Fix DCHECK when shutting down safe browsing DownloadMetadataManager.

chrome/browser/safe_browsing/incident_reporting/download_metadata_manager.cc

 // Copyright 2014 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/safe_browsing/incident_reporting/download_metadata_manager.h"
 
 #include <list>
 
 #include "base/bind.h"
 #include "base/callback.h"
@@ skipping 275 matching lines @@
   write_runner_ = worker_pool->GetSequencedTaskRunnerWithShutdownBehavior(
       token, base::SequencedWorkerPool::SKIP_ON_SHUTDOWN);
 }
 
 DownloadMetadataManager::DownloadMetadataManager(
     const scoped_refptr<base::SequencedTaskRunner>& task_runner)
     : read_runner_(task_runner), write_runner_(task_runner) {
 }
 
 DownloadMetadataManager::~DownloadMetadataManager() {
-  // All download managers must have gone down prior to this.
-  DCHECK(contexts_.empty());
+  // Destruction may have taken place before managers have gone down.
+  for (const auto& value : contexts_) {
+    value.first->RemoveObserver(this);
+    value.second->Detach();
+  }
+  contexts_.clear();
 }
 
 void DownloadMetadataManager::AddDownloadManager(
     content::DownloadManager* download_manager) {
   DCHECK_EQ(contexts_.count(download_manager), 0U);
   download_manager->AddObserver(this);
   contexts_[download_manager] =
       new ManagerContext(read_runner_, write_runner_, download_manager);
 }
 
@@ skipping 190 matching lines @@
     else
       iter->second.item = nullptr;
   } else if (item_) {
     // This item is no longer being observed.
     DCHECK_EQ(item_, download);
     item_ = nullptr;
   }
 }
 
 DownloadMetadataManager::ManagerContext::~ManagerContext() {
-  // All downloads must have been destroyed prior to deleting the context and
-  // all recorded operations and callbacks must have been handled.
+  // A context should not be deleted while waiting for a load to complete.
   DCHECK(pending_items_.empty());
-  DCHECK_EQ(item_, static_cast<content::DownloadItem*>(nullptr));
   DCHECK(get_details_callbacks_.empty());
+
+  // The context may have detached while still observing the item of interest
+  // since a DownloadManager announces that it's going down before it destroyes
+  // its items.
+  if (item_) {
+    item_->RemoveObserver(this);
+    item_ = nullptr;
+  }
 }
 
 void DownloadMetadataManager::ManagerContext::ClearPendingItems() {
   for (const auto& value : pending_items_) {
     if (value.second.item)
       value.second.item->RemoveObserver(this);
   }
   pending_items_.clear();
 }
 
@@ skipping 82 matching lines @@
 
 void DownloadMetadataManager::ManagerContext::WriteMetadata() {
   write_runner_->PostTask(
       FROM_HERE,
       base::Bind(&WriteMetadataOnWorkerPool,
                  metadata_path_,
                  base::Owned(new DownloadMetadata(*download_metadata_))));
 }
 
 }  // namespace safe_browsing