"Go back" on the block interstitial closes the tab if there is no previous URL.

chrome/browser/managed_mode/managed_mode_interstitial.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/managed_mode/managed_mode_interstitial.h"
 
 #include "base/i18n/rtl.h"
 #include "base/metrics/histogram.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 28 matching lines @@
     const base::Callback<void(bool)>& callback)
     : web_contents_(web_contents),
       interstitial_page_(NULL),
       url_(url),
       callback_(callback) {
   if (ShouldProceed()) {
     // It can happen that the site was only allowed very recently and the URL
     // filter on the IO thread had not been updated yet. Proceed with the
     // request without showing the interstitial.
     DispatchContinueRequest(true);
     delete this;

[Marc Treib, 2014/04/25 08:12:38]

Unrelated question: AFAIK, deleting an object in the constructor, i.e. before it
is fully constructed, gives undefined behavior (provided there is a non-trivial
destructor, which is the case here since our dtor is virtual). Am I missing
something?!

[Bernhard Bauer, 2014/04/25 08:30:13]

Hm, I think in this case the object is fully constructed (all the fields are
initialized), which would explain why it doesn't crash. You're absolutely right
though that we shouldn't do this. I would probably move the body of this
constructor into an Init() method. We could also make the constructor private
and instead expose a static method ManagedModeInterstitial::Show() that creates
a new instance, calls Init() on it and returns void.

[Marc Treib, 2014/04/25 08:50:34]

Officially, the object is fully constructed only when we leave the constructor
(afaik). I'm not worried about a crash in this case, but rather the compiler
deciding "hey, this case can never happen" and removing the whole if. There was
a similar problem a while back in the Linux kernel. I can't find anything
directly on that particular problem right now, but compare
http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html (first
example).
Anyway, I'll clean this up in a follow-up CL.

     return;
   }
 
   InfoBarService* service = InfoBarService::FromWebContents(web_contents);
   if (service) {
     // Remove all the infobars which are attached to |web_contents| and for
     // which ShouldExpire() returns true.
     content::LoadCommittedDetails details;
     // |details.is_in_page| is default false, and |details.is_main_frame| is
     // default true. This results in is_navigation_to_different_page() returning
@@ skipping 141 matching lines @@
   return url_filter->GetFilteringBehaviorForURL(url_) !=
          ManagedModeURLFilter::BLOCK;
 }
 
 void ManagedModeInterstitial::OnFilteringPrefsChanged() {
   if (ShouldProceed())
     interstitial_page_->Proceed();
 }
 
 void ManagedModeInterstitial::DispatchContinueRequest(bool continue_request) {
+  // If there is no history entry to go back to, close the tab instead.
+  int nav_entry_count = web_contents_->GetController().GetEntryCount();
+  if (!continue_request && nav_entry_count == 0) {

[Bernhard Bauer, 2014/04/25 08:30:13]

Nit: braces are optional for one-line bodies in general, and in this file they
are left out.

[Marc Treib, 2014/04/25 08:50:34]

Done.

+    web_contents_->Close();
+  }
+
   BrowserThread::PostTask(
       BrowserThread::IO, FROM_HERE, base::Bind(callback_, continue_request));
 }