Mojo EDK: Correctly handle EMSGSIZE on Mac.

mojo/edk/system/channel_posix.cc

 // Copyright 2016 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.
 
 #include "mojo/edk/system/channel.h"
 
 #include <errno.h>
 #include <sys/socket.h>
 
 #include <algorithm>
@@ skipping 382 matching lines @@
 #else
           handles.reset();
 #endif  // defined(OS_MACOSX)
         }
       } else {
         result = PlatformChannelWrite(handle_.get(), message_view.data(),
                                       message_view.data_num_bytes());
       }
 
       if (result < 0) {
-        if (errno != EAGAIN && errno != EWOULDBLOCK)
+        if (errno != EAGAIN && errno != EWOULDBLOCK
+#if defined(OS_MACOSX)
+            // On OS X if sendmsg() is trying to send fds between processes and
+            // there isn't enough room in the output buffer to send the fd
+            // structure over atomically then EMSGSIZE is returned.
+            //
+            // EMSGSIZE presents a problem since the system APIs can only call
+            // us when there's room in the socket buffer and not when there is
+            // "enough" room.
+            //
+            // The current behavior is to return to the event loop when EMSGSIZE
+            // is received and hopefull service another FD.  This is however
+            // still technically a busy wait since the event loop will call us
+            // right back until the receiver has read enough data to allow
+            // passing the FD over atomically.
+            && errno != EMSGSIZE
+#endif
+            ) {
           return false;
+        }
         message_view.SetHandles(std::move(handles));
         outgoing_messages_.emplace_front(std::move(message_view));
         WaitForWriteOnIOThreadNoLock();
         return true;
       }
 
       bytes_written = static_cast<size_t>(result);
     } while (bytes_written < message_view.data_num_bytes());
 
     return FlushOutgoingMessagesNoLock();
@@ skipping 126 matching lines @@
 // static
 scoped_refptr<Channel> Channel::Create(
     Delegate* delegate,
     ScopedPlatformHandle platform_handle,
     scoped_refptr<base::TaskRunner> io_task_runner) {
   return new ChannelPosix(delegate, std::move(platform_handle), io_task_runner);
 }
 
 }  // namespace edk
 }  // namespace mojo