net: Implement canceling of all async operations in FileStream.

net/base/file_stream.h

Index: net/base/file_stream.h
===================================================================
--- net/base/file_stream.h	(revision 145483)
+++ net/base/file_stream.h	(working copy)
@@ -41,7 +41,16 @@
   // |net_log| is the net log pointer to use to create a |BoundNetLog|.  May be
   // NULL if logging is not needed.
   // The already opened file will not be automatically closed when FileStream
-  // is destructed.
+  // is destroyed.
+  // Note: if you use this constructor you have 3 options of further usage:
+  // - Only sync operations are used on this stream;
+  // - You wait for completion of all async operations;
+  // - When you finished using stream you call any type of Close*() method
+  //   (including possibly CloseAndCancelAsync() or Close() with later
+  //   destruction without waiting for closing callback).
+  // If your usage doesn't fall into one of 3 options above you are introducing
+  // use-after-free bug when file descriptor could be used after you close it
+  // (and it possibly gets reopened to another file).
   FileStream(base::PlatformFile file, int flags, net::NetLog* net_log);
 
   // If the file stream was opened with Open() or OpenSync(), the underlying
@@ -62,10 +71,16 @@
 
   // Call this method to close the FileStream synchronously.
   // It is OK to call CloseSync() multiple times.  Redundant calls are
-  // ignored. Note that if there are any pending async operations, they'll
-  // be aborted.
+  // ignored. Note that if there are any pending async operations, then
+  // behavior of this method is equivalent to CloseAndCancelAsync().
   virtual void CloseSync();
 
+  // Call this method to cancel pending async operations and close FileStream
+  // asynchronously without any notification of completion. Method should be
+  // used when you want to quickly destroy FileStream without actually calling
+  // the destructor.
+  virtual void CloseAndCancelAsync();
+
   // Call this method to open the FileStream asynchronously.  The remaining
   // methods cannot be used unless the file is opened successfully. Returns
   // ERR_IO_PENDING if the operation is started. If the operation cannot be