Observe adding external extensions via windows registry.

chrome/browser/extensions/external_registry_loader_win.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/external_registry_loader_win.h"
 
 #include "base/bind.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_file.h"
@@ skipping 43 matching lines @@
 
 namespace extensions {
 
 void ExternalRegistryLoader::StartLoading() {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   BrowserThread::PostTask(
       BrowserThread::FILE, FROM_HERE,
       base::Bind(&ExternalRegistryLoader::LoadOnFileThread, this));
 }
 
-void ExternalRegistryLoader::LoadOnFileThread() {
+void ExternalRegistryLoader::LoadPrefsOnFileThread() {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
-  base::TimeTicks start_time = base::TimeTicks::Now();
   scoped_ptr<base::DictionaryValue> prefs(new base::DictionaryValue);
 
   // A map of IDs, to weed out duplicates between HKCU and HKLM.
   std::set<base::string16> keys;
   base::win::RegistryKeyIterator iterator_machine_key(
       HKEY_LOCAL_MACHINE,
       kRegistryExtensions,
       KEY_WOW64_32KEY);
   for (; iterator_machine_key.Valid(); ++iterator_machine_key)
     keys.insert(iterator_machine_key.Name());
@@ skipping 99 matching lines @@
         base::UTF16ToASCII(extension_version));
     prefs->SetString(
         MakePrefName(id, ExternalProviderImpl::kExternalCrx),
         extension_path_str);
     prefs->SetBoolean(
         MakePrefName(id, ExternalProviderImpl::kMayBeUntrusted),
         true);
   }
 
   prefs_.reset(prefs.release());
+}
+
+void ExternalRegistryLoader::LoadOnFileThread() {
+  base::TimeTicks start_time = base::TimeTicks::Now();
+  LoadPrefsOnFileThread();
   LOCAL_HISTOGRAM_TIMES("Extensions.ExternalRegistryLoaderWin",
                         base::TimeTicks::Now() - start_time);
   BrowserThread::PostTask(
       BrowserThread::UI, FROM_HERE,
-      base::Bind(&ExternalRegistryLoader::LoadFinished, this));
+      base::Bind(&ExternalRegistryLoader::CompleteLoadAndStartWatchingRegistry,
+                 this));
+}
+
+void ExternalRegistryLoader::CompleteLoadAndStartWatchingRegistry() {
+  CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  LoadFinished();
+
+  // Start watching registry.
+  // TODO(lazyboy): Is using KEY_WOW64_32KEY correct?
+  if (hklm_key_.Create(HKEY_LOCAL_MACHINE, kRegistryExtensions,
+                       KEY_NOTIFY | KEY_WOW64_32KEY) == ERROR_SUCCESS) {
+    base::win::RegKey::ChangeCallback callback =
+        base::Bind(&ExternalRegistryLoader::OnRegistryKeyChanged,
+                   base::Unretained(this), base::Unretained(&hklm_key_));
+    hklm_key_.StartWatching(callback);
+  } else {
+    // TODO(lazyboy): Error handling.

[asargent_no_longer_on_chrome, 2016/01/25 18:50:52]

Did you have any particular ideas in mind? I don't know that there's a lot more
you'd want to do in this case. Perhaps more details are available about the
error that you could add to the below log statement? 

[lazyboy, 2016/01/26 05:20:04]

No I don't, maybe a LOG(WARNING) is enough?
Perhaps more details are available about the

+    LOG(ERROR) << "Error observing HKLM.";
+  }
+
+  if (hkcu_key_.Create(HKEY_CURRENT_USER, kRegistryExtensions, KEY_NOTIFY) ==
+      ERROR_SUCCESS) {
+    base::win::RegKey::ChangeCallback callback =
+        base::Bind(&ExternalRegistryLoader::OnRegistryKeyChanged,
+                   base::Unretained(this), base::Unretained(&hkcu_key_));
+    hkcu_key_.StartWatching(callback);
+  } else {
+    // TODO(lazyboy): Error handling.
+    LOG(ERROR) << "Error observing HKCU.";
+  }
+}
+
+void ExternalRegistryLoader::OnRegistryKeyChanged(base::win::RegKey* key) {
+  // |OnRegistryKeyChanged| is removed as an observer when the ChangeCallback is
+  // called, so we need to re-register.
+  key->StartWatching(base::Bind(&ExternalRegistryLoader::OnRegistryKeyChanged,
+                                base::Unretained(this), base::Unretained(key)));
+
+  BrowserThread::PostTask(
+      BrowserThread::FILE, FROM_HERE,
+      base::Bind(&ExternalRegistryLoader::UpdatePrefsOnFileThread, this));
+}
+
+void ExternalRegistryLoader::UpdatePrefsOnFileThread() {
+  CHECK(BrowserThread::CurrentlyOn(BrowserThread::FILE));
+  base::TimeTicks start_time = base::TimeTicks::Now();
+  LoadPrefsOnFileThread();
+  LOCAL_HISTOGRAM_TIMES("Extensions.ExternalRegistryLoaderWinUpdate",
+                        base::TimeTicks::Now() - start_time);
+  BrowserThread::PostTask(BrowserThread::UI, FROM_HERE,
+                          base::Bind(&ExternalRegistryLoader::OnUpdated, this));
 }
 
 }  // namespace extensions