rust-pyo3: exposition of AncestorsIterator Compared to the early experiments, we have one less `RwLock` in the wrapping. Still that is somewhat redundant with `UnsafePyLeaked` being itself some kind of lock. In the original rust-cpython code, we were borrowing the `RefCell` with a method that can panic. Instead we are now converting the `PoisonError` that unlocking a `RwLock` can produce. Since all methods acquiring the `RwLock` are themselves protected by the GIL and do not release it before returning, nor do they leak RwLock guards, there is no risk of contention on these locks themselves.

--- a/rust/hg-pyo3/src/exceptions.rs	Sat Dec 07 18:24:24 2024 +0100
+++ b/rust/hg-pyo3/src/exceptions.rs	Sat Dec 07 18:38:37 2024 +0100
@@ -1,4 +1,4 @@
-use pyo3::exceptions::PyValueError;
+use pyo3::exceptions::{PyRuntimeError, PyValueError};
 use pyo3::import_exception;
 use pyo3::{create_exception, PyErr};
 
@@ -32,3 +32,7 @@
         }
     }
 }
+
+pub fn map_lock_error<T>(e: std::sync::PoisonError<T>) -> PyErr {
+    PyRuntimeError::new_err(format!("In Rust PyO3 bindings: {e}"))
+}