Extend load_from to allow loading from an absolute path

[evnu]

load_from already has a format to specify a path relative to an OTP application. This commit extends the type for load_from to also allow setting an absolute path. Note that no special check for the validity of the path is being done, as we do not know beforehand what the target operating system looks like. Setting this configuration correctly is thus the user's responsibility.

Fix #556.

[filmor]

Hmm, actually users should be able to just implement their own custom on_load handler right? I don't think it's a good idea to offer this in the library.

[evnu]

I don't think it's a good idea to offer this in the library.

You mean offering load_from at all, or the extension proposed here?

[filmor]

Passing the OTP application is fine, passing arbitrary paths is what I have an issue with. Nothing keeps users from just building their own on_load handler to achieve the same thing.

[evnu]

@filmor to check that I get you right, you propose that users overwrite @on_load and avoid using the rustler_init/0 function? That's prone to error as well I think. Erlang's :erlang.load_nif/2 function does not restrict the path where the NIF library resides, I am not sure if we should impose restriction on that path "artificially".

[filmor]

Yes... But that is exactly the problem with this PR as well. I suggest that if people want to load a NIF from an absolute path, they do it without use Rustler.

[evnu]

they do it without use Rustler.

That does not seem pratical. Users then also have to replicate the mechanism to auto-compile on changes with external resources and so on. I don't have a strong opinion on this topic, but I think users do not want to implement all of use Rustler just to load the library from a different path. But I see your point that using absolute paths introduces other problems for users. For example, the library is not necessarily versioned together with the application itself.

I will close this PR for now, we can discuss this further in #556 itself.