AU: Fix potential issues with premature destruction of HTTP fetchers.

http_fetcher.h

 // Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_HTTP_FETCHER_H__
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_HTTP_FETCHER_H__
 
 #include <string>
 #include <vector>
 #include <glib.h>
@@ skipping 27 matching lines @@
   void SetPostData(const void* data, size_t size) {
     post_data_set_ = true;
     post_data_.clear();
     const char *char_data = reinterpret_cast<const char*>(data);
     post_data_.insert(post_data_.end(), char_data, char_data + size);
   }
 
   // Downloading should resume from this offset
   virtual void SetOffset(off_t offset) = 0;
 
-  // Begins the transfer to the specified URL.
+  // Begins the transfer to the specified URL. This fetcher instance should not
+  // be destroyed until either TransferComplete, or TransferTerminated is
+  // called.
   virtual void BeginTransfer(const std::string& url) = 0;
 
-  // Aborts the transfer. TransferComplete() will not be called on the
-  // delegate.
+  // Aborts the transfer. The transfer may not abort right away -- delegate's
+  // TransferTerminated() will be called when the transfer is actually done.
   virtual void TerminateTransfer() = 0;
 
   // If data is coming in too quickly, you can call Pause() to pause the
   // transfer. The delegate will not have ReceivedBytes() called while
   // an HttpFetcher is paused.
   virtual void Pause() = 0;
 
   // Used to unpause an HttpFetcher and let the bytes stream in again.
   // If a delegate is set, ReceivedBytes() may be called on it before
   // Unpause() returns
@@ skipping 27 matching lines @@
 class HttpFetcherDelegate {
  public:
   // Called every time bytes are received.
   virtual void ReceivedBytes(HttpFetcher* fetcher,
                              const char* bytes,
                              int length) = 0;
 
   // Called if the fetcher seeks to a particular offset.
   virtual void SeekToOffset(off_t offset) {}
 
-  // Called when the transfer has completed successfully or been somehow
-  // aborted.
+  // Called when the transfer has completed successfully or been aborted through
+  // means other than TerminateTransfer. It's OK to destroy the |fetcher| object
+  // in this callback.
   virtual void TransferComplete(HttpFetcher* fetcher, bool successful) = 0;
+
+  // Called when the transfer has been aborted through TerminateTransfer. It's
+  // OK to destroy the |fetcher| object in this callback.
+  virtual void TransferTerminated(HttpFetcher* fetcher) {}
 };
 
 }  // namespace chromeos_update_engine
 
 #endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_HTTP_FETCHER_H__