutingID(),
                                 SessionTabHelper::IdForTab(web_contents())));
 }
 
 }  // namespace extensions