workaround for mac kernel bug

ipc/ipc_channel.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.
 
 #ifndef IPC_IPC_CHANNEL_H_
 #define IPC_IPC_CHANNEL_H_
 
 #include <string>
 
 #if defined(OS_POSIX)
@@ skipping 62 matching lines @@
     MODE_OPEN_NAMED_SERVER = MODE_OPEN_ACCESS_FLAG | MODE_SERVER_FLAG |
                              MODE_NAMED_FLAG
 #endif
   };
 
   // The Hello message is internal to the Channel class.  It is sent
   // by the peer when the channel is connected.  The message contains
   // just the process id (pid).  The message has a special routing_id
   // (MSG_ROUTING_NONE) and type (HELLO_MESSAGE_TYPE).
   enum {
-    HELLO_MESSAGE_TYPE = kuint16max  // Maximum value of message type (uint16),
+    HELLO_MESSAGE_TYPE = kuint16max,  // Maximum value of message type (uint16),
                                      // to avoid conflicting with normal
                                      // message types, which are enumeration
                                      // constants starting from 0.
+
+    // The CLOSE_FD_MESSAGE_TYPE is used internall in the IPC class to
+    // work around a bug in sendmsg() on Mac.
+    CLOSE_FD_MESSAGE_TYPE = kuint16max - 1

[Scott Hess - ex-Googler, 2013/10/09 20:18:33]

internall -> internally.

I think the comment is saying why this is here, but not what it does.  It seems
like if there's going to be a second one, maybe the comments should be
refactored to indicate the entire enum is about internal messages (with high
numbers to not conflict), and then have per-message comments with the
definitions.

Could you use HELLO_MESSAGE_TYPE - 1?  Presumably nobody else will define
another one, but you never know...

[hubbe, 2013/10/09 21:34:12]

Done.

   };
 
   // The maximum message size in bytes. Attempting to receive a message of this
   // size or bigger results in a channel error.
   static const size_t kMaximumMessageSize = 128 * 1024 * 1024;
 
   // Amount of data to read at once from the pipe.
   static const size_t kReadBufferSize = 4 * 1024;
 
   // Initialize a Channel.
@@ skipping 109 matching lines @@
 
  private:
   // PIMPL to which all channel calls are delegated.
   class ChannelImpl;
   ChannelImpl *channel_impl_;
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_CHANNEL_H_