The ReadFile API on Windows invoked by the FileStream::Context class which is used by URLRequestFil…

net/base/file_stream_context.h

 // 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.
 
 // This file defines FileStream::Context class.
 // The general design of FileStream is as follows: file_stream.h defines
 // FileStream class which basically is just an "wrapper" not containing any
 // specific implementation details. It re-routes all its method calls to
 // the instance of FileStream::Context (FileStream holds a scoped_ptr to
 // FileStream::Context instance). Context was extracted into a different class
@@ skipping 10 matching lines @@
 // located in file_stream_context.cc, and all platform-dependent methods are
 // in file_stream_context_{win,posix}.cc. This separation provides better
 // readability of Context's code. And we tried to make as much Context code
 // platform-independent as possible. So file_stream_context_{win,posix}.cc are
 // much smaller than file_stream_context.cc now.
 
 #ifndef NET_BASE_FILE_STREAM_CONTEXT_H_
 #define NET_BASE_FILE_STREAM_CONTEXT_H_
 
 #include "base/files/file.h"
+#include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/move.h"
 #include "base/task_runner.h"
 #include "net/base/completion_callback.h"
 #include "net/base/file_stream.h"
 
 #if defined(OS_POSIX)
 #include <errno.h>
 #endif
 
@@ skipping 111 matching lines @@
 
   void OnFileOpened();
 
 #if defined(OS_WIN)
   void IOCompletionIsPending(const CompletionCallback& callback, IOBuffer* buf);
 
   // Implementation of MessageLoopForIO::IOHandler.
   virtual void OnIOCompleted(base::MessageLoopForIO::IOContext* context,
                              DWORD bytes_read,
                              DWORD error) override;
+
+  // The ReadFile call on Windows can execute synchonously at times.
+  // http://support.microsoft.com/kb/156932. This ends up blocking the calling
+  // thread which is undesirable. To avoid this we execute the ReadFile call
+  // on a worker thread.
+  // The |context| parameter is a weak pointer instance passed to the worker
+  // pool

[ramant (doing other things), 2015/01/31 05:23:22]

nit: period on line# 169. "pool" -> "pool.".

[ananta, 2015/02/03 00:33:43]

Done.

+  // The |file| parameter is the handle to the file being read.
+  // The |buf| parameter is the buffer where we want the ReadFile to read the
+  // data into.
+  // The |buf_len| parameter contains the number of bytes to be read.
+  // The |overlapped| parameter is a pointer to the OVERLAPPED structure being
+  // used.
+  // The |origin_thread_loop| is a MessageLoopProxy instance used to post tasks
+  // back to the originating thread.
+  static void ReadAsync(
+      const base::WeakPtr<FileStream::Context>& context,
+      HANDLE file,
+      scoped_refptr<net::IOBuffer> buf,
+      int buf_len,
+      OVERLAPPED* overlapped,
+      scoped_refptr<base::MessageLoopProxy> origin_thread_loop);
+
+  // This callback executes on the main calling thread. It informs the caller
+  // about the result of the ReadFile call.
+  // The |os_error| parameter contains the value of the last error returned by
+  // the ReadFile API.
+  void ReadAsyncResult(DWORD os_error);
+
 #elif defined(OS_POSIX)
   // ReadFileImpl() is a simple wrapper around read() that handles EINTR
   // signals and calls RecordAndMapError() to map errno to net error codes.
   IOResult ReadFileImpl(scoped_refptr<IOBuffer> buf, int buf_len);
 
   // WriteFileImpl() is a simple wrapper around write() that handles EINTR
   // signals and calls MapSystemError() to map errno to net error codes.
   // It tries to write to completion.
   IOResult WriteFileImpl(scoped_refptr<IOBuffer> buf, int buf_len);
 #endif
 
   base::File file_;
   bool async_in_progress_;
   bool orphaned_;
   scoped_refptr<base::TaskRunner> task_runner_;
 
 #if defined(OS_WIN)
   base::MessageLoopForIO::IOContext io_context_;
   CompletionCallback callback_;
   scoped_refptr<IOBuffer> in_flight_buf_;
+  // WeakPtrFactory for posting tasks back to |this|.
+  base::WeakPtrFactory<Context> weak_ptr_factory_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(Context);
 };
 
 }  // namespace net
 
 #endif  // NET_BASE_FILE_STREAM_CONTEXT_H_