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 57 matching lines @@
     MODE_NAMED_CLIENT = MODE_CLIENT_FLAG | MODE_NAMED_FLAG,
 #if defined(OS_POSIX)
     // An "open" named server accepts connections from ANY client.
     // The caller must then implement their own access-control based on the
     // client process' user Id.
     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).
+  // Messages internal to the IPC implementation are defined here.
+  // Uses Maximum value of message type (uint16), to avoid conflicting
+  // with normal message types, which are enumeration constants starting from 0.
   enum {
-    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 Hello message 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).
+    HELLO_MESSAGE_TYPE = kuint16max,
+    // The CLOSE_FD_MESSAGE_TYPE is used in the IPC class to
+    // work around a bug in sendmsg() on Mac. When an FD is sent
+    // over the socket, a CLOSE_FD_MESSAGE is sent with hops = 2.
+    // The client will return the message with hops = 1, *after* it
+    // has received the message that contains the FD. When we
+    // receive it again on the sender side, we close the FD.
+    CLOSE_FD_MESSAGE_TYPE = HELLO_MESSAGE_TYPE - 1
   };
 
   // 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_