fix(py): a panic in the library raises clawhdf5.InternalError, not PanicException

PanicException derives from BaseException, so `except Exception` let a
library bug through. Every call from the bindings into the library now
runs under catch_unwind and a panic becomes InternalError (RuntimeError)
naming the object. Tests: a hidden hook panics inside the guard; and the
v4 chunk indexes are compared with h5py from Python — with the library
fix reverted, ds[0:30] of the implicit-index dataset now raises
InternalError instead of aborting the test run.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-py/src/lib.rs

@@ -24,6 +24,42 @@ pub(crate) use dataset::PyDataset;
 pub(crate) use file::PyFile;
 pub(crate) use group::PyGroup;
 
+pyo3::create_exception!(
+    clawhdf5,
+    InternalError,
+    pyo3::exceptions::PyRuntimeError,
+    "A bug in clawhdf5 met while reading or writing a file (a Rust panic, \
+     caught). Derived from RuntimeError, so `except Exception` handles it."
+);
+
+/// The text of a caught panic.
+pub(crate) fn panic_text(payload: &(dyn std::any::Any + Send)) -> String {
+    payload
+        .downcast_ref::<&str>()
+        .map(|s| (*s).to_string())
+        .or_else(|| payload.downcast_ref::<String>().cloned())
+        .unwrap_or_else(|| "unknown panic".to_string())
+}
+
+/// Run `f`, turning a panic in the library into [`InternalError`] instead of
+/// PyO3's `PanicException` (a `BaseException`, which `except Exception`
+/// does not catch). Wraps every call into the library.
+pub(crate) fn no_panic<T>(f: impl FnOnce() -> PyResult<T>) -> PyResult<T> {
+    std::panic::catch_unwind(std::panic::AssertUnwindSafe(f)).unwrap_or_else(|p| {
+        Err(InternalError::new_err(format!(
+            "clawhdf5 internal error (please report it): {}",
+            panic_text(&*p)
+        )))
+    })
+}
+
+/// A test hook: panics inside [`no_panic`], so the tests can check that a
+/// library panic reaches Python as an ordinary exception.
+#[pyfunction]
+fn _panic_for_test() -> PyResult<()> {
+    no_panic(|| panic!("deliberate panic for the test suite"))
+}
+
 /// Convert a `clawhdf5_rs::Error` into a `PyErr`.
 ///
 /// Maps different error variants to more specific Python exception types:
@@ -276,6 +312,8 @@ fn clawhdf5(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<PyGroup>()?;
     m.add_class::<PyAttrs>()?;
     m.add_class::<PyEmpty>()?;
+    m.add("InternalError", m.py().get_type::<InternalError>())?;
+    m.add_function(wrap_pyfunction!(_panic_for_test, m)?)?;
     Ok(())
 }