Fix tab discarding reload count metric.

chrome/browser/ui/tabs/tab_discard_state.cc

 // Copyright 2015 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/ui/tabs/tab_discard_state.h"
 
+#include "base/metrics/histogram.h"
+#include "base/threading/thread_checker.h"
 #include "content/public/browser/web_contents.h"
 
 using content::WebContents;
 
 namespace {
 
 const char kDiscardStateKey[] = "TabDiscardState";
 
+// Ensures that all access is done on the same thread.
+base::ThreadChecker thread_checker;

[chrisha, 2015/09/28 15:07:49]

The thread check remembers the thread on which it was constructed. This is a
static so it'll happen on the main thread in the C++ initializers. The general
pattern when using a thread_checker is to make it a member variable, but I just
realized that won't work for you, as the whole class has a 'static' interface.

Maybe just use BrowserThreads::CurrentlyOn and the associated DCHECKs?

https://code.google.com/p/chromium/codesearch#chromium/src/content/public/bro...

[Georges Khalil, 2015/09/28 18:41:13]

I was under the (wrong) impression that it remembers the thread on the first
call to CalledOnValidThread, my bad.

Changed to DCHECK_CURRENTLY_ON(content::BrowserThread::UI) instead.

+
 }  // namespace
 
 // static
 TabDiscardState* TabDiscardState::Get(WebContents* web_contents) {
+  DCHECK(thread_checker.CalledOnValidThread());
   TabDiscardState* discard_state = static_cast<TabDiscardState*>(
       web_contents->GetUserData(&kDiscardStateKey));
 
   // If this function is called, we probably need to query/change the discard
   // state. Let's go ahead a add one.
   if (!discard_state) {
     discard_state = new TabDiscardState;
     web_contents->SetUserData(&kDiscardStateKey, discard_state);
   }
 
   return discard_state;
 }
 
 // static
 void TabDiscardState::Set(WebContents* web_contents, TabDiscardState* state) {
   web_contents->SetUserData(&kDiscardStateKey, state);
 }
 
 // static
 bool TabDiscardState::IsDiscarded(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   return discard_state->is_discarded_;
 }
 
 // static
 void TabDiscardState::SetDiscardState(WebContents* web_contents, bool state) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
+  if (discard_state->is_discarded_ && !state) {
+    static int reload_count = 0;
+    UMA_HISTOGRAM_CUSTOM_COUNTS("Tabs.Discard.ReloadCount", ++reload_count, 1,
+                                1000, 50);
+  }
+
   discard_state->is_discarded_ = state;
 }
 
 // static
 int TabDiscardState::DiscardCount(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   return discard_state->discard_count_;
 }
 
 // static
 void TabDiscardState::IncrementDiscardCount(WebContents* web_contents) {
   TabDiscardState* discard_state = TabDiscardState::Get(web_contents);
   discard_state->discard_count_++;
 }
 
 // static
 void TabDiscardState::CopyState(content::WebContents* old_contents,
                                 content::WebContents* new_contents) {
   TabDiscardState* old_state = Get(old_contents);
   TabDiscardState* new_State = Get(new_contents);
   *new_State = *old_state;
 }