Implement a salt for MediaSource IDs that can be cleared by a user.

chrome/browser/media/media_device_id_salt.cc

+// Copyright 2013 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/media/media_device_id_salt.h"
+
+#include "base/logging.h"
+#include "base/prefs/pref_service.h"
+#include "base/rand_util.h"
+#include "chrome/browser/profiles/profile_io_data.h"
+#include "chrome/common/pref_names.h"
+#include "content/public/browser/browser_thread.h"
+
+using content::BrowserThread;
+
+namespace {
+
+std::string CreateSalt() {

[Jói, 2013/10/31 17:19:35]

This should be tested, if nothing more than a manually-inspected test. A basic
automated test could verify that you get a different salt each time.

[perkj_chrome, 2013/11/01 13:39:34]

I found another way. How about using a guid? I took this new implementation from
notifications_api.cc

[Jói, 2013/11/01 14:29:37]

Yep, that seems fine since our GUIDs are actually random (the ones Windows APIs
generate are not, see
http://blogs.msdn.com/b/oldnewthing/archive/2012/05/23/10309199.aspx).

+  const size_t kSaltLength = 8;
+  static const char kAlphabet[] = "0123456789"
+        "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+  std::string salt(kSaltLength, ' ');
+  for (size_t i = 0; i < salt.size(); ++i) {
+    int random_char = base::RandInt(0, sizeof(kAlphabet) - 1);
+    salt[i] = kAlphabet[random_char];
+  }
+  return salt;
+}
+
+}  // namespace
+
+MediaDeviceIDSalt::MediaDeviceIDSalt(PrefService* pref_service,
+                                     bool incognito) {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+
+  if (!incognito) {
+    media_device_id_salt_.Init(prefs::kMediaDeviceIdSalt, pref_service,
+                               base::Bind(&MediaDeviceIDSalt::OnValueReset,
+                                          base::Unretained(this)));
+    salt_ = media_device_id_salt_.GetValue();
+  }
+
+  if (salt_.empty()) {
+    salt_ = CreateSalt();
+  }
+
+  if (!incognito) {
+    media_device_id_salt_.SetValue(salt_);
+  }
+}
+
+MediaDeviceIDSalt::~MediaDeviceIDSalt() {
+}
+
+std::string MediaDeviceIDSalt::GetSalt() {
+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::IO));
+  base::AutoLock salt_lock(lock_);
+  return salt_;
+}
+
+void MediaDeviceIDSalt::RegisterProfilePrefs(
+    user_prefs::PrefRegistrySyncable* registry) {
+  registry->RegisterStringPref(
+      prefs::kMediaDeviceIdSalt,
+      std::string(),
+      user_prefs::PrefRegistrySyncable::UNSYNCABLE_PREF);
+}
+
+void MediaDeviceIDSalt::Reset(PrefService* pref_service) {
+  pref_service->ClearPref(prefs::kMediaDeviceIdSalt);
+}
+
+void MediaDeviceIDSalt::OnValueReset(const std::string& name) {

[Jói, 2013/10/31 17:19:35]

Are you sure this will trigger only when you reset the value, and not when you
change it a couple of lines down (leading to an endless loop)?

In either case this would go away if you switch to the approach we discussed
where Reset is a method instead of a static method.

[perkj_chrome, 2013/11/01 13:39:34]

YEs- it is only triggered once. I was afraid of the same thing but I never
looked up why this did not trigger a loop.

[Jói, 2013/11/01 14:29:37]

OK.

+  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  base::AutoLock salt_lock(lock_);
+  salt_ = CreateSalt();
+  media_device_id_salt_.SetValue(salt_);
+}