[mojo-edk] Support transferring null Mach ports.

mojo/edk/system/channel_posix.cc

Index: mojo/edk/system/channel_posix.cc
diff --git a/mojo/edk/system/channel_posix.cc b/mojo/edk/system/channel_posix.cc
index bc759e0c159f4af0ed1b3f6a89661d8a35e79b95..8dac516bf02d77dab27c5751f70692a365041fa9 100644
--- a/mojo/edk/system/channel_posix.cc
+++ b/mojo/edk/system/channel_posix.cc
@@ -140,20 +140,19 @@ class ChannelPosix : public Channel,
     // On OSX, we can have mach ports which are located in the extra header
     // section.
     using MachPortsEntry = Channel::Message::MachPortsEntry;
-    CHECK(extra_header_size >= num_handles * sizeof(MachPortsEntry));
-    size_t num_mach_ports = 0;
-    const MachPortsEntry* mach_ports =
-        reinterpret_cast<const MachPortsEntry*>(extra_header);
-    for (size_t i = 0; i < num_handles; i++) {
-      if (mach_ports[i].mach_port != MACH_PORT_NULL)
-        num_mach_ports++;
-    }
+    using MachPortsExtraHeader = Channel::Message::MachPortsExtraHeader;
+    CHECK(extra_header_size >=
+          sizeof(MachPortsExtraHeader) + num_handles * sizeof(MachPortsEntry));
+    const MachPortsExtraHeader* mach_ports_header =
+        reinterpret_cast<const MachPortsExtraHeader*>(extra_header);
+    size_t num_mach_ports = mach_ports_header->num_ports;
     CHECK(num_mach_ports <= num_handles);
     if (incoming_platform_handles_.size() + num_mach_ports < num_handles)
       return nullptr;
 
     ScopedPlatformHandleVectorPtr handles(
         new PlatformHandleVector(num_handles));
+    const MachPortsEntry* mach_ports = mach_ports_header->entries;
     for (size_t i = 0, mach_port_index = 0; i < num_handles; ++i) {
       if (mach_port_index < num_mach_ports &&
           mach_ports[mach_port_index].index == i) {