PyO3 v0.21 beta pyfunction annotation coverage

[reaperhulk]

In PyO3 v0.21.0-beta.0 we (pyca/cryptography) are seeing an issue where some #[pyo3::prelude::pyfunction] annotations on functions are marked as uncovered lines. This appears to occur only on functions where the function cannot error.

An example function:
https://github.com/pyca/cryptography/blob/e9954a0a31db22201b96d62535f51a5f0316e218/src/rust/src/backend/x25519.rs#L19-L24

I'm sure I can build a minimal reproducer if necessary, but if this is sufficient information then great 😄

[reaperhulk]

#1726 may be relevant as a past similar issue

[alex]

My specific hypothesis in this case is that the error code paths are never covered.

[davidhewitt]

My specific hypothesis in this case is that the error code paths are never covered.

I guess this is probably true, though it's tricky to know what we can do about it. #[coverage(off)] isn't available. Should we be adding #[automatically_derived]? I think in 1.77 they no longer get instrumented: rust-lang/rust#120185

We might be able to modify the generated code be smarter to make the compiler not care about the error paths, if that is indeed the case.

Would you be willing to use cargo expand to test against a manually expanded form of the problematic functions and see what lines are reported as missing coverage?

[alex]

I think adding automatically_derived for the generated functions makes sense (I assume pyo3 generates a function which calls the user defined function?). That'd be consistent with what we want at any rate.

[reaperhulk]

I used cargo expand (and some judicious copy/paste) to pull out (hopefully) the expanded form of generate_key on one of our uncovered paths:

[davidhewitt]

Aha that red block is the new code we added to support declarative modules, cc @Tpt

I think what we can probably do is refactor that code into traits so that we don't need to emit a function directly in that position. I'm happy to have a go at this but might not be until the weekend or so, so if someone is happy to try sooner please do!

[alex]

FYI, I think there's two other situations in which we aren't getting coverage -- one on pyclass and one on import_exception. I've poked @reaperhulk to see if we can get clean screenshots on those as well.

[davidhewitt]

It is very likely the same code for supporting those types. 👍

[alex]

It looks like for import_exception this is already in an impl PyAddToModule.

Is the goal of moving them to an impl that it should be easy to add the #[automatically_derived]?

[Tpt]

I think what we can probably do is refactor that code into traits

The tricky thing here is that traits can be implemented by stucts and enum but not function and modules so it won't work with #[pyfunction] and #[pymodule]

[davidhewitt]

Agreed that the functions and modules can't directly implement traits, but their helper MakeDef structs can.

I think rather than adding #[automatically_derived] my ideal would be that the trait doesn't even need a function implementation emitted.

Maybe something like this:

#[doc(hidden)]
mod generate_key 1 {
    pub (crate) struct MakeDef;
    pub const DEF: ::pyo3::impl_::pymethods::PyMethodDef = MakeDef::DEF;
｝
// change this bit to a trait implementation?
impl ::pyo3::impl::pymodule::PyAddToModule for generate_key::MakeDef {
    const DEF: ::pyo3::impl_::pymethods::PyMethodDef = :: pyo3::impl_::pymethods::PyMethodDef::noargs (...);
}

I think that code isn't exactly right, but hopefully it conveys the idea. Really all we need to do is get a constant exposed, and if the namespaces look the same, (e.g. function::DEF, module::DEF, Class::DEF) then the proc macros should be able to build something which some traits inside PyO3's impl_ module can then handle.

[reaperhulk]

Alex attempted implementing some of this in main...alex:pyo3:add-to-module-auto but pyclass/import_exception coverage did not improve in our tests. 😢