Setting up Child Policy explicitly for a child PD which will announce a service.

[Norman Feske]

Hi Sid,

On 2023-06-27 22:39, Sid Agrawal wrote:
> I am trying to setup a scenario where the parent starts a child PD in 
> the C code, i.e., there is no <start> node for the child in the config 
> file. It is similar to the rm_nested test example [1].
> 
> But in my scenario, I would like to start a server in the child and have 
> the parent make RPC calls to it, and I need some help setting that up. I 
> think the answer should be in some configuration of the ChildPolicy used 
> to start the child, but I am missing something.

we explored this idea in the past [1] but ultimately disregarded it [2].

[1] 
https://genode.org/documentation/release-notes/12.02#API_support_for_enslaving_services
[2] https://github.com/genodelabs/genode/issues/3754

You can still find a trace of it left as part of the qt_avplay component 
in the genode-world repository [3]. There you can find the slave.h 
header that implements the mechanism for hosting a service as a child.

[3] https://github.com/genodelabs/genode-world/tree/master/src/app/qt_avplay

For hosting and managing child components, I warmly recommend the use of 
the sandbox library [4], which is far more flexible while easier to use 
than the raw child.h interfaces. We plan to replace all manual uses of 
child.h by the use of the sandbox library eventually.

[4] 
https://genode.org/documentation/release-notes/20.02#New_sandbox_library_based_on_the_init_component

> Any help pointing me in the right direction would be very helpful. 
> Thanks as always!

As a general advice, please try to avoid making a parent depend on a 
child. It is better to either use an session interface that can be 
provided by the parent and invoked by the child, or to use a second 
child to interact with a child service. The latter approach is 
extensively used by the depot_download_manager, which orchestrates the 
downloading, verification, extraction, and installation of software in 
Sculpt OS.

Cheers
Norman

-- 
Dr.-Ing. Norman Feske
Genode Labs

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