The Implementation
------------------

This section is intended for developing messaging internals.  If any of this is useful to you as a client of IPDL then you probably made a wrong turn down a dark alley somewhere.

TODO:

* Define Chromium and Gecko entities
    * ``MessageLoop``: Provides basic functionality for threads that run a loop that processes messages.  Uses system primitives to avoid wastefully running wait loops.
        * ``MessageLoopForUI`` is a ``MessageLoop`` that adds the ability for the Windows event queue to process events, in addition to handling its own local messages.  It is just a ``MessageLoop`` outside of Windows.
        * ``MessageLoopForIO`` is a ``MessageLoop`` that adds the ability for supported platforms to read from an OS-level IPC queue, in addition to handling its own local messages.
    * The I/O Thread: Has a ``MessageLoopForIO``, used for all IPC between the process and all other processes.  NB: Chromium code refers to this thread as the "main" thread, which it never is in our parlance.
    * ``MessagePump`` and ``base::MessagePump``: Strategies for running messages queued in an ``nsIEventTarget``.  There are a ton of these -- for different platforms, UI vs non-UI-capable handlers, main vs child processes, main thread vs off-main-thread handlers, etc.  ``MessagePumpForIO`` is used by cross-process actors on the I/O thread.
    * ``MessageLink``: abstract base class representing the two ways that actors communicate:
        * ``ThreadLink``: Same-process actors use a queue between threads.  Messages may even be dispatched in-line.
        * ``ProcessLink``: Two-process actors use `sockets <https://man7.org/linux/man-pages/man2/socket.2.html>`_ (POSIX) or `completion ports <https://docs.microsoft.com/en-us/windows/win32/fileio/i-o-completion-ports>`_ (Windows).
    * ``MessageChannel``: Very beefy class where much of the Gecko (platform-agnostic) complexity resides.  Boy, this sure could use some documentation...
* Discuss platform implementations + issues.
* Give a high-level overview of the IPDL compiler.
* IPDLParamTraits (vs ParamTraits)