mozilla-central/ipc/docs/utility_process.rst

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Utility Process
===============
.. warning::
As of january 2022, this process is under heavy work, and many things can
evolve. Documentation might not always be as accurate as it should be.
Please reach to #ipc if you intent to add a new utility.
The utility process is used to provide a simple way to implement IPC actor with
some more specific sandboxing properties, in case where you don't need or want
to deal with the extra complexity of adding a whole new process type but you
just want to apply different sandboxing policies.
To implement such an actor, you will have to follow a few steps like for
implementing the trivial example visible in `EmptyUtil
- Define a new IPC actor, e.g., ``PEmptyUtil`` that allows to get some string
via ``GetSomeString()`` from the child to the parent
- In the ``PUtilityProcess`` definition, expose a new child-level method,
e.g., ``StartEmptyUtilService(Endpoint<PEmptyUtilChild>)``
- Implement ``EmptyUtilChild`` and ``EmptyUtilParent`` classes both deriving
from their ``PEmptyUtilXX``. If you want or need to run things from a
different thread, you can have a look at ``UtilityProcessGenericActor``
- Make sure both are refcounted
- Expose your new service on ``UtilityProcessManager`` with a method
performing the heavy lifting of starting your process, you can take
inspiration from ``StartEmptyUtil()`` in the sample.
- To use ``StartUtility()`` mentioned above, please ensure that you provide
a ``nsresult BindToUtilityProcess(RefPtr<UtilityProcessParent>
aUtilityParent)``. Usually, it should be in charge of creating a set of
endpoints and performing ``Bind()`` to setup the IPC. You can see some example for `Utility AudioDecoder <https://searchfox.org/mozilla-central/rev/4b3039b48c3cb67774270ebcc2a7d8624d888092/ipc/glue/UtilityAudioDecoderChild.h#31-51>`_
- For proper user-facing exposition in ``about:processes`` you will have to also provide an actor
name via a method ``UtilityActorName GetActorName() { return UtilityActorName::EmptyUtil; }``
- Handle reception of ``StartEmptyUtilService`` on the child side of
``UtilityProcess`` within ``RecvStartEmptyUtilService()``
- In ``UtilityProcessChild::ActorDestroy``, release any resources that
you stored a reference to in ``RecvStartEmptyUtilService()``. This
will probably include a reference to the ``EmptyUtilChild``.
- The specific sandboxing requirements can be implemented by tracking
``SandboxingKind``, and it starts within `UtilityProcessSandboxing header
- Try and make sure you at least add some ``gtest`` coverage of your new
actor, for example like in `existing gtest
- Also ensure actual sandbox testing within
+ ``SandboxTest`` to start your new process,
+ ``SandboxTestingChildTests`` to define the test
+ ``SandboxTestingChild`` to run your test