Populate `$callers` with `start_supervised`

[LostKobrakai]

Elixir and Erlang/OTP versions

Erlang/OTP 26 [erts-14.2.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]

Elixir 1.15.4 (compiled with Erlang/OTP 26)

Operating system

Mac OS

Current behavior

Using ex_units start_supervised the started process doesn't :"$callers" set correctly.

Expected behavior

Have :"$callers" set correctly.

[josevalim]

Can you expand more? $callers and $ancestors are separate things. Supervision typically set ancestors.

[LostKobrakai]

I think as a user of start_supervised I shouldn‘t need to care that there‘s a separate supervisor process involved. Currently one cannot use start_supervised if $callers is required for a test to work properly, which I guess isn‘t as relevant to Task, but imo is to use cases like Mox or other tools, which link processes to the context of a test.

[josevalim]

It is not about start_supervised starting a process, but rather that callers is never set when going through the child_spec start.

[LostKobrakai]

Tbh to me that's totally fine. My interest in $callers is purely practical in that it is useful for a bunch of test tooling and I'd love to see it used in more places. As it stands right now that tooling can only be used if I manually call an API for starting processes, which harnesses $callers, e.g. Task.async.

I'm looking into using $callers for processes, which actually are "long running" (contrary to Task), which are scoped to a test when started. That started would be MyModule.start_link([]). If I replace MyModule.start_link([]) with start_supervised(MyModule) it breaks however. I think that shouldn't be the case, because to someone not intimate with the details the difference likely won't be obvious. I also doubt they'll know to replace start_supervised(MyModule) with MyModule.start_link([]) to get it working.

The same case can be made with Task, which also sets $callers on start_link/1, but again it it only works wenn calling start_link manually, but not when calling it through start_supervised. It just happens to be, that calling Task via a child_spec is rare.

[ityonemo]

I've run into this a lot. I thought about generally suggesting a change to the GenServer options to allow an add_callers option, but I have since realized the only real use case I've run into is with start_supervised.

[ityonemo]

Apologies for the walled garden, but: https://twitter.com/DNAutics/status/1750236523579003182

[josevalim]

Moving discussion to PR, as we will discuss the final details there. :)