Fix possible deadlock during shutdown by not using the FILE thread.

content/browser/browser_context.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 "content/public/browser/browser_context.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
 #include <limits>
 #include <memory>
 #include <utility>
 #include <vector>
 
 #include "base/base64.h"
 #include "base/command_line.h"
 #include "base/guid.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/rand_util.h"
+#include "base/task_scheduler/post_task.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/browser/blob_storage/chrome_blob_storage_context.h"
 #include "content/browser/browsing_data/browsing_data_remover_impl.h"
 #include "content/browser/download/download_manager_impl.h"
 #include "content/browser/indexed_db/indexed_db_context_impl.h"
 #include "content/browser/loader/resource_dispatcher_host_impl.h"
 #include "content/browser/push_messaging/push_messaging_router.h"
 #include "content/browser/storage_partition_impl_map.h"
 #include "content/common/child_process_host_impl.h"
@@ skipping 438 matching lines @@
                                  base::WrapUnique(connection_holder));
 
     ServiceManagerConnection* connection =
         connection_holder->service_manager_connection();
 
     // New embedded service factories should be added to |connection| here.
 
     if (base::CommandLine::ForCurrentProcess()->HasSwitch(
             switches::kMojoLocalStorage)) {
       ServiceInfo info;
+      // TODO(mek): Use sequenced task runner rather than single thread task
+      // runner when mojo supports that.

[gab, 2017/05/03 20:18:57]

Add http://crbug.com/678155 for ease of searching.

But overall curious why this has to do with Mojo? Mojo isn't happy running its
services per se on sequences but it shouldn't care that the
"file_service_runner_" is a SequencedTaskRunner (just requires adjusting
FileService's interface to allow that? it doesn't appear to be truly
thread-affine, rather just thread-unsafe == sequence-affine?)

       info.factory =
           base::Bind(&file::CreateFileService,
-                     BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
+                     base::CreateSingleThreadTaskRunnerWithTraits(
+                         base::TaskTraits()
+                             .MayBlock()
+                             .WithBaseSyncPrimitives()
+                             .WithShutdownBehavior(
+                                 base::TaskShutdownBehavior::BLOCK_SHUTDOWN)),
                      BrowserThread::GetTaskRunnerForThread(BrowserThread::DB));
       connection->AddEmbeddedService(file::mojom::kServiceName, info);
     }
 
     ContentBrowserClient::StaticServiceMap services;
     browser_context->RegisterInProcessServices(&services);
     for (const auto& entry : services) {
       connection->AddEmbeddedService(entry.first, entry.second);
     }
     connection->Start();
@@ skipping 69 matching lines @@
   base::Base64Encode(base::RandBytesAsString(16), &salt);
   DCHECK(!salt.empty());
   return salt;
 }
 
 BrowsingDataRemoverDelegate* BrowserContext::GetBrowsingDataRemoverDelegate() {
   return nullptr;
 }
 
 }  // namespace content