Tiny: Post PerformFullHashCheck on IO thread, instead of calling it synchronously.

components/safe_browsing_db/v4_local_database_manager.cc

 // Copyright 2016 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.
 
 // This file should not be build on Android but is currently getting built.
 // TODO(vakh): Fix that: http://crbug.com/621647
 
 #include "components/safe_browsing_db/v4_local_database_manager.h"
 
 #include <vector>
@@ skipping 129 matching lines @@
   if (!v4_database_) {
     queued_checks_.push_back(std::move(check));
     return false;
   }
 
   FullHashToStoreAndHashPrefixesMap full_hash_to_store_and_hash_prefixes;
   if (!GetPrefixMatches(check, &full_hash_to_store_and_hash_prefixes)) {
     return true;
   }
 
-  PerformFullHashCheck(std::move(check), full_hash_to_store_and_hash_prefixes);
+  BrowserThread::PostTask(
+      BrowserThread::IO, FROM_HERE,
+      base::Bind(&V4LocalDatabaseManager::PerformFullHashCheck, this,
+                 base::Passed(std::move(check)),
+                 full_hash_to_store_and_hash_prefixes));

[Scott Hess - ex-Googler, 2016/10/14 00:40:59]

I was thinking this might introduce an extra spin of the loop versus v3, but ...
v3 transitions through the UI thread on fetch?  To get the extended-reporting
flag.  OMG.

Anyhow, where I was going with that is that I think right now this would post a
task, and that task might startup a fetcher which would post another task, so
worst-case you'll have two additional tasks to process.  Best case you'll have
one.  I think right now you have enough info to reduce that to one in either
case (either post the task leading directly to RespondToClient, or start the
fetcher leading to RespondToClient later).

[I am open to the argument "That's too complicated" or "It can be a TODO" or
"The fetcher can callback synchronously, and it's hard to detect."]

[[I'm not fresh enough to reason about interactions with StopOnIOThread().]]

   return false;
 }
 
 bool V4LocalDatabaseManager::CheckDownloadUrl(
     const std::vector<GURL>& url_chain,
     Client* client) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   // TODO(vakh): Implement this skeleton.
   return true;
 }
@@ skipping 325 matching lines @@
 }
 
 void V4LocalDatabaseManager::UpdateRequestCompleted(
     std::unique_ptr<ParsedServerResponse> parsed_server_response) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   v4_database_->ApplyUpdate(std::move(parsed_server_response),
                             db_updated_callback_);
 }
 
 }  // namespace safe_browsing