Add an IO thread index bitmap to disk cache.

net/disk_cache/sparse_control.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 "net/disk_cache/sparse_control.h"
 
 #include "base/bind.h"
 #include "base/format_macros.h"
 #include "base/logging.h"
 #include "base/message_loop.h"
@@ skipping 513 matching lines @@
 // We were not able to open this child; see what we can do.
 bool SparseControl::ContinueWithoutChild(const std::string& key) {
   if (kReadOperation == operation_)
     return false;
   if (kGetRangeOperation == operation_)
     return true;
 
   if (!entry_->backend_)
     return false;
 
+  // This CreateEntry doesn't need to modify the index bitmap, because child
+  // entries are only ever created from the cache thread for use on the cache
+  // thread.
   child_ = entry_->backend_->CreateEntryImpl(key);
   if (!child_) {
     child_ = NULL;
     result_ = net::ERR_CACHE_READ_FAILURE;
     return false;
   }
   // Write signature.
   InitChildData();
   return true;
 }
@@ skipping 331 matching lines @@
     CompletionCallback cb = abort_callbacks_[i];
     if (i == abort_callbacks_.size() - 1)
       abort_callbacks_.clear();
 
     entry_->Release();  // Don't touch object after this line.
     cb.Run(net::OK);
   }
 }
 
 }  // namespace disk_cache