ipc: Update MachPortMac ownership semantics.

ipc/mach_port_attachment_mac.h

 // Copyright 2015 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_MACH_PORT_ATTACHMENT_MAC_H_
 #define IPC_MACH_PORT_ATTACHMENT_MAC_H_
 
 #include <mach/mach.h>
 #include <stdint.h>
 
@@ skipping 27 matching lines @@
     uint32_t mach_port;
     static_assert(sizeof(mach_port_t) <= sizeof(uint32_t),
                   "mach_port_t must be smaller than uint32_t");
 
     // The id of the destination process that the handle is duplicated into.
     base::ProcessId destination_process;
 
     AttachmentId attachment_id;
   };
 
+  // This constructor increments the ref count of |mach_port_| and takes
+  // ownership of the result. Should only be called by the sender of a Chrome
+  // IPC message.
   explicit MachPortAttachmentMac(mach_port_t mach_port);
+
+  // These constructors do not take ownership of |mach_port|, and should only be
+  // called by the receiver of a Chrome IPC message.
   explicit MachPortAttachmentMac(const WireFormat& wire_format);
   explicit MachPortAttachmentMac(const BrokerableAttachment::AttachmentId& id);
 
   BrokerableType GetBrokerableType() const override;
 
   // Returns the wire format of this attachment.
   WireFormat GetWireFormat(const base::ProcessId& destination) const;
 
   mach_port_t get_mach_port() const { return mach_port_; }
 
+  // The caller of this method has taken ownership of |mach_port_|.
+  void reset_mach_port_ownership() { owns_mach_port_ = false; }
+
  private:
   ~MachPortAttachmentMac() override;
   mach_port_t mach_port_;
+
+  // In the sender process, the attachment owns the Mach port of a newly created
+  // message. The attachment broker will eventually take ownership, and set
+  // this member to |false|.
+  // In the destination process, the attachment never owns the Mach port. The
+  // client code that receives the Chrome IPC message is always expected to take
+  // ownership.
+  bool owns_mach_port_;
+  DISALLOW_COPY_AND_ASSIGN(MachPortAttachmentMac);
 };
 
 }  // namespace internal
 }  // namespace IPC
 
 #endif  // IPC_MACH_PORT_ATTACHMENT_MAC_H_