Implement layer to allow adapter mocking.

[aarongreig]

No description provided.

[aarongreig]

I haven't hooked up the storage and data members of this yet, I'm kind of hoping that when it comes to porting the particular sycl unittests that were using them I can achieve the same effect with callbacks defined in the test rather than default behavior in our layer. I've left them in for now in case that isn't so.

[aarongreig]

I'm going to leave this in draft until I've ported most/all of the PI unittests just to make sure our initial version satisfies all the requirements there, but this is a complete PR with documentation and testing for the new layer. It isn't urgent since it'll take me a bit to port those tests I mentioned but I'd appreciate some feedback on the interface, scripting and docs @pbalcer

[pbalcer]

A general comment: if we focus on the mocking use-case instead of creating a generic callback mechanism, it might be easier to instead create a mock adapter, with its own include header that the application would use to modify its behavior. We could even modify the existing null adapter for this purpose.

A side note - did you considering just using something existing like using -Wl,--wrap for the mocked tests?

[pbalcer]

This is sort of ugly, and will get worse the more layers need some state from the loader. Ideally we'd have a generic abstraction here that would allow the layer to somehow "collect" loader state, but that's probably an abstraction too far. Here's a simpler thought:

struct {
  layer_type type;
  union {
    struct {
      codeloc codeloc;
    } tracing;
    struct {
      callbacks callbacks;
    } callbacks;
  }
}

Or, even simpler, we can drop the union/layer_type entirely, and just pass this data as a struct - which would eliminate the need to update all the layers whenever this changes.

[aarongreig]

A general comment: if we focus on the mocking use-case instead of creating a generic callback mechanism, it might be easier to instead create a mock adapter, with its own include header that the application would use to modify its behavior. We could even modify the existing null adapter for this purpose.

A side note - did you considering just using something existing like using -Wl,--wrap for the mocked tests?

I actually didn't know about --wrap, I think it could work with some clever cmake but at this point I don't really want to spend the time to try and get a whole new approach working.

I have hit some stuff while porting tests that looks like it's going to necessitate a mock header from the UR side being included there.. the only reason this is still done in a layer is that it started as the generic callback layer before I realised we'd need to focus in on the mock element so I might need to look at moving the mock stuff into the null adapter (I think the only thing we use that for in its current state is layer testing right?)

[pbalcer]

I think the only thing we use that for in its current state is layer testing right?

yes.

at this point I don't really want to spend the time to try and get a whole new approach working.

My experience is that --wrap is a bit fiddly, and given that you've invested so much time in this approach, sticking with it is probably the right choice.

[aarongreig]

closed in favour of #1780