interfaces: introduce and use a protocol class for the `bdiff` module This is allowed by PEP 544[1], and we basically follow the example there. The class here is copied from `mercurial.pure.bdiff`, and the implementation removed. There are several modules that have a few different implementations, and the implementation chosen is controlled by `HGMODULEPOLICY`. The module is loaded via `mercurial/policy.py`, and has been inferred by pytype as `Any` up to this point. Therefore it and PyCharm were blind to all functions on the module, and their signatures. Also, having multiple instances of the same module allows their signatures to get out of sync. Introducing a protocol class allows the loaded module that is stored in a variable to be given type info, which cascades through the various places it is used. This change alters 11 *.pyi files, for example. In theory, this would also allow us to ensure the various implementations of the same module are kept in alignment- simply import the module in a test module, attempt to pass it to a function that uses the corresponding protocol as an argument, and run pytype on it. In practice, this doesn't work (yet). PyCharm (erroneously) flags imported modules being passed where a protocol class is used[2]. Pytype has problems the other way- it fails to detect when a module that doesn't adhere to the protocol is passed to a protocol argument. The good news is that mypy properly detects this case. The bad news is that mypy spews a bunch of other errors when importing even simple modules, like the various `bdiff` modules. Therefore I'm punting on the tests for now because the type info around a loaded module in PyCharm is a clear win by itself. [1] https://peps.python.org/pep-0544/#modules-as-implementations-of-protocols [2] https://youtrack.jetbrains.com/issue/PY-58679/Support-modules-implementing-protocols

use cpython::{PyBytes, Python};
use stable_deref_trait::StableDeref;

/// Safe abstraction over a `PyBytes` together with the `&[u8]` slice
/// that borrows it. Implements `Deref<Target = [u8]>`.
///
/// Calling `PyBytes::data` requires a GIL marker but we want to access the
/// data in a thread that (ideally) does not need to acquire the GIL.
/// This type allows separating the call an the use.
///
/// It also enables using a (wrapped) `PyBytes` in GIL-unaware generic code.
pub struct PyBytesDeref {
    #[allow(unused)]
    keep_alive: PyBytes,

    /// Borrows the buffer inside `self.keep_alive`,
    /// but the borrow-checker cannot express self-referential structs.
    data: *const [u8],
}

impl PyBytesDeref {
    pub fn new(py: Python, bytes: PyBytes) -> Self {
        Self {
            data: bytes.data(py),
            keep_alive: bytes,
        }
    }

    pub fn unwrap(self) -> PyBytes {
        self.keep_alive
    }
}

impl std::ops::Deref for PyBytesDeref {
    type Target = [u8];

    fn deref(&self) -> &[u8] {
        // Safety: the raw pointer is valid as long as the PyBytes is still
        // alive, and the returned slice borrows `self`.
        unsafe { &*self.data }
    }
}

unsafe impl StableDeref for PyBytesDeref {}

fn require_send<T: Send>() {}

#[allow(unused)]
fn static_assert_pybytes_is_send() {
    #[allow(clippy::no_effect)]
    require_send::<PyBytes>;
}

// Safety: PyBytes is Send. Raw pointers are not by default,
// but here sending one to another thread is fine since we ensure it stays
// valid.
unsafe impl Send for PyBytesDeref {}