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 439 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 (http://crbug.com/678155).
       info.factory =
           base::Bind(&file::CreateFileService,
-                     BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
+                     base::CreateSingleThreadTaskRunnerWithTraits(
+                         base::TaskTraits()

[gab, 2017/05/08 17:14:32]

See brand new way to specify TaskTraits as a bag instead of builder pattern
(constexpr and much less verbose :)):
https://cs.chromium.org/chromium/src/base/task_scheduler/task_traits.h?rcl=58...

[Marijn Kruisselbrink, 2017/05/08 18:18:01]

Ah, nice. Done. The sample code in post_task.h should probably be updated to the
new pattern too?

[gab, 2017/05/08 19:25:14]

Ah oops looks like we forgot to update that, will do, thanks!

+                             .MayBlock()
+                             .WithBaseSyncPrimitives()

[gab, 2017/05/08 17:14:32]

Do you really need WithBaseSyncPrimitives()? (see its API)

[Marijn Kruisselbrink, 2017/05/08 18:18:01]

Ah yes, it indeed seems that to signal on a waitable event you don't need
WithBaseSyncPrimitives, only to wait for it it's needed? Removed it. The other
task runner here (which currently still uses the DB thread), as well as a third
thread internal in the leveldb mojo code do need WithBaseSyncPrimitives as they
actually wait on waitable events, but this indeed doesn't.

+                             .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