rust-pyo3: simplified API to get core Index references

Given the amount of conversion and arcane internal detail, it
is easier for the caller and makes a better separation of concerns
to just introduce a new unsafe helper. It is actually possible that
it would be safe, and we can decide about that later.

Actually the reason given in the `cpython` crate for unsafety of the
`try_borrow()` method is the following:

```
    // try_borrow() and try_borrow_mut() are unsafe because self.data may
    // have a function returning the inner &'static reference.
    // If T is &'static U, its lifetime can be easily coerced to &'a U, but
    // how could we do that for Whatever<'static> in general?
```

Given that we coerce the Index reference to the GIL lifetime and that
it is unlikely that the inner data would contain a function returning the
a `'static` reference, it is possible that it is actually even safe.

rust/hg-pyo3/src/convert_cpython.rs

@@ -1,173 +1,186
 //! This module takes care of all conversions involving `rusthg` (hg-cpython)
 //! objects in the PyO3 call context.
 //!
 //! For source code clarity, we only import (`use`) [`cpython`] traits and not
 //! any of its data objects. We are instead using full qualifiers, such as
 //! `cpython::PyObject`, and believe that the added heaviness is an acceptatble
 //! price to pay to avoid confusion.
 //!
 //! Also it, is customary in [`cpython`] to label the GIL lifetime as `'p`,
 //! whereas it is `'py` in PyO3 context. We keep both these conventions in
 //! the arguments side of function signatures when they are not simply elided.
 use pyo3::exceptions::PyTypeError;
 use pyo3::prelude::*;
 
 use cpython::ObjectProtocol;
 use cpython::PythonObject;
 use lazy_static::lazy_static;
 
+use hg::revlog::index::Index as CoreIndex;
 use rusthg::revlog::{InnerRevlog, PySharedIndex};
 
 /// Force cpython's GIL handle with the appropriate lifetime
 ///
 /// In `pyo3`, the fact that we have the GIL is expressed by the lifetime of
 /// the incoming [`Bound`] smart pointer. We therefore simply instantiate
 /// the `cpython` handle and coerce its lifetime by the function signature.
 ///
 /// Reacquiring the GIL is also a possible alternative, as the CPython
 /// documentation explicitely states that "recursive calls are allowed"
 /// (we interpret that as saying that acquiring the GIL within a thread that
 /// already has it works) *as long as it is properly released*
 /// reference:
 /// <https://docs.python.org/3.8/c-api/init.html#c.PyGILState_Ensure>
 pub(crate) fn cpython_handle<'py, T>(
     _bound: &Bound<'py, T>,
 ) -> cpython::Python<'py> {
     // safety: this is safe because the returned object has the 'py lifetime
     unsafe { cpython::Python::assume_gil_acquired() }
 }
 
 /// Force PyO3 GIL handle from cpython's.
 ///
 /// Very similar to [`cpython_handle`]
 pub fn pyo3_handle(_py: cpython::Python<'_>) -> Python<'_> {
     // safety: this is safe because the returned object has the same lifetime
     // as the incoming object.
     unsafe { Python::assume_gil_acquired() }
 }
 
 /// Convert a PyO3 [`PyObject`] into a [`cpython::PyObject`]
 ///
 /// During this process, the reference count is increased, then decreased.
 /// This means that the GIL (symbolized by the lifetime on the `obj`
 /// argument) is needed.
 ///
 /// We could make something perhaps more handy by simply stealing the
 /// pointer, forgetting the incoming and then implement `From` with "newtype".
 /// It would be worth the effort for a generic cpython-to-pyo3 crate, perhaps
 /// not for the current endeavour.
 pub(crate) fn to_cpython_py_object<'py>(
     obj: &Bound<'py, PyAny>,
 ) -> (cpython::Python<'py>, cpython::PyObject) {
     let py = cpython_handle(obj);
     // public alias of the private cpython::fii::PyObject (!)
     let raw = obj.as_ptr() as *mut python3_sys::PyObject;
     // both pyo3 and rust-cpython will decrement the refcount on drop.
     // If we use from_owned_ptr, that's a segfault.
     (py, unsafe { cpython::PyObject::from_borrowed_ptr(py, raw) })
 }
 
 /// Convert a [`cpython::PyObject`] into a PyO3 [`PyObject`]
 ///
 /// During this process, the reference count is increased, then decreased.
 /// This means that the GIL (symbolized by the PyO3 [`Python`] handle is
 /// needed.
 ///
 /// We could make something perhaps more handy by simply stealing the
 /// pointer, forgetting the incoming and then implement `From` with "newtype".
 /// It would be worth the effort for a generic cpython-to-pyo3 crate, perhaps
 /// not for the current endeavour.
 pub(crate) fn from_cpython_py_object(
     py: Python<'_>,
     obj: cpython::PyObject,
 ) -> PyObject {
     let raw = obj.as_ptr() as *mut pyo3::ffi::PyObject;
     unsafe { Py::from_borrowed_ptr(py, raw) }
 }
 
 /// Convert [`cpython::PyErr`] into [`pyo3::PyErr`]
 ///
 /// The exception class remains the same as the original exception,
 /// hence if it is also defined in another dylib based on `cpython` crate,
 /// it will need to be converted to be downcasted in this crate.
 pub(crate) fn from_cpython_pyerr(
     py: cpython::Python<'_>,
     mut e: cpython::PyErr,
 ) -> PyErr {
     let pyo3_py = pyo3_handle(py);
     let cpython_exc_obj = e.instance(py);
     let pyo3_exc_obj = from_cpython_py_object(pyo3_py, cpython_exc_obj);
     PyErr::from_value(pyo3_exc_obj.into_bound(pyo3_py))
 }
 
 /// Retrieve the PyType for objects from the `mercurial.rustext` crate.
 fn retrieve_cpython_py_type(
     submodule_name: &str,
     type_name: &str,
 ) -> cpython::PyResult<cpython::PyType> {
     let guard = cpython::Python::acquire_gil();
     let py = guard.python();
     let module = py.import(&format!("mercurial.rustext.{submodule_name}"))?;
     module.get(py, type_name)?.extract::<cpython::PyType>(py)
 }
 
 lazy_static! {
     static ref INNER_REVLOG_PY_TYPE: cpython::PyType = {
         retrieve_cpython_py_type("revlog", "InnerRevlog")
             .expect("Could not import InnerRevlog in Python")
     };
 }
 
 /// Downcast [`InnerRevlog`], with the appropriate Python type checking.
 ///
 /// The PyType object representing the `InnerRevlog` Python class is not the
 /// the same in this dylib as it is in the `mercurial.rustext` module.
 /// This is because the code created with the [`cpython::py_class!`]
 /// macro is itself duplicated in both dylibs. In the case of this crate, this
 /// happens by linking to the [`rusthg`] crate and provides the `InnerRevlog`
 /// that is visible from this crate. The `InnerRevlog::get_type` associated
 /// function turns out to return a `static mut` (look for `TYPE_OBJECT` in
 /// `py_class_impl3.rs`), which obviously is different in both dylibs.
 ///
 /// The consequence of that is that downcasting an `InnerRevlog` originally
 /// from the `mecurial.rustext` module to our `InnerRevlog` cannot be done with
 /// the usual `extract::<InnerRevlog>(py)`, as it would perform the type
 /// checking with the `PyType` that is embedded in `mercurial.pyo3_rustext`.
 /// We must check the `PyType` that is within `mercurial.rustext` instead.
 /// This is what this function does.
 fn extract_inner_revlog(
     py: cpython::Python,
     inner_revlog: cpython::PyObject,
 ) -> PyResult<InnerRevlog> {
     if !(*INNER_REVLOG_PY_TYPE).is_instance(py, &inner_revlog) {
         return Err(PyTypeError::new_err("Not an InnerRevlog instance"));
     }
     // Safety: this is safe because we checked the PyType already, with the
     // value embedded in `mercurial.rustext`.
     Ok(unsafe { InnerRevlog::unchecked_downcast_from(inner_revlog) })
 }
 
 /// This is similar to [`rusthg.py_rust_index_to_graph`], with difference in
 /// how we retrieve the [`InnerRevlog`].
 pub fn py_rust_index_to_graph(
     py: cpython::Python,
     index_proxy: cpython::PyObject,
 ) -> PyResult<cpython::UnsafePyLeaked<PySharedIndex>> {
     let inner_revlog = extract_inner_revlog(
         py,
         index_proxy
             .getattr(py, "inner")
             .map_err(|e| from_cpython_pyerr(py, e))?,
     )?;
 
     let leaked = inner_revlog.pub_inner(py).leak_immutable();
     // Safety: we don't leak the "faked" reference out of the `UnsafePyLeaked`
     Ok(unsafe { leaked.map(py, |idx| PySharedIndex { inner: &idx.index }) })
 }
 
-pub(crate) fn proxy_index_extract<'py>(
+/// Full extraction of the proxy index object as received in PyO3 to a
+/// [`CoreIndex`] reference.
+///
+/// The safety invariants to maintain are those of the underlying
+/// [`UnsafePyLeaked::try_borrow`]: the caller must not leak the inner
+/// reference.
+pub(crate) unsafe fn proxy_index_extract<'py>(
     index_proxy: &Bound<'py, PyAny>,
-) -> PyResult<(cpython::Python<'py>, cpython::UnsafePyLeaked<PySharedIndex>)> {
+) -> PyResult<&'py CoreIndex> {
     let (py, idx_proxy) = to_cpython_py_object(index_proxy);
-    Ok((py, py_rust_index_to_graph(py, idx_proxy)?))
+    let py_leaked = py_rust_index_to_graph(py, idx_proxy)?;
+    let py_shared = &*unsafe {
+        py_leaked
+            .try_borrow(py)
+            .map_err(|e| from_cpython_pyerr(py, e))?
+    };
+    Ok(py_shared.inner)
 }

rust/hg-pyo3/src/dagops.rs

@@ -1,73 +1,67
 // dagops.rs
 //
 // Copyright 2024 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 for the `hg::dagops` module provided by the
 //! `hg-core` package.
 //!
 //! From Python, this will be seen as `mercurial.pyo3-rustext.dagop`
 use pyo3::prelude::*;
 
 use std::collections::HashSet;
 
 use hg::{dagops, Revision};
 
-use crate::convert_cpython::{from_cpython_pyerr, proxy_index_extract};
+use crate::convert_cpython::proxy_index_extract;
 use crate::exceptions::GraphError;
 use crate::revision::{rev_pyiter_collect, PyRevision};
 use crate::util::new_submodule;
 
 /// Using the the `index_proxy`, return heads out of any Python iterable of
 /// Revisions
 ///
 /// This is the Rust counterpart for `mercurial.dagop.headrevs`
 #[pyfunction]
 pub fn headrevs(
     index_proxy: &Bound<'_, PyAny>,
     revs: &Bound<'_, PyAny>,
 ) -> PyResult<HashSet<PyRevision>> {
-    let (py, py_leaked) = proxy_index_extract(index_proxy)?;
     // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
-    let index = &*unsafe {
-        py_leaked
-            .try_borrow(py)
-            .map_err(|e| from_cpython_pyerr(py, e))?
-    };
-
+    let index = unsafe { proxy_index_extract(index_proxy)? };
     let mut as_set: HashSet<Revision> = rev_pyiter_collect(revs, index)?;
     dagops::retain_heads(index, &mut as_set).map_err(GraphError::from_hg)?;
     Ok(as_set.into_iter().map(Into::into).collect())
 }
 
 /// Computes the rank, i.e. the number of ancestors including itself,
 /// of a node represented by its parents.
 ///
 /// Currently, the pure Rust index supports only the REVLOGV1 format, hence
 /// the only possible return value is that the rank is unknown.
 ///
 /// References:
 /// - C implementation, function `index_fast_rank()`.
 /// - `impl vcsgraph::graph::RankedGraph for Index` in `crate::cindex`.
 #[pyfunction]
 pub fn rank(
     _index: &Bound<'_, PyAny>,
     _p1r: PyRevision,
     _p2r: PyRevision,
 ) -> PyResult<()> {
     Err(GraphError::from_vcsgraph(
         vcsgraph::graph::GraphReadError::InconsistentGraphData,
     ))
 }
 
 pub fn init_module<'py>(
     py: Python<'py>,
     package: &str,
 ) -> PyResult<Bound<'py, PyModule>> {
     let m = new_submodule(py, package, "dagop")?;
     m.add_function(wrap_pyfunction!(headrevs, &m)?)?;
     m.add_function(wrap_pyfunction!(rank, &m)?)?;
     Ok(m)
 }