view rust/hg-cpython/src/cindex.rs @ 42748:7cae6bc29ff9

rust-parsers: switch to parse/pack_dirstate to mutate-on-loop

Both `parse_dirstate` and `pack_dirstate` can operate directly on the data
they're passed, which prevents the creation of intermediate data structures,
simplifies the function signatures and reduces boilerplate. They are exposed
directly to the Python for now, but a later patch will make use of them inside
`hg-core`.

Differential Revision: https://phab.mercurial-scm.org/D6628

// cindex.rs
//
// Copyright 2018 Georges Racinet <[e-mail]>
//
// This software may be used and distributed according to the terms of the
// GNU General Public License version 2 or any later version.

//! Bindings to use the Index defined by the parsers C extension
//!
//! Ideally, we should use an Index entirely implemented in Rust,
//! but this will take some time to get there.
#[cfg(feature = "python27")]
use python27_sys as python_sys;
#[cfg(feature = "python3")]
use python3_sys as python_sys;

use cpython::{PyClone, PyErr, PyObject, PyResult, Python};
use hg::{Graph, GraphError, Revision, WORKING_DIRECTORY_REVISION};
use libc::c_int;
use python_sys::PyCapsule_Import;
use std::ffi::CStr;
use std::mem::transmute;

type IndexParentsFn = unsafe extern "C" fn(
    index: *mut python_sys::PyObject,
    rev: c_int,
    ps: *mut [c_int; 2],
) -> c_int;

/// A `Graph` backed up by objects and functions from revlog.c
///
/// This implementation of the `Graph` trait, relies on (pointers to)
/// - the C index object (`index` member)
/// - the `index_get_parents()` function (`parents` member)
///
/// # Safety
///
/// The C index itself is mutable, and this Rust exposition is **not
/// protected by the GIL**, meaning that this construct isn't safe with respect
/// to Python threads.
///
/// All callers of this `Index` must acquire the GIL and must not release it
/// while working.
///
/// # TODO find a solution to make it GIL safe again.
///
/// This is non trivial, and can wait until we have a clearer picture with
/// more Rust Mercurial constructs.
///
/// One possibility would be to a `GILProtectedIndex` wrapper enclosing
/// a `Python<'p>` marker and have it be the one implementing the
/// `Graph` trait, but this would mean the `Graph` implementor would become
/// likely to change between subsequent method invocations of the `hg-core`
/// objects (a serious change of the `hg-core` API):
/// either exposing ways to mutate the `Graph`, or making it a non persistent
/// parameter in the relevant methods that need one.
///
/// Another possibility would be to introduce an abstract lock handle into
/// the core API, that would be tied to `GILGuard` / `Python<'p>`
/// in the case of the `cpython` crate bindings yet could leave room for other
/// mechanisms in other contexts.
pub struct Index {
    index: PyObject,
    parents: IndexParentsFn,
}

impl Index {
    pub fn new(py: Python, index: PyObject) -> PyResult<Self> {
        Ok(Index {
            index: index,
            parents: decapsule_parents_fn(py)?,
        })
    }
}

impl Clone for Index {
    fn clone(&self) -> Self {
        let guard = Python::acquire_gil();
        Index {
            index: self.index.clone_ref(guard.python()),
            parents: self.parents.clone(),
        }
    }
}

impl Graph for Index {
    /// wrap a call to the C extern parents function
    fn parents(&self, rev: Revision) -> Result<[Revision; 2], GraphError> {
        if rev == WORKING_DIRECTORY_REVISION {
            return Err(GraphError::WorkingDirectoryUnsupported);
        }
        let mut res: [c_int; 2] = [0; 2];
        let code = unsafe {
            (self.parents)(
                self.index.as_ptr(),
                rev as c_int,
                &mut res as *mut [c_int; 2],
            )
        };
        match code {
            0 => Ok(res),
            _ => Err(GraphError::ParentOutOfRange(rev)),
        }
    }
}

/// Return the `index_get_parents` function of the parsers C Extension module.
///
/// A pointer to the function is stored in the `parsers` module as a
/// standard [Python capsule](https://docs.python.org/2/c-api/capsule.html).
///
/// This function retrieves the capsule and casts the function pointer
///
/// Casting function pointers is one of the rare cases of
/// legitimate use cases of `mem::transmute()` (see
/// https://doc.rust-lang.org/std/mem/fn.transmute.html of
/// `mem::transmute()`.
/// It is inappropriate for architectures where
/// function and data pointer sizes differ (so-called "Harvard
/// architectures"), but these are nowadays mostly DSPs
/// and microcontrollers, hence out of our scope.
fn decapsule_parents_fn(py: Python) -> PyResult<IndexParentsFn> {
    unsafe {
        let caps_name = CStr::from_bytes_with_nul_unchecked(
            b"mercurial.cext.parsers.index_get_parents_CAPI\0",
        );
        let from_caps = PyCapsule_Import(caps_name.as_ptr(), 0);
        if from_caps.is_null() {
            return Err(PyErr::fetch(py));
        }
        Ok(transmute(from_caps))
    }
}