Supervised user blacklist: Don't download the blacklist file on every startup

chrome/browser/supervised_user/supervised_user_service.cc

Index: chrome/browser/supervised_user/supervised_user_service.cc
diff --git a/chrome/browser/supervised_user/supervised_user_service.cc b/chrome/browser/supervised_user/supervised_user_service.cc
index 76c4288a97c1b46f832eaf287e15b51873a69699..21c1b4ce290fc562053c5f8469f30aa3e7330669 100644
--- a/chrome/browser/supervised_user/supervised_user_service.cc
+++ b/chrome/browser/supervised_user/supervised_user_service.cc
@@ -6,6 +6,7 @@
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
+#include "base/files/file_util.h"
 #include "base/memory/ref_counted.h"
 #include "base/path_service.h"
 #include "base/prefs/pref_service.h"
@@ -620,8 +621,18 @@ void SupervisedUserService::OnSiteListUpdated() {
 
 void SupervisedUserService::LoadBlacklist(const base::FilePath& path,
                                           const GURL& url) {
-  // TODO(treib): Don't re-download the blacklist if the local file exists!
-  if (!url.is_valid()) {
+  BrowserThread::PostTaskAndReplyWithResult(
+      BrowserThread::IO,

[Bernhard Bauer, 2015/04/16 13:58:06]

I don't think you want to do this on the IO thread (which, sadly, is not the
thread to do disk I/O on: it's only for non-blocking I/O, which in practice
means IPC...). You probably want BrowserThread::GetBlockingPool() (with
base::PostTaskAndReplyWithResult()).

[Marc Treib, 2015/04/16 14:04:16]

Argh, yeah, I ran into this before (the IO thread not being allowed to do IO ;)
)
I meant to use BrowserThread::FILE. Is GetBlockingPool preferable to that?

[Bernhard Bauer, 2015/04/16 14:09:16]

Yes, because it will use more than one thread, so there is less risk of
contention. It also doesn't block shutdown by default (I think).

[Marc Treib, 2015/04/16 14:40:49]

Got it, thanks! Done.

+      FROM_HERE,
+      base::Bind(&base::PathExists, path),
+      base::Bind(&SupervisedUserService::OnBlacklistFileChecked,
+                 weak_ptr_factory_.GetWeakPtr(), path, url));
+}
+
+void SupervisedUserService::OnBlacklistFileChecked(const base::FilePath& path,
+                                                   const GURL& url,
+                                                   bool file_exists) {
+  if (file_exists) {
     LoadBlacklistFromFile(path);
     return;
   }