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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+IPDL: Inter-Thread and Inter-Process Message Passing
+====================================================
+
+The Idea
+--------
+
+**IPDL**, the "Inter-[thread|process] Protocol Definition Language", is the
+Mozilla-specific language that allows code to communicate between system
+threads or processes in a standardized, efficient, safe, secure and
+platform-agnostic way.  IPDL communications take place between *parent* and
+*child* objects called *actors*.  The architecture is inspired by the `actor
+model <https://en.wikipedia.org/wiki/Actor_model>`_.
+
+.. note::
+    IPDL actors differ from the actor model in one significant way -- all
+    IPDL communications are *only* between a parent and its only child.
+
+The actors that constitute a parent/child pair are called **peers**.  Peer
+actors communicate through an **endpoint**, which is an end of a message pipe.
+An actor is explicitly bound to its endpoint, which in turn is bound to a
+particular thread soon after it is constructed.  An actor never changes its
+endpoint and may only send and receive predeclared **messages** from/to that
+endpoint, on that thread.  Violations result in runtime errors.  A thread may
+be bound to many otherwise unrelated actors but an endpoint supports
+**top-level** actors and any actors they **manage** (see below).
+
+.. note::
+     More precisely, endpoints can be bound to any ``nsISerialEventTarget``,
+     which are themselves associated with a specific thread.  By default,
+     IPDL will bind to the current thread's "main" serial event target,
+     which, if it exists, is retrieved with ``GetCurrentSerialEventTarget``.
+     For the sake of clarity, this document will frequently refer to actors
+     as bound to threads, although the more precise interpretation of serial
+     event targets is also always valid.
+
+.. note::
+    Internally, we use the "Ports" component of the `Chromium Mojo`_ library
+    to *multiplex* multiple endpoints (and, therefore, multiple top-level
+    actors).  This means that the endpoints communicate over the same native
+    pipe, which conserves limited OS resources.  The implications of this are
+    discussed in `IPDL Best Practices`_.
+
+Parent and child actors may be bound to threads in different processes, in
+different threads in the same process, or even in the same thread in the same
+process.  That last option may seem unreasonable but actors are versatile and
+their layout can be established at run-time so this could theoretically arise
+as the result of run-time choices.  One large example of this versatility is
+``PCompositorBridge`` actors, which in different cases connect endpoints in the
+main process and the GPU process (for UI rendering on Windows), in a content
+process and the GPU process (for content rendering on Windows), in the main
+process and the content process (for content rendering on Mac, where there is
+no GPU process), or between threads on the main process (UI rendering on Mac).
+For the most part, this does not require elaborate or redundant coding; it
+just needs endpoints to be bound judiciously at runtime.  The example in
+:ref:`Connecting With Other Processes` shows one way this can be done.  It
+also shows that, without proper plain-language documentation of *all* of the
+ways endpoints are configured, this can quickly lead to unmaintainable code.
+Be sure to document your endpoint bindings throroughly!!!
+
+.. _Chromium Mojo: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/mojo/core/README.md#Port
+
+The Approach
+------------
+
+The actor framework will schedule tasks to run on its associated event target,
+in response to messages it receives.  Messages are specified in an IPDL
+**protocol** file and the response handler tasks are defined per-message by C++
+methods.  As actors only communicate in pairs, and each is bound to one thread,
+sending is always done sequentially, never concurrently (same for receiving).
+This means that it can, and does, guarantee that an actor will always receive
+messages in the same order they were sent by its related actor -- and that this
+order is well defined since the related actor can only send from one thread.
+
+.. warning::
+    There are a few (rare) exceptions to the message order guarantee.  They
+    include  `synchronous nested`_  messages, `interrupt`_ messages, and
+    messages with a ``[Priority]`` or ``[Compress]`` annotation.
+
+An IPDL protocol file specifies the messages that may be sent between parent
+and child actors, as well as the direction and payload of those messages.
+Messages look like function calls but, from the standpoint of their caller,
+they may start and end at any time in the future -- they are *asynchronous*,
+so they won't block their sending actors or any other components that may be
+running in the actor's thread's ``MessageLoop``.
+
+.. note::
+    Not all IPDL messages are asynchronous.  Again, we run into exceptions for
+    messages that are synchronous, `synchronous nested`_ or `interrupt`_.  Use
+    of synchronous and nested messages is strongly discouraged but may not
+    always be avoidable.  They will be defined later, along with superior
+    alternatives to both that should work in nearly all cases.  Interrupt
+    messages were prone to misuse and are deprecated, with removal expected in
+    the near future
+    (`Bug 1729044 <https://bugzilla.mozilla.org/show_bug.cgi?id=1729044>`_).
+
+Protocol files are compiled by the *IPDL compiler* in an early stage of the
+build process.  The compiler generates C++ code that reflects the protocol.
+Specifically, it creates one C++ class that represents the parent actor and one
+that represents the child.  The generated files are then automatically included
+in the C++ build process.  The generated classes contain public methods for
+sending the protocol messages, which client code will use as the entry-point to
+IPC communication.  The generated methods are built atop our IPC framework,
+defined in `/ipc <https://searchfox.org/mozilla-central/source/ipc>`_, that
+standardizes the safe and secure use of sockets, pipes, shared memory, etc on
+all supported platforms.  See `Using The IPDL compiler`_ for more on
+integration with the build process.
+
+Client code must be written that subclasses these generated classes, in order
+to add handlers for the tasks generated to respond to each message.  It must
+also add routines (``ParamTraits``) that define serialization and
+deserialization for any types used in the payload of a message that aren't
+already known to the IPDL system.  Primitive types, and a bunch of Mozilla
+types, have predefined ``ParamTraits`` (`here
+<https://searchfox.org/mozilla-central/source/ipc/glue/IPCMessageUtils.h>`__
+and `here
+<https://searchfox.org/mozilla-central/source/ipc/glue/IPCMessageUtilsSpecializations.h>`__).
+
+.. note::
+    Among other things, client code that uses the generated code must include
+    ``chromium-config.mozbuild`` in its ``moz.build`` file.  See `Using The
+    IPDL compiler`_ for a complete list of required build changes.
+
+.. _interrupt: `The Old Ways`_
+.. _synchronous nested: `The Rest`_
+
+The Steps To Making A New Actor
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+#. Decide what folder you will work in and create:
+
+    #. An IPDL protocol file, named for your actor (e.g. ``PMyActor.ipdl`` --
+       actor protocols must begin with a ``P``).  See `The Protocol Language`_.
+    #. Properly-named source files for your actor's parent and child
+       implementations (e.g. ``MyActorParent.h``, ``MyActorChild.h`` and,
+       optionally, adjacent .cpp files).  See `The C++ Interface`_.
+    #. IPDL-specific updates to the ``moz.build`` file.  See `Using The IPDL
+       compiler`_.
+#. Write your actor protocol (.ipdl) file:
+
+    #. Decide whether you need a top-level actor or a managed actor.  See
+       `Top Level Actors`_.
+    #. Find/write the IPDL and C++ data types you will use in communication.
+       Write ``ParamTraits`` for C++ data types that don't have them.  See
+       `Generating IPDL-Aware C++ Data Types: IPDL Structs and Unions`_ for IPDL
+       structures.  See `Referencing Externally Defined Data Types: IPDL
+       Includes`_ and `ParamTraits`_ for C++ data types.
+    #. Write your actor and its messages.  See `Defining Actors`_.
+#. Write C++ code to create and destroy instances of your actor at runtime.
+
+    * For managed actors, see `Actor Lifetimes in C++`_.
+    * For top-level actors, see `Creating Top Level Actors From Other Actors`_.
+      The first actor in a process is a very special exception -- see `Creating
+      First Top Level Actors`_.
+#. Write handlers for your actor's messages.  See `Actors and Messages in
+   C++`_.
+#. Start sending messages through your actors!  Again, see `Actors and Messages
+   in C++`_.
+
+The Protocol Language
+---------------------
+
+This document will follow the integration of two actors into Firefox --
+``PMyManager`` and ``PMyManaged``.  ``PMyManager`` will manage ``PMyManaged``.
+A good place to start is with the IPDL actor definitions.  These are files
+that are named for the actor (e.g. ``PMyManager.ipdl``) and that declare the
+messages that a protocol understands.  These actors are for demonstration
+purposes and involve quite a bit of functionality.  Most actors will use a very
+small fraction of these features.
+
+.. literalinclude:: _static/PMyManager.ipdl
+   :language: c++
+   :name: PMyManager.ipdl
+
+.. literalinclude:: _static/PMyManaged.ipdl
+   :language: c++
+   :name: PMyManaged.ipdl
+
+These files reference three additional files.  ``MyTypes.ipdlh`` is an "IPDL
+header" that can be included into ``.ipdl`` files as if it were inline, except
+that it also needs to include any external actors and data types it uses:
+
+.. literalinclude:: _static/MyTypes.ipdlh
+   :language: c++
+   :name: MyTypes.ipdlh
+
+``MyActorUtils.h`` and ``MyDataTypes.h`` are normal C++ header files that
+contain definitions for types passed by these messages, as well as instructions
+for serializing them.  They will be covered in `The C++ Interface`_.
+
+Using The IPDL compiler
+~~~~~~~~~~~~~~~~~~~~~~~
+
+To build IPDL files, list them (alphabetically sorted) in a ``moz.build`` file.
+In this example, the ``.ipdl`` and ``.ipdlh`` files would be alongside a
+``moz.build`` containing:
+
+.. code-block:: c++
+
+    IPDL_SOURCES += [
+        "MyTypes.ipdlh",
+        "PMyManaged.ipdl",
+        "PMyManager.ipdl",
+    ]
+
+    UNIFIED_SOURCES += [
+        "MyManagedChild.cpp",
+        "MyManagedParent.cpp",
+        "MyManagerChild.cpp",
+        "MyManagerParent.cpp",
+    ]
+
+    include("/ipc/chromium/chromium-config.mozbuild")
+
+``chromium-config.mozbuild`` sets up paths so that generated IPDL header files
+are in the proper scope.  If it isn't included, the build will fail with
+``#include`` errors in both your actor code and some internal ipc headers.  For
+example:
+
+.. code-block:: c++
+
+    c:/mozilla-src/mozilla-unified/obj-64/dist/include\ipc/IPCMessageUtils.h(13,10): fatal error: 'build/build_config.h' file not found
+
+``.ipdl`` files are compiled to C++ files as one of the earliest post-configure
+build steps.  Those files are, in turn, referenced throughout the source code
+and build process.  From ``PMyManager.ipdl`` the compiler generates two header
+files added to the build context and exported globally:
+``mozilla/myns/PMyManagerParent.h`` and ``mozilla/myns/PMyManagerChild.h``, as
+discussed in `Namespaces`_ below.  These files contain the base classes for the
+actors.  It also makes several other files, including C++ source files and
+another header, that are automatically included into the build and should not
+require attention.
+
+C++ definions of the actors are required for IPDL.  They define the actions
+that are taken in response to messages -- without this, they would have no
+value.  There will be much more on this when we discuss `Actors and Messages in
+C++`_ but note here that C++ header files named for the actor are required by
+the IPDL `compiler`.  The example would expect
+``mozilla/myns/MyManagedChild.h``, ``mozilla/myns/MyManagedParent.h``,
+``mozilla/myns/MyManagerChild.h`` and ``mozilla/myns/MyManagerParent.h`` and
+will not build without them.
+
+Referencing Externally Defined Data Types: IPDL Includes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Let's begin with ``PMyManager.ipdl``.  It starts by including types that it
+will need from other places:
+
+.. code-block:: c++
+
+    include protocol PMyManaged;
+    include MyTypes;                          // for MyActorPair
+
+    using MyActorEnum from "mozilla/myns/MyActorUtils.h";
+    using struct mozilla::myns::MyData from "mozilla/MyDataTypes.h";
+    [MoveOnly] using mozilla::myns::MyOtherData from "mozilla/MyDataTypes.h";
+    [RefCounted] using class mozilla::myns::MyThirdData from "mozilla/MyDataTypes.h";
+
+The first line includes a protocol that PMyManager will manage.  That protocol
+is defined in its own ``.ipdl`` file.  Cyclic references are expected and pose
+no concern.
+
+The second line includes the file ``MyTypes.ipdlh``, which defines types like
+structs and unions, but in IPDL, which means they have behavior that goes
+beyond the similar C++ concepts.  Details can be found in `Generating
+IPDL-Aware C++ Data Types: IPDL Structs and Unions`_.
+
+The final lines include types from C++ headers.  Additionally, the [RefCounted]
+and [MoveOnly] attributes tell IPDL that the types have special functionality
+that is important to operations.  These are the data type attributes currently
+understood by IPDL:
+
+================ ==============================================================
+``[RefCounted]`` Type ``T`` is reference counted (by ``AddRef``/``Release``).
+                 As a parameter to a message or as a type in IPDL
+                 structs/unions, it is referenced as a ``RefPtr<T>``.
+``[MoveOnly]``   The type ``T`` is treated as uncopyable.  When used as a
+                 parameter in a message or an IPDL struct/union, it is as an
+                 r-value ``T&&``.
+================ ==============================================================
+
+Finally, note that ``using``, ``using class`` and ``using struct`` are all
+valid syntax.  The ``class`` and ``struct`` keywords are optional.
+
+Namespaces
+~~~~~~~~~~
+
+From the IPDL file:
+
+.. code-block:: c++
+
+    namespace mozilla {
+    namespace myns {
+
+        // ... data type and actor definitions ...
+
+    }    // namespace myns
+    }    // namespace mozilla
+
+
+Namespaces work similar to the way they do in C++.  They also mimic the
+notation, in an attempt to make them comfortable to use.  When IPDL actors are
+compiled into C++ actors, the namespace scoping is carried over.  As previously
+noted, when C++ types are included into IPDL files, the same is true.  The most
+important way in which they differ is that IPDL also uses the namespace to
+establish the path to the generated files.  So, the example defines the IPDL
+data type ``mozilla::myns::MyUnion`` and the actors
+``mozilla::myns::PMyManagerParent`` and  ``mozilla::myns::PMyManagerChild``,
+which can be included from ``mozilla/myns/PMyManagerParent.h``,
+``mozilla/myns/PMyManagerParent.h`` and ``mozilla/myns/PMyManagerChild.h``,
+respectively.  The namespace becomes part of the path.
+
+Generating IPDL-Aware C++ Data Types: IPDL Structs and Unions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+``PMyManager.ipdl`` and ``MyTypes.ipdlh`` define:
+
+.. code-block:: c++
+
+    [Comparable] union MyUnion {
+        float;
+        MyOtherData;
+    };
+
+    struct MyActorPair {
+        PMyManaged actor1;
+        nullable PMyManaged actor2;
+    };
+
+From these descriptions, IPDL generates C++ classes that approximate the
+behavior of C++ structs and unions but that come with pre-defined
+``ParamTraits`` implementations.  These objects can also be used as usual
+outside of IPDL, although the lack of control over the generated code means
+they are sometimes poorly suited to use as plain data.  See `ParamTraits`_ for
+details.
+
+The ``[Comparable]`` attribute tells IPDL to generate ``operator==`` and
+``operator!=`` for the new type.  In order for it to do that, the fields inside
+the new type need to define both of those operators.
+
+Finally, the ``nullable`` keyword indicates that, when serialized, the actor
+may be null.  It is intended to help users avoid null-object dereference
+errors.  It only applies to actor types and may also be attached to parameters
+in message declarations.
+
+Defining Actors
+~~~~~~~~~~~~~~~
+
+The real point of any ``.ipdl`` file is that each defines exactly one actor
+protocol.  The definition always matches the ``.ipdl`` filename.  Repeating the
+one in ``PMyManager.ipdl``:
+
+.. code-block:: c++
+
+    sync protocol PMyManager {
+        manages PMyManaged;
+
+        async PMyManaged();
+        // ... more message declarations ...
+    };
+
+.. important::
+    A form of reference counting is `always` used internally by IPDL to make
+    sure that it and its clients never address an actor the other component
+    deleted but this becomes fragile, and sometimes fails, when the client code
+    does not respect the reference count.  For example, when IPDL detects that
+    a connection died due to a crashed remote process, deleting the actor could
+    leave dangling pointers, so IPDL `cannot` delete it.  On the other hand,
+    there are many cases where IPDL is the only entity to have references to
+    some actors (this is very common for one side of a managed actor) so IPDL
+    `must` delete it.  If all of those objects were reference counted then
+    there would be no complexity here.  Indeed, new actors using
+    ``[ManualDealloc]`` should not be approved without a very compelling
+    reason.  New ``[ManualDealloc]`` actors may soon be forbidden.
+
+The ``sync`` keyword tells IPDL that the actor contains messages that block the
+sender using ``sync`` blocking, so the sending thread waits for a response to
+the message.  There is more on what it and the other blocking modes mean in
+`IPDL messages`_.  For now, just know that this is redundant information whose
+value is primarily in making it easy for other developers to know that there
+are ``sync`` messages defined here.  This list gives preliminary definitions of
+the options for the actor-blocking policy of messages:
+
+======================= =======================================================
+``async``               Actor may contain only asynchronous messages.
+``sync``                Actor has ``async`` capabilities and adds  ``sync``
+                        messages.  ``sync`` messages
+                        can only be sent from the child actor to the parent.
+``intr`` (deprecated)   Actor has ``sync`` capabilities and adds ``intr``
+                        messages.  Some messages can be received while an actor
+                        waits for an ``intr`` response.  This type will be
+                        removed soon.
+======================= =======================================================
+
+Beyond these protocol blocking strategies, IPDL supports annotations that
+indicate the actor has messages that may be received in an order other than
+the one they were sent in.  These orderings attempt to handle messages in
+"message thread" order (as in e.g. mailing lists).  These behaviors can be
+difficult to design for.  Their use is discouraged but is sometimes warranted.
+They will be discussed further in `Nested messages`_.
+
+============================== ================================================
+``[NestedUpTo=inside_sync]``   Actor has high priority messages that can be
+                               handled while waiting for a ``sync`` response.
+``[NestedUpTo=inside_cpow]``   Actor has the highest priority messages that
+                               can be handled while waiting for a ``sync``
+                               response.
+============================== ================================================
+
+The ``manages`` clause tells IPDL that ``PMyManager`` manages the
+``PMyManaged`` actor that was previously ``include`` d.  As with any managed
+protocol, it must also be the case that ``PMyManaged.ipdl`` includes
+``PMyManager`` and declares that ``PMyManaged`` is ``managed`` by
+``PMyManager``.  Recalling the code:
+
+.. code-block:: c++
+
+    // PMyManaged.ipdl
+    include protocol PMyManager;
+    // ...
+
+    protocol PMyManaged {
+      manager PMyManager;
+      // ...
+    };
+
+An actor has a ``manager`` (e.g. ``PMyManaged``) or else it is a top-level
+actor (e.g. ``PMyManager``).  An actor protocol may be managed by more than one
+actor type.  For example, ``PMyManaged`` could have also been managed by some
+``PMyOtherManager`` not shown here.  In that case, ``manager`` s are presented
+in a list, separated by ``or`` -- e.g. ``manager PMyManager or
+PMyOtherManager``.  Of course, an **instance** of a managed actor type has only
+one manager actor (and is therefore managed by only one of the types of
+manager).  The manager of an instance of a managee is always the actor that
+constructed that managee.
+
+Finally, there is the message declaration ``async PMyManaged()``.  This message
+is a constructor for ``MyManaged`` actors; unlike C++ classes, it is found in
+``MyManager``.  Every manager will need to expose constructors to create its
+managed types.  These constructors are the only way to create an actor that is
+managed.  They can take parameters and return results, like normal messages.
+The implementation of IPDL constructors are discussed in `Actor Lifetimes in
+C++`_.
+
+We haven't discussed a way to construct new top level actors.  This is a more
+advanced topic and is covered separately in `Top Level Actors`_.
+
+.. _IPDL messages: `Declaring IPDL Messages`_
+
+Declaring IPDL Messages
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The final part of the actor definition is the declaration of messages:
+
+.. code-block:: c++
+
+    sync protocol PMyManager {
+      // ...
+      parent:
+        async __delete__(nsString aNote);
+        sync SomeMsg(MyActorPair? aActors, MyData[] aMyData)
+            returns (int32_t x, int32_t y, MyUnion aUnion);
+        async PMyManaged();
+      both:
+        [Tainted] async AnotherMsg(MyActorEnum aEnum, int32_t aNumber)
+            returns (MyOtherData aOtherData);
+    };
+
+The messages are grouped into blocks by ``parent:``, ``child:`` and ``both:``.
+These labels work the way ``public:`` and ``private:`` work in C++ -- messages
+after these descriptors are sent/received (only) in the direction specified.
+
+.. note::
+    As a mnemonic to remember which direction they indicate, remember to put
+    the word "to" in front of them.  So, for example, ``parent:`` precedes
+    ``__delete__``, meaning ``__delete__`` is sent from the child **to** the
+    parent, and ``both:`` states that ``AnotherMsg`` can be sent **to** either
+    endpoint.
+
+IPDL messages support the following annotations:
+
+======================== ======================================================
+``[Compress]``           Indicates repeated messages of this type will
+                         consolidate.
+``[Tainted]``            Parameters are required to be validated before using
+                         them.
+``[Priority=Foo]``       Priority of ``MessageTask`` that runs the C++ message
+                         handler.  ``Foo`` is one of: ``normal``, ``input``,
+                         ``vsync``, ``mediumhigh``, or ``control``.
+                         See the ``IPC::Message::PriorityValue`` enum.
+``[Nested=inside_sync]`` Indicates that the message can sometimes be handled
+                         while a sync message waits for a response.
+``[Nested=inside_cpow]`` Indicates that the message can sometimes be handled
+                         while a sync message waits for a response.
+``[LazySend]``           Messages with this annotation will be queued up to be
+                         sent together either immediately before a non-LazySend
+                         message, or from a direct task.
+======================== ======================================================
+
+``[Compress]`` provides crude protection against spamming with a flood of
+messages.  When messages of type ``M`` are compressed, the queue of unprocessed
+messages between actors will never contain an ``M`` beside another one; they
+will always be separated by a message of a different type.  This is achieved by
+throwing out the older of the two messages if sending the new one would break
+the rule.  This has been used to throttle pointer events between the main and
+content processes.
+
+``[Compress=all]`` is similar but applies whether or not the messages are
+adjacent in the message queue.
+
+``[Tainted]`` is a C++ mechanism designed to encourage paying attentiton to
+parameter security.  The values of tainted parameters cannot be used until you
+vouch for their safety.  They are discussed in `Actors and Messages in C++`_.
+
+The ``Nested`` annotations are deeply related to the message's blocking policy
+that follows it and which was briefly discussed in `Defining Actors`_.  See
+`Nested messages`_ for details.
+
+``[LazySend]`` indicates the message doesn't need to be sent immediately, and
+can be sent later, from a direct task. Worker threads which do not support
+direct task dispatch will ignore this attribute. Messages with this annotation
+will still be delivered in-order with other messages, meaning that if a normal
+message is sent, any queued ``[LazySend]`` messages will be sent first. The
+attribute allows the transport layer to combine messages to be sent together,
+potentially reducing thread wake-ups for I/O and receiving threads.
+
+The following is a complete list of the available blocking policies.  It
+resembles the list in `Defining Actors`_:
+
+====================== ========================================================
+``async``              Actor may contain only asynchronous messages.
+``sync``               Actor has ``async`` capabilities and adds  ``sync``
+                       messages.  ``sync`` messages can only be sent from the
+                       child actor to the parent.
+``intr`` (deprecated)  Actor has ``sync`` capabilities and adds ``intr``
+                       messages.  This type will be removed soon.
+====================== ========================================================
+
+The policy defines whether an actor will wait for a response when it sends a
+certain type of message.  A ``sync`` actor will wait immediately after sending
+a ``sync`` message, stalling its thread, until a response is received.  This is
+an easy source of browser stalls.  It is rarely required that a message be
+synchronous.  New ``sync`` messages are therefore required to get approval from
+an IPC peer.  The IPDL compiler will require such messages to be listed in the
+file ``sync-messages.ini``.
+
+The notion that only child actors can send ``sync`` messages was introduced to
+avoid potential deadlocks.  It relies on the belief that a cycle (deadlock) of
+sync messages is impossible because they all point in one direction.  This is
+no longer the case because any endpoint can be a child `or` parent and some,
+like the main process, sometimes serve as both.  This means that sync messages
+should be used with extreme care.
+
+.. note::
+    The notion of sync messages flowing in one direction is still the main
+    mechanism IPDL uses to avoid deadlock.  New actors should avoid violating
+    this rule as the consequences are severe (and complex).  Actors that break
+    these rules should not be approved without **extreme** extenuating
+    circumstances.  If you think you need this, check with the IPC team on
+    Element first (#ipc).
+
+An ``async`` actor will not wait.  An ``async`` response is essentially
+identical to sending another ``async`` message back.  It may be handled
+whenever received messages are handled.  The value over an ``async`` response
+message comes in the ergonomics -- async responses are usually handled by C++
+lambda functions that are more like continuations than methods.  This makes
+them easier to write and to read.  Additionally, they allow a response to
+return message failure, while there would be no such response if we were
+expecting to send a new async message back, and it failed.
+
+Following synchronization is the name of the message and its parameter list.
+The message ``__delete__`` stands out as strange -- indeed, it terminates the
+actor's connection.  `It does not delete any actor objects itself!`  It severs
+the connections of the actor `and any actors it manages` at both endpoints.  An
+actor will never send or receive any messages after it sends or receives a
+``__delete__``.  Note that all sends and receives have to happen on a specific
+*worker* thread for any actor tree so the send/receive order is well defined.
+Anything sent after the actor processes ``__delete__`` is ignored (send returns
+an error, messages yet to be received fail their delivery).  In other words,
+some future operations may fail but no unexpected behavior is possible.
+
+In our example, the child can break the connection by sending ``__delete__`` to
+the parent.  The only thing the parent can do to sever the connection is to
+fail, such as by crashing.  This sort of unidirectional control is both common
+and desirable.
+
+``PMyManaged()`` is a managed actor constructor.  Note the asymmetry -- an
+actor contains its managed actor's constructors but its own destructor.
+
+The list of parameters to a message is fairly straight-forward.  Parameters
+can be any type that has a C++ ``ParamTraits`` specialization and is imported
+by a directive.  That said, there are some surprises in the list of messages:
+
+================= =============================================================
+``int32_t``,...   The standard primitive types are included.  See `builtin.py`_
+                  for a list.  Pointer types are, unsurprisingly, forbidden.
+``?``             When following a type T, the parameter is translated into
+                  ``Maybe<T>`` in C++.
+``[]``            When following a type T, the parameter is translated into
+                  ``nsTArray<T>`` in C++.
+================= =============================================================
+
+Finally, the returns list declares the information sent in response, also as a
+tuple of typed parameters.  As previously mentioned, even ``async`` messages
+can receive responses.  A ``sync`` message will always wait for a response but
+an ``async`` message will not get one unless it has a ``returns`` clause.
+
+This concludes our tour of the IPDL example file.  The connection to C++ is
+discussed in the next chapter; messages in particular are covered in `Actors
+and Messages in C++`_.  For suggestions on best practices when designing your
+IPDL actor approach, see `IPDL Best Practices`_.
+
+.. _builtin.py: https://searchfox.org/mozilla-central/source/ipc/ipdl/ipdl/builtin.py
+
+IPDL Syntax Quick Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following is a list of the keywords and operators that have been introduced
+for use in IPDL files:
+
+============================= =================================================
+``include``                   Include a C++ header (quoted file name) or
+                              ``.ipdlh`` file (unquoted with no file suffix).
+``using (class|struct) from`` Similar to ``include`` but imports only a
+                              specific data type.
+``include protocol``          Include another actor for use in management
+                              statements, IPDL data types or as parameters to
+                              messages.
+``[RefCounted]``              Indicates that the imported C++ data types are
+                              reference counted. Refcounted types require a
+                              different ``ParamTraits`` interface than
+                              non-reference-counted types.
+``[ManualDealloc]``           Indicates that the IPDL interface uses the legacy
+                              manual allocation/deallocation interface, rather
+                              than modern reference counting.
+``[MoveOnly]``                Indicates that an imported C++ data type should
+                              not be copied.  IPDL code will move it instead.
+``namespace``                 Specifies the namespace for IPDL generated code.
+``union``                     An IPDL union definition.
+``struct``                    An IPDL struct definition.
+``[Comparable]``              Indicates that IPDL should generate
+                              ``operator==`` and ``operator!=`` for the given
+                              IPDL struct/union.
+``nullable``                  Indicates that an actor reference in an IPDL type
+                              may be null when sent over IPC.
+``protocol``                  An IPDL protocol (actor) definition.
+``sync/async``                These are used in two cases: (1) to indicate
+                              whether a message blocks as it waits for a result
+                              and (2) because an actor that contains ``sync``
+                              messages must itself be labeled ``sync`` or
+                              ``intr``.
+``[NestedUpTo=inside_sync]``  Indicates that an actor contains
+                              [Nested=inside_sync] messages, in addition to
+                              normal messages.
+``[NestedUpTo=inside_cpow]``  Indicates that an actor contains
+                              [Nested=inside_cpow] messages, in addition to
+                              normal messages.
+``intr``                      Used to indicate either that (1) an actor
+                              contains ``sync``, ``async`` and (deprecated)
+                              ``intr`` messages, or (2) a message is ``intr``
+                              type.
+``[Nested=inside_sync]``      Indicates that the message can be handled while
+                              waiting for lower-priority, or in-message-thread,
+                              sync responses.
+``[Nested=inside_cpow]``      Indicates that the message can be handled while
+                              waiting for lower-priority, or in-message-thread,
+                              sync responses.  Cannot be sent by the parent
+                              actor.
+``manager``                   Used in a protocol definition to indicate that
+                              this actor manages another one.
+``manages``                   Used in a protocol definition to indicate that
+                              this actor is managed by another one.
+``or``                        Used in a ``manager`` clause for actors that have
+                              multiple potential managers.
+``parent: / child: / both:``  Indicates direction of subsequent actor messages.
+                              As a mnemonic to remember which direction they
+                              indicate, put the word "to" in front of them.
+``returns``                   Defines return values for messages.  All types
+                              of message, including ``async``, support
+                              returning values.
+``__delete__``                A special message that destroys the related
+                              actors at both endpoints when sent.
+                              ``Recv__delete__`` and ``ActorDestroy`` are
+                              called before destroying the actor at the other
+                              endpoint, to allow for cleanup.
+``int32_t``,...               The standard primitive types are included.
+``String``                    Translated into ``nsString`` in C++.
+``?``                         When following a type T in an IPDL data structure
+                              or message parameter,
+                              the parameter is translated into ``Maybe<T>`` in
+                              C++.
+``[]``                        When following a type T in an IPDL data structure
+                              or message parameter,
+                              the parameter is translated into ``nsTArray<T>``
+                              in C++.
+``[Tainted]``                 Used to indicate that a message's handler should
+                              receive parameters that it is required to
+                              manually validate.  Parameters of type ``T``
+                              become ``Tainted<T>`` in C++.
+``[Compress]``                Indicates repeated messages of this type will
+                              consolidate.  When two messages of this type are
+                              sent and end up side-by-side in the message queue
+                              then the older message is discarded (not sent).
+``[Compress=all]``            Like ``[Compress]`` but discards the older
+                              message regardless of whether they are adjacent
+                              in the message queue.
+``[Priority=Foo]``            Priority of ``MessageTask`` that runs the C++
+                              message handler.  ``Foo`` is one of: ``normal``,
+                              ``input``, ``vsync``, ``mediumhigh``, or
+                              ``control``.
+``[LazySend]``                Messages with this annotation will be queued up to
+                              be sent together immediately before a non-LazySend
+                              message, or from a direct task.
+``[ChildImpl="RemoteFoo"]``   Indicates that the child side implementation of
+                              the actor is a class named ``RemoteFoo``, and the
+                              definition is included by one of the
+                              ``include "...";`` statements in the file.
+                              *New uses of this attribute are discouraged.*
+``[ParentImpl="FooImpl"]``    Indicates that the parent side implementation of
+                              the actor is a class named ``FooImpl``, and the
+                              definition is included by one of the
+                              ``include "...";`` statements in the file.
+                              *New uses of this attribute are discouraged.*
+``[ChildImpl=virtual]``       Indicates that the child side implementation of
+                              the actor is not exported by a header, so virtual
+                              ``Recv`` methods should be used instead of direct
+                              function calls.  *New uses of this attribute are
+                              discouraged.*
+``[ParentImpl=virtual]``      Indicates that the parent side implementation of
+                              the actor is not exported by a header, so virtual
+                              ``Recv`` methods should be used instead of direct
+                              function calls.  *New uses of this attribute are
+                              discouraged.*
+============================= =================================================
+
+
+The C++ Interface
+-----------------
+
+ParamTraits
+~~~~~~~~~~~
+
+Before discussing how C++ represents actors and messages, we look at how IPDL
+connects to the imported C++ data types.  In order for any C++ type to be
+(de)serialized, it needs an implementation of the ``ParamTraits`` C++ type
+class.  ``ParamTraits`` is how your code tells IPDL what bytes to write to
+serialize your objects for sending, and how to convert those bytes back to
+objects at the other endpoint.  Since ``ParamTraits`` need to be reachable by
+IPDL code, they need to be declared in a C++ header and imported by your
+protocol file.  Failure to do so will result in a build error.
+
+Most basic types and many essential Mozilla types are always available for use
+without inclusion.  An incomplete list includes: C++ primitives, strings
+(``std`` and ``mozilla``), vectors (``std`` and ``mozilla``), ``RefPtr<T>``
+(for serializable ``T``), ``UniquePtr<T>``, ``nsCOMPtr<T>``, ``nsTArray<T>``,
+``std::unordered_map<T>``, ``nsresult``, etc.  See `builtin.py
+<https://searchfox.org/mozilla-central/source/ipc/ipdl/ipdl/builtin.py>`_,
+`ipc_message_utils.h
+<https://searchfox.org/mozilla-central/source/ipc/chromium/src/chrome/common/ipc_message_utils.h>`_
+and `IPCMessageUtilsSpecializations.h
+<https://searchfox.org/mozilla-central/source/ipc/glue/IPCMessageUtilsSpecializations.h>`_.
+
+``ParamTraits`` typically bootstrap with the ``ParamTraits`` of more basic
+types, until they hit bedrock (e.g. one of the basic types above).  In the most
+extreme cases, a ``ParamTraits`` author may have to resort to designing a
+binary data format for a type.  Both options are available.
+
+We haven't seen any of this C++ yet.  Let's look at the data types included
+from ``MyDataTypes.h``:
+
+.. code-block:: c++
+
+    // MyDataTypes.h
+    namespace mozilla::myns {
+        struct MyData {
+            nsCString s;
+            uint8_t bytes[17];
+            MyData();      // IPDL requires the default constructor to be public
+        };
+
+        struct MoveonlyData {
+            MoveonlyData();
+            MoveonlyData& operator=(const MoveonlyData&) = delete;
+
+            MoveonlyData(MoveonlyData&& m);
+            MoveonlyData& operator=(MoveonlyData&& m);
+        };
+
+        typedef MoveonlyData MyOtherData;
+
+        class MyUnusedData {
+        public:
+            NS_INLINE_DECL_REFCOUNTING(MyUnusedData)
+            int x;
+        };
+    };
+
+    namespace IPC {
+        // Basic type
+        template<>
+        struct ParamTraits<mozilla::myns::MyData> {
+            typedef mozilla::myns::MyData paramType;
+            static void Write(MessageWriter* m, const paramType& in);
+            static bool Read(MessageReader* m, paramType* out);
+        };
+
+        // [MoveOnly] type
+        template<>
+        struct ParamTraits<mozilla::myns::MyOtherData> {
+            typedef mozilla::myns::MyOtherData paramType;
+            static void Write(MessageWriter* m, const paramType& in);
+            static bool Read(MessageReader* m, paramType* out);
+        };
+
+        // [RefCounted] type
+        template<>
+        struct ParamTraits<mozilla::myns::MyUnusedData*> {
+            typedef mozilla::myns::MyUnusedData paramType;
+            static void Write(MessageWriter* m, paramType* in);
+            static bool Read(MessageReader* m, RefPtr<paramType>* out);
+        };
+    }
+
+MyData is a struct and MyOtherData is a typedef.  IPDL is fine with both.
+Additionally, MyOtherData is not copyable, matching its IPDL ``[MoveOnly]``
+annotation.
+
+``ParamTraits`` are required to be defined in the ``IPC`` namespace.  They must
+contain a ``Write`` method with the proper signature that is used for
+serialization and a ``Read`` method, again with the correct signature, for
+deserialization.
+
+Here we have three examples of declarations: one for an unannotated type, one
+for ``[MoveOnly]`` and a ``[RefCounted]`` one.  Notice the difference in the
+``[RefCounted]`` type's method signatures.  The only difference that may not be
+clear from the function types is that, in the non-reference-counted case, a
+default-constructed object is supplied to ``Read`` but, in the
+reference-counted case, ``Read`` is given an empty ``RefPtr<MyUnusedData>`` and
+should only allocate a ``MyUnusedData`` to return if it so desires.
+
+These are straight-forward implementations of the ``ParamTraits`` methods for
+``MyData``:
+
+.. code-block:: c++
+
+    /* static */ void IPC::ParamTraits<MyData>::Write(MessageWriter* m, const paramType& in) {
+        WriteParam(m, in.s);
+        m->WriteBytes(in.bytes, sizeof(in.bytes));
+    }
+    /* static */ bool IPC::ParamTraits<MyData>::Read(MessageReader* m, paramType* out) {
+        return ReadParam(m, &out->s) &&
+               m->ReadBytesInto(out->bytes, sizeof(out->bytes));
+    }
+
+``WriteParam`` and ``ReadParam`` call the ``ParamTraits`` for the data you pass
+them, determined using the type of the object as supplied.  ``WriteBytes`` and
+``ReadBytesInto`` work on raw, contiguous bytes as expected.  ``MessageWriter``
+and ``MessageReader`` are IPDL internal objects which hold the incoming/outgoing
+message as a stream of bytes and the current spot in the stream.  It is *very*
+rare for client code to need to create or manipulate these objects. Their
+advanced use is beyond the scope of this document.
+
+.. important::
+    Potential failures in ``Read`` include everyday C++ failures like
+    out-of-memory conditions, which can be handled as usual.  But ``Read`` can
+    also fail due to things like data validation errors.  ``ParamTraits`` read
+    data that is considered insecure.  It is important that they catch
+    corruption and properly handle it.  Returning false from ``Read`` will
+    usually result in crashing the process (everywhere except in the main
+    process).  This is the right behavior as the browser would be in an
+    unexpected state, even if the serialization failure was not malicious
+    (since it cannot process the message).  Other responses, such as failing
+    with a crashing assertion, are inferior.  IPDL fuzzing relies on
+    ``ParamTraits`` not crashing due to corruption failures.
+    Occasionally, validation will require access to state that ``ParamTraits``
+    can't easily reach.  (Only) in those cases, validation can be reasonably
+    done in the message handler.  Such cases are a good use of the ``Tainted``
+    annotation.  See `Actors and Messages in C++`_ for more.
+
+.. note::
+    In the past, it was required to specialize ``mozilla::ipc::IPDLParamTraits<T>``
+    instead of ``IPC::ParamTraits<T>`` if you needed the actor object itself during
+    serialization or deserialization. These days the actor can be fetched using
+    ``IPC::Message{Reader,Writer}::GetActor()`` in ``IPC::ParamTraits``, so that
+    trait should be used for all new serializations.
+
+A special case worth mentioning is that of enums.  Enums are a common source of
+security holes since code is rarely safe with enum values that are not valid.
+Since data obtained through IPDL messages should be considered tainted, enums
+are of principal concern.  ``ContiguousEnumSerializer`` and
+``ContiguousEnumSerializerInclusive`` safely implement ``ParamTraits`` for
+enums that are only valid for a contiguous set of values, which is most of
+them.  The generated ``ParamTraits`` confirm that the enum is in valid range;
+``Read`` will return false otherwise.  As an example, here is the
+``MyActorEnum`` included from ``MyActorUtils.h``:
+
+.. code-block:: c++
+
+    enum MyActorEnum { e1, e2, e3, e4, e5 };
+
+    template<>
+    struct ParamTraits<MyActorEnum>
+      : public ContiguousEnumSerializerInclusive<MyActorEnum, MyActorEnum::e1, MyActorEnum::e5> {};
+
+IPDL Structs and Unions in C++
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+IPDL structs and unions become C++ classes that provide interfaces that are
+fairly self-explanatory.  Recalling ``MyUnion`` and ``MyActorPair`` from
+`IPDL Structs and Unions`_ :
+
+.. code-block:: c++
+
+    union MyUnion {
+        float;
+        MyOtherData;
+    };
+
+    struct MyActorPair {
+        PMyManaged actor1;
+        nullable PMyManaged actor2;
+    };
+
+These compile to:
+
+.. code-block:: c++
+
+    class MyUnion {
+        enum Type { Tfloat, TMyOtherData };
+        Type type();
+        MyUnion(float f);
+        MyUnion(MyOtherData&& aOD);
+        MyUnion& operator=(float f);
+        MyUnion& operator=(MyOtherData&& aOD);
+        operator float&();
+        operator MyOtherData&();
+    };
+
+    class MyActorPair {
+        MyActorPair(PMyManagedParent* actor1Parent, PMyManagedChild* actor1Child,
+                    PMyManagedParent* actor2Parent, PMyManagedChild* actor2Child);
+        // Exactly one of { actor1Parent(), actor1Child() } must be non-null.
+        PMyManagedParent*& actor1Parent();
+        PMyManagedChild*& actor1Child();
+        // As nullable, zero or one of { actor2Parent(), actor2Child() } will be non-null.
+        PMyManagedParent*& actor2Parent();
+        PMyManagedChild*& actor2Child();
+    }
+
+The generated ``ParamTraits`` use the ``ParamTraits`` for the types referenced
+by the IPDL struct or union.  Fields respect any annotations for their type
+(see `IPDL Includes`_).  For example, a ``[RefCounted]`` type ``T`` generates
+``RefPtr<T>`` fields.
+
+Note that actor members result in members of both the parent and child actor
+types, as seen in ``MyActorPair``.  When actors are used to bridge processes,
+only one of those could ever be used at a given endpoint.  IPDL makes sure
+that, when you send one type (say, ``PMyManagedChild``), the adjacent actor of
+the other type (``PMyManagedParent``) is received.  This is not only true for
+message parameters and IPDL structs/unions but also for custom ``ParamTraits``
+implementations.  If you ``Write`` a ``PFooParent*`` then you must ``Read`` a
+``PFooChild*``.  This is hard to confuse in message handlers since they are
+members of a class named for the side they operate on, but this cannot be
+enforced by the compiler.  If you are writing
+``MyManagerParent::RecvSomeMsg(Maybe<MyActorPair>&& aActors, nsTArray<MyData>&& aMyData)``
+then the ``actor1Child`` and ``actor2Child`` fields cannot be valid since the
+child (usually) exists in another process.
+
+.. _IPDL Structs and Unions: `Generating IPDL-Aware C++ Data Types: IPDL Structs and Unions`_
+.. _IPDL Includes: `Referencing Externally Defined Data Types: IPDL Includes`_
+
+Actors and Messages in C++
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As mentioned in `Using The IPDL compiler`_, the IPDL compiler generates two
+header files for the protocol ``PMyManager``: ``PMyManagerParent.h`` and
+``PMyManagerChild.h``, which declare the actor's base classes.  There, we
+discussed how the headers are visible to C++ components that include
+``chromium-config.mozbuild``.  We, in turn, always need to define two files
+that declare our actor implementation subclasses (``MyManagerParent.h`` and
+``MyManagerChild.h``).  The IPDL file looked like this:
+
+.. literalinclude:: _static/PMyManager.ipdl
+   :language: c++
+   :name: PMyManager.ipdl
+
+So ``MyManagerParent.h`` looks like this:
+
+.. code-block:: c++
+
+    #include "PMyManagerParent.h"
+
+    namespace mozilla {
+    namespace myns {
+
+    class MyManagerParent : public PMyManagerParent {
+        NS_INLINE_DECL_REFCOUNTING(MyManagerParent, override)
+    protected:
+        IPCResult Recv__delete__(const nsString& aNote);
+        IPCResult RecvSomeMsg(const Maybe<MyActorPair>& aActors, const nsTArray<MyData>& aMyData,
+                              int32_t* x, int32_t* y, MyUnion* aUnion);
+        IPCResult RecvAnotherMsg(const Tainted<MyActorEnum>& aEnum, const Tainted<int32_t>& aNumber,
+                                 AnotherMsgResolver&& aResolver);
+
+        already_AddRefed<PMyManagerParent> AllocPMyManagedParent();
+        IPCResult RecvPMyManagedConstructor(PMyManagedConstructor* aActor);
+
+        // ... etc ...
+    };
+
+    } // namespace myns
+    } // namespace mozilla
+
+All messages that can be sent to the actor must be handled by ``Recv`` methods
+in the proper actor subclass.  They should return ``IPC_OK()`` on success and
+``IPC_FAIL(actor, reason)`` if an error occurred (where ``actor`` is ``this``
+and ``reason`` is a human text explanation) that should be considered a failure
+to process the message.  The handling of such a failure is specific to the
+process type.
+
+``Recv`` methods are called by IPDL by enqueueing a task to run them on the
+``MessageLoop`` for the thread on which they are bound.  This thread is the
+actor's *worker thread*.  All actors in a managed actor tree have the same
+worker thread -- in other words, actors inherit the worker thread from their
+managers.  Top level actors establish their worker thread when they are
+*bound*.  More information on threads can be found in `Top Level Actors`_.  For
+the most part, client code will never engage with an IPDL actor outside of its
+worker thread.
+
+Received parameters become stack variables that are ``std::move``-d into the
+``Recv`` method.  They can be received as a const l-value reference,
+rvalue-reference, or by value (type-permitting).  ``[MoveOnly]`` types should
+not be received as const l-values.  Return values for sync messages are
+assigned by writing to non-const (pointer) parameters.  Return values for async
+messages are handled differently -- they are passed to a resolver function.  In
+our example, ``AnotherMsgResolver`` would be a ``std::function<>`` and
+``aResolver`` would be given the value to return by passing it a reference to a
+``MyOtherData`` object.
+
+``MyManagerParent`` is also capable of ``sending`` an async message that
+returns a value: ``AnotherMsg``.  This is done with ``SendAnotherMsg``, which
+is defined automatically by IPDL in the base class ``PMyManagerParent``.  There
+are two signatures for ``Send`` and they look like this:
+
+.. code-block:: c++
+
+    // Return a Promise that IPDL will resolve with the response or reject.
+    RefPtr<MozPromise<MyOtherData, ResponseRejectReason, true>>
+    SendAnotherMsg(const MyActorEnum& aEnum, int32_t aNumber);
+
+    // Provide callbacks to process response / reject.  The callbacks are just
+    // std::functions.
+    void SendAnotherMsg(const MyActorEnum& aEnum, int32_t aNumber,
+        ResolveCallback<MyOtherData>&& aResolve, RejectCallback&& aReject);
+
+The response is usually handled by lambda functions defined at the site of the
+``Send`` call, either by attaching them to the returned promise with e.g.
+``MozPromise::Then``, or by passing them as callback parameters.  See docs on
+``MozPromise`` for more on its use.  The promise itself is either resolved or
+rejected by IPDL when a valid reply is received or when the endpoint determines
+that the communication failed.  ``ResponseRejectReason`` is an enum IPDL
+provides to explain failures.
+
+Additionally, the ``AnotherMsg`` handler has ``Tainted`` parameters, as a
+result of the [Tainted] annotation in the protocol file.  Recall that
+``Tainted`` is used to force explicit validation of parameters in the message
+handler before their values can be used (as opposed to validation in
+``ParamTraits``).  They therefore have access to any state that the message
+handler does.  Their APIs, along with a list of macros that are used to
+validate them, are detailed `here
+<https://searchfox.org/mozilla-central/source/mfbt/Tainting.h>`__.
+
+Send methods that are not for async messages with return values follow a
+simpler form; they return a ``bool`` indicating success or failure and return
+response values in non-const parameters, as the ``Recv`` methods do.  For
+example, ``PMyManagerChild`` defines this to send the sync message ``SomeMsg``:
+
+.. code-block:: c++
+
+    // generated in PMyManagerChild
+    bool SendSomeMsg(const Maybe<MyActorPair>& aActors, const nsTArray<MyData>& aMyData,
+                     int32_t& x, int32_t& y, MyUnion& aUnion);
+
+Since it is sync, this method will not return to its caller until the response
+is received or an error is detected.
+
+All calls to ``Send`` methods, like all messages handler ``Recv`` methods, must
+only be called on the worker thread for the actor.
+
+Constructors, like the one for ``MyManaged``, are clearly an exception to these
+rules.  They are discussed in the next section.
+
+.. _Actor Lifetimes in C++:
+
+Actor Lifetimes in C++
+~~~~~~~~~~~~~~~~~~~~~~
+
+The constructor message for ``MyManaged`` becomes *two* methods at the
+receiving end.  ``AllocPMyManagedParent`` constructs the managed actor, then
+``RecvPMyManagedConstructor`` is called to update the new actor.  The following
+diagram shows the construction of the ``MyManaged`` actor pair:
+
+.. mermaid::
+    :align: center
+    :caption: A ``MyManaged`` actor pair being created by some ``Driver``
+              object.  Internal IPC objects in the parent and child processes
+              are combined for compactness.  Connected **par** blocks run
+              concurrently.  This shows that messages can be safely sent while
+              the parent is still being constructed.
+
+    %%{init: {'sequence': {'boxMargin': 4, 'actorMargin': 10} }}%%
+    sequenceDiagram
+        participant d as Driver
+        participant mgdc as MyManagedChild
+        participant mgrc as MyManagerChild
+        participant ipc as IPC Child/Parent
+        participant mgrp as MyManagerParent
+        participant mgdp as MyManagedParent
+        d->>mgdc: new
+        mgdc->>d: [mgd_child]
+        d->>mgrc: SendPMyManagedConstructor<br/>[mgd_child, params]
+        mgrc->>ipc: Form actor pair<br/>[mgd_child, params]
+        par
+        mgdc->>ipc: early PMyManaged messages
+        and
+        ipc->>mgrp: AllocPMyManagedParent<br/>[params]
+        mgrp->>mgdp: new
+        mgdp->>mgrp: [mgd_parent]
+        ipc->>mgrp: RecvPMyManagedConstructor<br/>[mgd_parent, params]
+        mgrp->>mgdp: initialization
+        ipc->>mgdp: early PMyManaged messages
+        end
+        Note over mgdc,mgdp: Bi-directional sending and receiving will now happen concurrently.
+
+The next diagram shows the destruction of the ``MyManaged`` actor pair, as
+initiated by a call to ``Send__delete__``.  ``__delete__`` is sent from the
+child process because that is the only side that can call it, as declared in
+the IPDL protocol file.
+
+.. mermaid::
+    :align: center
+    :caption: A ``MyManaged`` actor pair being disconnected due to some
+              ``Driver`` object in the child process sending ``__delete__``.
+
+    %%{init: {'sequence': {'boxMargin': 4, 'actorMargin': 10} }}%%
+    sequenceDiagram
+        participant d as Driver
+        participant mgdc as MyManagedChild
+        participant ipc as IPC Child/Parent
+        participant mgdp as MyManagedParent
+        d->>mgdc: Send__delete__
+        mgdc->>ipc: Disconnect<br/>actor pair
+        par
+        ipc->>mgdc: ActorDestroy
+        ipc->>mgdc: Release
+        and
+        ipc->>mgdp: Recv__delete__
+        ipc->>mgdp: ActorDestroy
+        ipc->>mgdp: Release
+        end
+
+Finally, let's take a look at the behavior of an actor whose peer has been lost
+(e.g. due to a crashed process).
+
+.. mermaid::
+    :align: center
+    :caption: A ``MyManaged`` actor pair being disconnected when its peer is
+              lost due to a fatal error.  Note that ``Recv__delete__`` is not
+              called.
+
+    %%{init: {'sequence': {'boxMargin': 4, 'actorMargin': 10} }}%%
+    sequenceDiagram
+        participant mgdc as MyManagedChild
+        participant ipc as IPC Child/Parent
+        participant mgdp as MyManagedParent
+        Note over mgdc: CRASH!!!
+        ipc->>ipc: Notice fatal error.
+        ipc->>mgdp: ActorDestroy
+        ipc->>mgdp: Release
+
+The ``Alloc`` and ``Recv...Constructor`` methods are somewhat mirrored by
+``Recv__delete__`` and ``ActorDestroy`` but there are a few differences.
+First, the ``Alloc`` method really does create the actor but the
+``ActorDestroy`` method does not delete it.  Additionally, ``ActorDestroy``
+is run at *both* endpoints, during ``Send__delete__`` or after
+``Recv__delete__``.  Finally and most importantly, ``Recv__delete__`` is only
+called if the ``__delete__`` message is received but it may not be if, for
+example, the remote process crashes.  ``ActorDestroy``, on the other hand, is
+guaranteed to run for *every* actor unless the process terminates uncleanly.
+For this reason, ``ActorDestroy`` is the right place for most actor shutdown
+code.  ``Recv__delete__`` is rarely useful, although it is occasionally
+beneficial to have it receive some final data.
+
+The relevant part of the parent class looks like this:
+
+.. code-block:: c++
+
+    class MyManagerParent : public PMyManagerParent {
+        already_AddRefed<PMyManagedParent> AllocPMyManagedParent();
+        IPCResult RecvPMyManagedConstructor(PMyManagedParent* aActor);
+
+        IPCResult Recv__delete__(const nsString& aNote);
+        void ActorDestroy(ActorDestroyReason why);
+
+        // ... etc ...
+    };
+
+The ``Alloc`` method is required for managed actors that are constructed by
+IPDL receiving a ``Send`` message.  It is not required for the actor at the
+endpoint that calls ``Send``.  The ``Recv...Constructor`` message is not
+required -- it has a base implementation that does nothing.
+
+If the constructor message has parameters, they are sent to both methods.
+Parameters are given to the ``Alloc`` method by const reference but are moved
+into the ``Recv`` method.  They differ in that messages can be sent from the
+``Recv`` method but, in ``Alloc``, the newly created actor is not yet
+operational.
+
+The ``Send`` method for a constructor is similarly different from other
+``Send`` methods.  In the child actor, ours looks like this:
+
+.. code-block:: c++
+
+    IPCResult SendPMyManagedConstructor(PMyManagedChild* aActor);
+
+The method expects a ``PMyManagedChild`` that the caller will have constructed,
+presumably using ``new`` (this is why it does not require an ``Alloc`` method).
+Once ``Send...Constructor`` is called, the actor can be used to send and
+receive messages.  It does not matter that the remote actor may not have been
+created yet due to asynchronicity.
+
+The destruction of actors is as unusual as their construction.  Unlike
+construction, it is the same for managed and top-level actors.  Avoiding
+``[ManualDealloc]`` actors removes a lot of the complexity but there is still
+a process to understand.  Actor destruction begins when an ``__delete__``
+message is sent.  In ``PMyManager``, this message is declared from child to
+parent.  The actor calling ``Send__delete__`` is no longer connected to
+anything when the method returns.  Future calls to ``Send`` return an error
+and no future messages will be received.  This is also the case for an actor
+that has run ``Recv__delete__``; it is no longer connected to the other
+endpoint.
+
+.. note::
+    Since ``Send__delete__`` may release the final reference to itself, it
+    cannot safely be a class instance method.  Instead, unlike other ``Send``
+    methods, it's a ``static`` class method and takes the actor as a parameter:
+
+    .. code-block:: c++
+
+        static IPCResult Send__delete__(PMyManagerChild* aToDelete);
+
+    Additionally, the ``__delete__`` message tells IPDL to disconnect both the
+    given actor *and all of its managed actors*.  So it is really deleting the
+    actor subtree, although ``Recv__delete__`` is only called for the actor it
+    was sent to.
+
+During the call to ``Send__delete__``, or after the call to ``Recv__delete__``,
+the actor's ``ActorDestroy`` method is called.  This method gives client code a
+chance to do any teardown that must happen in `all` circumstances were it is
+possible -- both expected and unexpected.  This means that ``ActorDestroy``
+will also be called when, for example, IPDL detects that the other endpoint has
+terminated unexpectedly, so it is releasing its reference to the actor, or
+because an ancestral manager (manager or manager's manager...) received a
+``__delete__``.  The only way for an actor to avoid ``ActorDestroy`` is for its
+process to crash first.  ``ActorDestroy`` is always run after its actor is
+disconnected so it is pointless to try to send messages from it.
+
+Why use ``ActorDestroy`` instead of the actor's destructor?  ``ActorDestroy``
+gives a chance to clean up things that are only used for communication and
+therefore don't need to live for however long the actor's (reference-counted)
+object does.  For example, you might have references to shared memory (Shmems)
+that are no longer valid.  Or perhaps the actor can now release a cache of data
+that was only needed for processing messages.  It is cleaner to deal with
+communication-related objects in ``ActorDestroy``, where they become invalid,
+than to leave them in limbo until the destructor is run.
+
+Consider actors to be like normal reference-counted objects, but where IPDL
+holds a reference while the connection will or does exist.  One common
+architecture has IPDL holding the `only` reference to an actor.  This is common
+with actors created by sending constructor messages but the idea is available to
+any actor.  That only reference is then released when the ``__delete__``
+message is sent or received.
+
+The dual of IPDL holding the only reference is to have client code hold the
+only reference.  A common pattern to achieve this has been to override the
+actor's ``AddRef`` to have it send ``__delete__`` only when it's count is down
+to one reference (which must be IPDL if ``actor.CanSend()`` is true).  A better
+approach would be to create a reference-counted delegate for your actor that
+can send ``__delete__`` from its destructor.  IPDL does not guarantee that it
+will not hold more than one reference to your actor.
+
+.. _Top Level Actors:
+
+Top Level Actors
+----------------
+
+Recall that top level actors are actors that have no manager.  They are at the
+root of every actor tree.  There are two settings in which we use top-level
+actors that differ pretty dramatically.  The first type are top-level actors
+that are created and maintained in a way that resembles managed actors, but
+with some important differences we will cover in this section.  The second type
+of top-level actors are the very first actors in a new process -- these actors
+are created through different means and closing them (usually) terminates the
+process.  The `new process example
+<https://phabricator.services.mozilla.com/D119038>`_ demonstrates both of
+these.  It is discussed in detail in :ref:`Adding a New Type of Process`.
+
+Value of Top Level Actors
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Top-level actors are harder to create and destroy than normal actors.  They
+used to be more heavyweight than managed actors but this has recently been
+dramatically reduced.
+
+.. note::
+    Top-level actors previously required a dedicated *message channel*, which
+    are limited OS resources.  This is no longer the case -- message channels
+    are now shared by actors that connect the same two processes.  This
+    *message interleaving* can affect message delivery latency but profiling
+    suggests that the change was basically inconsequential.
+
+So why use a new top level actor?
+
+* The most dramatic property distinguishing top-level actors is the ability to
+  *bind* to whatever ``EventTarget`` they choose.  This means that any thread
+  that runs a ``MessageLoop`` can use the event target for that loop as the
+  place to send incoming messages.  In other words, ``Recv`` methods would be
+  run by that message loop, on that thread.  The IPDL apparatus will
+  asynchronously dispatch messages to these event targets, meaning that
+  multiple threads can be handling incoming messages at the same time.  The
+  `PBackground`_ approach was born of a desire to make it easier to exploit
+  this, although it has some complications, detailed in that section, that
+  limit its value.
+* Top level actors suggest modularity.  Actor protocols are tough to debug, as
+  is just about anything that spans process boundaries.  Modularity can give
+  other developers a clue as to what they need to know (and what they don't)
+  when reading an actor's code.  The alternative is proverbial *dumpster
+  classes* that are as critical to operations (because they do so much) as they
+  are difficult to learn (because they do so much).
+* Top level actors are required to connect two processes, regardless of whether
+  the actors are the first in the process or not.  As said above, the first
+  actor is created through special means but other actors are created through
+  messages.  In Gecko, apart from the launcher and main processes, all new
+  processes X are created with their first actor being between X and the main
+  process.  To create a connection between X and, say, a content process, the
+  main process has to send connected ``Endpoints`` to X and to the content
+  process, which in turn use those endpoints to create new top level actors
+  that form an actor pair.  This is discussed at length in :ref:`Connecting
+  With Other Processes`.
+
+Top-level actors are not as frictionless as desired but they are probably
+under-utilized relative to their power.  In cases where it is supported,
+``PBackground`` is sometimes a simpler alternative to achieve the same goals.
+
+Creating Top Level Actors From Other Actors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The most common way to create new top level actors is by creating a pair of
+connected Endpoints and sending one to the other actor.  This is done exactly
+the way it sounds.  For example:
+
+.. code-block:: c++
+
+    bool MyPreexistingActorParent::MakeMyActor() {
+        Endpoint<PMyActorParent> parentEnd;
+        Endpoint<PMyActorChild> childEnd;
+        if (NS_WARN_IF(NS_FAILED(PMyActor::CreateEndpoints(
+              base::GetCurrentProcId(), OtherPid(), &parentEnd, &childEnd)))) {
+            // ... handle failure ...
+            return false;
+        }
+        RefPtr<MyActorParent> parent = new MyActorParent;
+        if (!parentEnd.Bind(parent)) {
+            // ... handle failure ...
+            delete parent;
+            return false;
+        }
+        // Do this second so we skip child if parent failed to connect properly.
+        if (!SendCreateMyActorChild(std::move(childEnd))) {
+            // ... assume an IPDL error will destroy parent.  Handle failure beyond that ...
+            return false;
+        }
+        return true;
+    }
+
+Here ``MyPreexistingActorParent`` is used to send a child endpoint for the new
+top level actor to ``MyPreexistingActorChild``, after it hooks up the parent
+end.  In this example, we bind our new actor to the same thread we are running
+on -- which must be the same thread ``MyPreexistingActorParent`` is bound to
+since we are sending ``CreateMyActorChild`` from it.  We could have bound on a
+different thread.
+
+At this point, messages can be sent on the parent.  Eventually, it will start
+receiving them as well.
+
+``MyPreexistingActorChild`` still has to receive the create message.  The code
+for that handler is pretty similar:
+
+.. code-block:: c++
+
+    IPCResult MyPreexistingActorChild::RecvCreateMyActorChild(Endpoint<PMyActorChild>&& childEnd) {
+        RefPtr<MyActorChild> child = new MyActorChild;
+        if (!childEnd.Bind(child)) {
+            // ... handle failure and return ok, assuming a related IPDL error will alert the other side to failure ...
+            return IPC_OK();
+        }
+        return IPC_OK();
+    }
+
+Like the parent, the child is ready to send as soon as ``Bind`` is complete.
+It will start receiving messages soon afterward on the event target for the
+thread on which it is bound.
+
+Creating First Top Level Actors
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The first actor in a process is an advanced topic that is covered in
+:ref:`the documentation for adding a new process<Adding a New Type of Process>`.
+
+PBackground
+-----------
+
+Developed as a convenient alternative to top level actors, ``PBackground`` is
+an IPDL protocol whose managees choose their worker threads in the child
+process and share a thread dedicated solely to them in the parent process.
+When an actor (parent or child) should run without hogging the main thread,
+making that actor a managee of ``PBackground`` (aka a *background actor*) is an
+option.
+
+.. warning::
+    Background actors can be difficult to use correctly, as spelled out in this
+    section.  It is recommended that other options -- namely, top-level actors
+    -- be adopted instead.
+
+Background actors can only be used in limited circumstances:
+
+* ``PBackground`` only supports the following process connections (where
+  ordering is parent <-> child): main <-> main, main <-> content,
+  main <-> socket and socket <-> content.
+
+.. important::
+
+    Socket process ``PBackground`` actor support was added after the other
+    options.  It has some rough edges that aren't easy to anticipate.  In the
+    future, their support may be broken out into a different actor or removed
+    altogether.  You are strongly encouraged to use new `Top Level Actors`_
+    instead of ``PBackground`` actor when communicating with socket process
+    worker threads.
+
+* Background actor creation is always initiated by the child.  Of course, a
+  request to create one can be sent to the child by any other means.
+* All parent background actors run in the same thread.  This thread is
+  dedicated to serving as the worker for parent background actors.  While it
+  has no other functions, it should remain responsive to all connected
+  background actors.  For this reason, it is a bad idea to conduct long
+  operations in parent background actors.  For such cases, create a top level
+  actor and an independent thread on the parent side instead.
+* Background actors are currently *not* reference-counted.  IPDL's ownership
+  has to be carefully respected and the (de-)allocators for the new actors have
+  to be defined.  See `The Old Ways`_ for details.
+
+A hypothetical layout of ``PBackground`` threads, demonstrating some of the
+process-type limitations, is shown in the diagram below.
+
+.. mermaid::
+    :align: center
+    :caption: Hypothetical ``PBackground`` thread setup.  Arrow direction
+              indicates child-to-parent ``PBackground``-managee relationships.
+              Parents always share a thread and may be connected to multiple
+              processes.  Child threads can be any thread, including main.
+
+    flowchart LR
+        subgraph content #1
+            direction TB
+            c1tm[main]
+            c1t1[worker #1]
+            c1t2[worker #2]
+            c1t3[worker #3]
+        end
+        subgraph content #2
+            direction TB
+            c2tm[main]
+            c2t1[worker #1]
+            c2t2[worker #2]
+        end
+        subgraph socket
+            direction TB
+            stm[main]
+            st1[background parent /\nworker #1]
+            st2[worker #2]
+        end
+        subgraph main
+            direction TB
+            mtm[main]
+            mt1[background parent]
+        end
+
+        %% PBackground connections
+        c1tm --> mt1
+        c1t1 --> mt1
+        c1t2 --> mt1
+
+        c1t3 --> mt1
+        c1t3 --> st1
+
+        c2t1 --> st1
+        c2t1 --> mt1
+
+        c2t2 --> mt1
+
+        c2tm --> st1
+
+        stm --> mt1
+        st1 --> mt1
+        st2 --> mt1
+
+Creating background actors is done a bit differently than normal managees.  The
+new managed type and constructor are still added to ``PBackground.ipdl`` as
+with normal managees but, instead of ``new``-ing the child actor and then
+passing it in a ``SendFooConstructor`` call, background actors issue the send
+call to the ``BackgroundChild`` manager, which returns the new child:
+
+.. code-block:: c++
+
+    // Bind our new PMyBackgroundActorChild to the current thread.
+    PBackgroundChild* bc = BackgroundChild::GetOrCreateForCurrentThread();
+    if (!bc) {
+        return false;
+    }
+    PMyBackgroundActorChild* pmyBac = bac->SendMyBackgroundActor(constructorParameters);
+    if (!pmyBac) {
+        return false;
+    }
+    auto myBac = static_cast<MyBackgroundActorChild*>(pmyBac);
+
+.. note::
+    ``PBackgroundParent`` still needs a ``RecvMyBackgroundActorConstructor``
+    handler, as usual.  This must be done in the ``ParentImpl`` class.
+    ``ParentImpl`` is the non-standard name used for the implementation of
+    ``PBackgroundParent``.
+
+To summarize, ``PBackground`` attempts to simplify a common desire in Gecko:
+to run tasks that communicate between the main and content processes but avoid
+having much to do with the main thread of either.  Unfortunately, it can be
+complicated to use correctly and has missed on some favorable IPDL
+improvements, like reference counting.  While top level actors are always a
+complete option for independent jobs that need a lot of resources,
+``PBackground`` offers a compromise for some cases.
+
+IPDL Best Practices
+-------------------
+
+IPC performance is affected by a lot of factors.  Many of them are out of our
+control, like the influence of the system thread scheduler on latency or
+messages whose travel internally requires multiple legs for security reasons.
+On the other hand, some things we can and should control for:
+
+* Messages incur inherent performance overhead for a number of reasons: IPDL
+  internal thread latency (e.g. the I/O thread), parameter (de-)serialization,
+  etc.  While not usually dramatic, this cost can add up.  What's more, each
+  message generates a fair amount of C++ code.  For these reasons, it is wise
+  to reduce the number of messages being sent as far as is reasonable.  This
+  can be as simple as consolidating two asynchronous messages that are always
+  in succession.  Or it can be more complex, like consolidating two
+  somewhat-overlapping messages by merging their parameter lists and marking
+  parameters that may not be needed as optional.  It is easy to go too far down
+  this path but careful message optimization can show big gains.
+* Even ``[moveonly]`` parameters are "copied" in the sense that they are
+  serialized.  The pipes that transmit the data are limited in size and require
+  allocation.  So understand that the performance of your transmission will be
+  inversely proportional to the size of your content.  Filter out data you
+  won't need.  For complex reasons related to Linux pipe write atomicity, it is
+  highly desirable to keep message sizes below 4K (including a small amount for
+  message metadata).
+* On the flip side, very large messages are not permitted by IPDL and will
+  result in a runtime error.  The limit is currently 256M but message failures
+  frequently arise even with slightly smaller messages.
+* Parameters to messages are C++ types and therefore can be very complex in the
+  sense that they generally represent a tree (or graph) of objects.  If this
+  tree has a lot of objects in it, and each of them is serialized by
+  ``ParamTraits``, then we will find that serialization is allocating and
+  constructing a lot of objects, which will stress the allocator and cause
+  memory fragmentation.  Avoid this by using larger objects or by sharing this
+  kind of data through careful use of shared memory.
+* As it is with everything, concurrency is critical to the performance of IPDL.
+  For actors, this mostly manifests in the choice of bound thread.  While
+  adding a managed actor to an existing actor tree may be a quick
+  implementation, this new actor will be bound to the same thread as the old
+  one.  This contention may be undesirable.  Other times it may be necessary
+  since message handlers may need to use data that isn't thread safe or may
+  need a guarantee that the two actors' messages are received in order.  Plan
+  up front for your actor hierarchy and its thread model.  Recognize when you
+  are better off with a new top level actor or ``PBackground`` managee that
+  facilitates processing messages simultaneously.
+* Remember that latency will slow your entire thread, including any other
+  actors/messages on that thread.  If you have messages that will need a long
+  time to be processed but can run concurrently then they should use actors
+  that run on a separate thread.
+* Top-level actors decide a lot of properties for their managees.  Probably the
+  most important are the process layout of the actor (including which process
+  is "Parent" and which is "Child") and the thread.  Every top-level actor
+  should clearly document this, ideally in their .ipdl file.
+
+The Old Ways
+------------
+
+TODO:
+
+The FUD
+-------
+
+TODO:
+
+The Rest
+--------
+
+Nested messages
+~~~~~~~~~~~~~~~
+
+The ``Nested`` message annotations indicate the nesting type of the message.
+They attempt to process messages in the nested order of the "conversation
+thread", as found in e.g. a mailing-list client.  This is an advanced concept
+that should be considered to be discouraged, legacy functionality.
+Essentially, ``Nested`` messages can make other ``sync`` messages break the
+policy of blocking their thread -- nested messages are allowed to be received
+while a sync messagee is waiting for a response.  The rules for when a nested
+message can be handled are somewhat complex but they try to safely allow a
+``sync`` message ``M`` to handle and respond to some special (nested) messages
+that may be needed for the other endpoint to finish processing ``M``.  There is
+a `comment in MessageChannel`_ with info on how the decision to handle nested
+messages is made.  For sync nested messages, note that this implies a relay
+between the endpoints, which could dramatically affect their throughput.
+
+Declaring messages to nest requires an annotation on the actor and one on the
+message itself.  The nesting annotations were listed in `Defining Actors`_ and
+`Declaring IPDL Messages`_.  We repeat them here.  The actor annotations
+specify the maximum priority level of messages in the actor.  It is validated
+by the IPDL compiler.  The annotations are:
+
+============================== ================================================
+``[NestedUpTo=inside_sync]``   Indicates that an actor contains messages of
+                               priority [Nested=inside_sync] or lower.
+``[NestedUpTo=inside_cpow]``   Indicates that an actor contains messages of
+                               priority [Nested=inside_cpow] or lower.
+============================== ================================================
+
+.. note::
+
+    The order of the nesting priorities is:
+    (no nesting priority) < ``inside_sync`` < ``inside_cpow``.
+
+The message annotations are:
+
+========================== ====================================================
+``[Nested=inside_sync]``   Indicates that the message can be handled while
+                           waiting for lower-priority, or in-message-thread,
+                           sync responses.
+``[Nested=inside_cpow]``   Indicates that the message can be handled while
+                           waiting for lower-priority, or in-message-thread,
+                           sync responses.  Cannot be sent by the parent actor.
+========================== ====================================================
+
+.. note::
+
+    ``[Nested=inside_sync]`` messages must be sync (this is enforced by the
+    IPDL compiler) but ``[Nested=inside_cpow]`` may be async.
+
+Nested messages are obviously only interesting when sent to an actor that is
+performing a synchronous wait.  Therefore, we will assume we are in such a
+state.  Say ``actorX`` is waiting for a sync reply from ``actorY`` for message
+``m1`` when ``actorY`` sends ``actorX`` a message ``m2``.  We distinguish two
+cases here: (1) when ``m2`` is sent while processing ``m1`` (so ``m2`` is sent
+by the ``RecvM1()`` method -- this is what we mean when we say "nested") and
+(2) when ``m2`` is unrelated to ``m1``.  Case (2) is easy; ``m2`` is only
+dispatched while ``m1`` waits if
+``priority(m2) > priority(m1) > (no priority)`` and the message is being
+received by the parent, or if ``priority(m2) >= priority(m1) > (no priority)``
+and the message is being received by the child.  Case (1) is less
+straightforward.
+
+To analyze case (1), we again distinguish the two possible ways we can end up
+in the nested case: (A) ``m1`` is sent by the parent to the child and ``m2``
+is sent by the child to the parent, or (B) where the directions are reversed.
+The following tables explain what happens in all cases:
+
+.. |strike| raw:: html
+
+   <strike>
+
+.. |endstrike| raw:: html
+
+   </strike>
+
+.. |br| raw:: html
+
+   <br/>
+
+.. table :: Case (A): Child sends message to a parent that is awaiting a sync response
+    :align: center
+
+    ==============================     ========================      ========================================================
+    sync ``m1`` type (from parent)     ``m2`` type (from child)      ``m2`` handled or rejected
+    ==============================     ========================      ========================================================
+    sync (no priority)                 \*                            IPDL compiler error: parent cannot send sync (no priority)
+    sync inside_sync                   async (no priority)           |strike| ``m2`` delayed until after ``m1`` completes |endstrike| |br|
+                                                                     Currently ``m2`` is handled during the sync wait (bug?)
+    sync inside_sync                   sync (no priority)            |strike| ``m2`` send fails: lower priority than ``m1`` |endstrike| |br|
+                                                                     Currently ``m2`` is handled during the sync wait (bug?)
+    sync inside_sync                   sync inside_sync              ``m2`` handled during ``m1`` sync wait: same message thread and same priority
+    sync inside_sync                   async inside_cpow             ``m2`` handled during ``m1`` sync wait: higher priority
+    sync inside_sync                   sync inside_cpow              ``m2`` handled during ``m1`` sync wait: higher priority
+    sync inside_cpow                   \*                            IPDL compiler error: parent cannot use inside_cpow priority
+    ==============================     ========================      ========================================================
+
+.. table :: Case (B): Parent sends message to a child that is awaiting a sync response
+    :align: center
+
+    =============================      =========================      ========================================================
+    sync ``m1`` type (from child)      ``m2`` type (from parent)      ``m2`` handled or rejected
+    =============================      =========================      ========================================================
+    \*                                 async (no priority)            ``m2`` delayed until after ``m1`` completes
+    \*                                 sync (no priority)             IPDL compiler error: parent cannot send sync (no priority)
+    sync (no priority)                 sync inside_sync               ``m2`` send fails: no-priority sync messages cannot handle
+                                                                      incoming messages during wait
+    sync inside_sync                   sync inside_sync               ``m2`` handled during ``m1`` sync wait: same message thread and same priority
+    sync inside_cpow                   sync inside_sync               ``m2`` send fails: lower priority than ``m1``
+    \*                                 async inside_cpow              IPDL compiler error: parent cannot use inside_cpow priority
+    \*                                 sync inside_cpow               IPDL compiler error: parent cannot use inside_cpow priority
+    =============================      =========================      ========================================================
+
+We haven't seen rule #2 from the `comment in MessageChannel`_ in action but, as
+the comment mentions, it is needed to break deadlocks in cases where both the
+parent and child are initiating message-threads simultaneously.  It
+accomplishes this by favoring the parent's sent messages over the child's when
+deciding which message-thread to pursue first (and blocks the other until the
+first completes).  Since this distinction is entirely thread-timing based,
+client code needs only to be aware that IPDL internals will not deadlock
+because of this type of race, and that this protection is limited to a single
+actor tree -- the parent/child messages are only well-ordered when under the
+same top-level actor so simultaneous sync messages across trees are still
+capable of deadlock.
+
+Clearly, tight control over these types of protocols is required to predict how
+they will coordinate within themselves and with the rest of the application
+objects.  Control flow, and hence state, can be very difficult to predict and
+are just as hard to maintain.  This is one of the key reasons why we have
+stressed that message priorities should be avoided whenever possible.
+
+.. _comment in MessageChannel: https://searchfox.org/mozilla-central/rev/077501b34cca91763ae04f4633a42fddd919fdbd/ipc/glue/MessageChannel.cpp#54-118
+
+.. _Message Logging:
+
+Message Logging
+~~~~~~~~~~~~~~~
+
+The environment variable ``MOZ_IPC_MESSAGE_LOG`` controls the logging of IPC
+messages.  It logs details about the transmission and reception of messages.
+This isn't controlled by ``MOZ_LOG`` -- it is a separate system.  Set this
+variable to ``1`` to log information on all IPDL messages, or specify a
+comma-separated list of **top-level** protocols to log (e.g.
+``MOZ_IPC_MESSAGE_LOG="PMyManagerChild,PMyManagedParent,PMyManagedChild"``).
+:ref:`Debugging with IPDL Logging` has an example where IPDL logging is useful
+in tracking down a bug.
+
+.. important::
+    The preceding ``P`` and the ``Parent`` or ``Child`` suffix are required
+    when listing individual protocols in ``MOZ_IPC_MESSAGE_LOG``.