[Ext Registry] Do not try to re-watch registry if we are already watching it.

chrome/browser/extensions/external_registry_loader_win.cc

Index: chrome/browser/extensions/external_registry_loader_win.cc
diff --git a/chrome/browser/extensions/external_registry_loader_win.cc b/chrome/browser/extensions/external_registry_loader_win.cc
index b54835aff5923dc09df2e599efa9688b7d330261..02466345cedb9592cfa048c5e9b0d34a3d476d8c 100644
--- a/chrome/browser/extensions/external_registry_loader_win.cc
+++ b/chrome/browser/extensions/external_registry_loader_win.cc
@@ -54,6 +54,9 @@ std::string MakePrefName(const std::string& extension_id,
 
 namespace extensions {
 
+ExternalRegistryLoader::ExternalRegistryLoader()
+    : is_watching_hklm_(false), is_watching_hkcu_(false) {}
+
 void ExternalRegistryLoader::StartLoading() {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   BrowserThread::PostTask(
@@ -200,27 +203,33 @@ void ExternalRegistryLoader::CompleteLoadAndStartWatchingRegistry() {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   LoadFinished();
 
-  // Start watching registry.
+  // Start watching registry if we are not already.
   LONG result = ERROR_SUCCESS;
-  if ((result = 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 {
-    LOG(WARNING) << "Error observing HKLM: " << result;
+  if (!is_watching_hklm_) {

[Devlin, 2017/04/04 01:44:07]

Do we want these (this and is_watching_hkcu_) as separate bools?  If so, why? 
It doesn't seem like we have logic to try again in the case of an error result
(unless we happen to call StartLoading() again).

[lazyboy, 2017/04/04 18:45:10]

These can independently fail. Or at least HKLM can easily fail, that's why I
resorted to two bools.

Right, we do not have logic to retry and only we have an opportunity to retry if
StartLoading is called.

[Devlin, 2017/04/04 20:48:35]

So, my question here is more around whether or not calling StartLoading() twice
*should* have potential side effects.  Originally, I'd think not, but as it is
now, it could - a watcher that previously failed could be restarted.  I'm not
sure that's a bad thing; it just seems interesting that this would be how we
address it.  Is that change intentional?  If so, maybe worth a comment in the
code or the CL?

[lazyboy, 2017/04/04 21:47:23]

Note that a watcher that previously failed would actually have been restarted
without this CL. But that was not intentional, more like a side effect we
inherited when we allowed StartLoading  to be called more than once.

Since this wasn't intentional, I think it's better to remove this one-off case
that doesn't seem too deterministic and is hard to reason about.

 it just seems interesting that this would be how we

+    if ((result = 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);
+      is_watching_hklm_ = true;
+    } else {
+      LOG(WARNING) << "Error observing HKLM: " << result;
+    }
   }
 
-  if ((result = 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 {
-    LOG(WARNING) << "Error observing HKCU: " << result;
+  if (!is_watching_hkcu_) {
+    if ((result = 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);
+      is_watching_hkcu_ = true;
+    } else {
+      LOG(WARNING) << "Error observing HKCU: " << result;
+    }
   }
 }