AU: Watch for writes on write fds only, and for reads on read fds.

libcurl_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_LIBCURL_HTTP_FETCHER_H__
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_LIBCURL_HTTP_FETCHER_H__
 
 #include <map>
 #include <string>
 #include <curl/curl.h>
@@ skipping 129 matching lines @@
   // expensive.
   bool ConnectionIsExpensive() const;
 
   // Returns whether or not the current build is official.
   bool IsOfficialBuild() const;
 
   // Handles for the libcurl library
   CURLM *curl_multi_handle_;
   CURL *curl_handle_;
 
-  // a list of all file descriptors that we're waiting on from the
-  // glib main loop
+  // Lists of all read(0)/write(1) file descriptors that we're waiting on from
+  // the glib main loop. libcurl may open/close descriptors and switch their
+  // directions so maintain two separate lists so that watch conditions can be
+  // set appropriately.
   typedef std::map<int, std::pair<GIOChannel*, guint> > IOChannels;
-  IOChannels io_channels_;
+  IOChannels io_channels_[2];
 
   // if non-NULL, a timer we're waiting on. glib main loop will call us back
   // when it fires.
   GSource* timeout_source_;
 
   bool transfer_in_progress_;
 
   // The transfer size. -1 if not known.
   off_t transfer_size_;
 
@@ skipping 31 matching lines @@
   // We can't clean everything up while we're in a write callback, so
   // if we get a terminate request, queue it until we can handle it.
   bool terminate_requested_;
 
   DISALLOW_COPY_AND_ASSIGN(LibcurlHttpFetcher);
 };
 
 }  // namespace chromeos_update_engine
 
 #endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_LIBCURL_HTTP_FETCHER_H__