rust-pyo3: error propagation for UnsafePyLeaked wrapping a result

This `py_leaked_or_map_err` is the PyO3 version of the function
of the same name in `hg-cpython/src/ancestors.rs`.

rust/hg-pyo3/src/convert_cpython.rs

@@ -1,214 +1,252
 //! 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 pyo3::{pyclass::boolean_struct::False, PyClass};
 
 use cpython::ObjectProtocol;
 use cpython::PythonObject;
 use lazy_static::lazy_static;
 
 use hg::revlog::index::Index as CoreIndex;
 use rusthg::revlog::{InnerRevlog, PySharedIndex};
 
 /// Marker trait for PyO3 objects with a lifetime representing the acquired GIL
 ///
 /// # Safety
 ///
 /// This trait must not be implemented for objects with lifetimes that
 /// do not imply in PyO3 that the GIL is acquired during the whole lifetime.
 pub unsafe trait WithGIL<'py> {}
 
 // Safety: the lifetime on these PyO3 objects all represent the acquired GIL
 unsafe impl<'py> WithGIL<'py> for Python<'py> {}
 unsafe impl<'py, T> WithGIL<'py> for Bound<'py, T> {}
 unsafe impl<'py, T: PyClass> WithGIL<'py> for PyRef<'py, T> {}
 unsafe impl<'py, T: PyClass<Frozen = False>> WithGIL<'py>
     for PyRefMut<'py, T>
 {
 }
 
 /// 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: WithGIL<'py>>(
     _with_gil: &T,
 ) -> cpython::Python<'py> {
     // safety: this is safe because the returned object has the same lifetime
     // as the incoming object.
     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_py_leak<'py>(
     index_proxy: &Bound<'py, PyAny>,
 ) -> PyResult<(cpython::Python<'py>, cpython::UnsafePyLeaked<PySharedIndex>)> {
     let (py, idx_proxy) = to_cpython_py_object(index_proxy);
     let py_leaked = py_rust_index_to_graph(py, idx_proxy)?;
     Ok((py, py_leaked))
 }
 
 /// Full extraction of the proxy index object as received in PyO3 to a
 /// [`CoreIndex`] reference.
 ///
 /// # Safety
 ///
 /// The 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<&'py CoreIndex> {
     let (py, py_leaked) = proxy_index_py_leak(index_proxy)?;
     let py_shared = &*unsafe {
         py_leaked
             .try_borrow(py)
             .map_err(|e| from_cpython_pyerr(py, e))?
     };
     Ok(py_shared.inner)
 }
+
+/// Error propagation for an [`UnsafePyLeaked`] wrapping a [`Result`]
+///
+/// TODO (will consider when implementing UnsafePyLeaked in PyO3):
+/// It would be nice for UnsafePyLeaked to provide this directly as a variant
+/// of the `map` method with a signature such as:
+///
+/// ```
+///   unsafe fn map_or_err(&self,
+///                        py: Python,
+///                        f: impl FnOnce(T) -> Result(U, E),
+///                        convert_err: impl FnOnce(E) -> PyErr)
+/// ```
+///
+/// This would spare users of the `cpython` crate the additional `unsafe` deref
+/// to inspect the error and return it outside `UnsafePyLeaked`, and the
+/// subsequent unwrapping that this function performs.
+#[allow(dead_code)]
+pub(crate) fn py_leaked_or_map_err<T, E: std::fmt::Debug + Copy>(
+    py: cpython::Python,
+    leaked: cpython::UnsafePyLeaked<Result<T, E>>,
+    convert_err: impl FnOnce(E) -> PyErr,
+) -> PyResult<cpython::UnsafePyLeaked<T>> {
+    // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+    if let Err(e) = *unsafe {
+        leaked
+            .try_borrow(py)
+            .map_err(|e| from_cpython_pyerr(py, e))?
+    } {
+        return Err(convert_err(e));
+    }
+    // Safety: we don't leak the "faked" reference out of `UnsafePyLeaked`
+    Ok(unsafe {
+        leaked.map(py, |res| {
+            res.expect("Error case should have already be treated")
+        })
+    })
+}