Add catch_unwind! macro to prevent panics crossing ffi boundaries

Author: davidhewitt

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Changed
 
+* Panics from Rust will now be caught and raised as Python errors. [#797](https://github.com/PyO3/pyo3/pull/797)
 * `PyObject` and `Py<T>` reference counts are now decremented sooner after `drop()`. [#851](https://github.com/PyO3/pyo3/pull/851)
   * When the GIL is held, the refcount is now decreased immediately on drop. (Previously would wait until just before releasing the GIL.)
   * When the GIL is not held, the refcount is now decreased when the GIL is next acquired. (Previously would wait until next time the GIL was released.)

src/callback.rs

@@ -155,10 +155,10 @@ where
 #[doc(hidden)]
 pub fn run_callback<T, F>(py: Python, callback: F) -> T
 where
-    F: FnOnce() -> PyResult<T>,
+    F: FnOnce() -> PyResult<T> + std::panic::UnwindSafe,
     T: PyCallbackOutput,
 {
-    callback().unwrap_or_else(|e| {
+    crate::catch_unwind!(py, callback()).unwrap_or_else(|e| {
         e.restore(py);
         T::ERR_VALUE
     })

src/err.rs

@@ -1,11 +1,12 @@
 // Copyright (c) 2017-present PyO3 Project and Contributors
 
+use crate::panic::PanicException;
 use crate::type_object::PyTypeObject;
 use crate::types::PyType;
 use crate::{exceptions, ffi};
 use crate::{
-    AsPyPointer, FromPy, IntoPy, IntoPyPointer, Py, PyAny, PyObject, Python, ToBorrowedObject,
-    ToPyObject,
+    AsPyPointer, FromPy, FromPyPointer, IntoPy, IntoPyPointer, ObjectProtocol, Py, PyAny, PyObject,
+    Python, ToBorrowedObject, ToPyObject,
 };
 use libc::c_int;
 use std::ffi::CString;
@@ -168,12 +169,20 @@ impl PyErr {
     ///
     /// The error is cleared from the Python interpreter.
     /// If no error is set, returns a `SystemError`.
-    pub fn fetch(_: Python) -> PyErr {
+    pub fn fetch(py: Python) -> PyErr {
         unsafe {
             let mut ptype: *mut ffi::PyObject = std::ptr::null_mut();
             let mut pvalue: *mut ffi::PyObject = std::ptr::null_mut();
             let mut ptraceback: *mut ffi::PyObject = std::ptr::null_mut();
             ffi::PyErr_Fetch(&mut ptype, &mut pvalue, &mut ptraceback);
+
+            if ptype == PanicException::type_object().as_ptr() {
+                let msg: String = PyAny::from_borrowed_ptr_or_opt(py, pvalue)
+                    .and_then(|obj| obj.extract().ok())
+                    .unwrap_or_else(|| String::from("Unwrapped panic from Python code"));
+                panic!(msg)
+            }
+
             PyErr::new_from_ffi_tuple(ptype, pvalue, ptraceback)
         }
     }
@@ -564,6 +573,7 @@ pub fn error_on_minusone(py: Python, result: c_int) -> PyResult<()> {
 #[cfg(test)]
 mod tests {
     use crate::exceptions;
+    use crate::panic::PanicException;
     use crate::{PyErr, Python};
 
     #[test]
@@ -575,4 +585,15 @@ mod tests {
         assert!(PyErr::occurred(py));
         drop(PyErr::fetch(py));
     }
+
+    #[test]
+    fn fetching_panic_exception_panics() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let err: PyErr = PanicException::py_err("new panic");
+        err.restore(py);
+        assert!(PyErr::occurred(py));
+        let started_unwind = std::panic::catch_unwind(|| PyErr::fetch(py)).is_err();
+        assert!(started_unwind);
+    }
 }

src/lib.rs

@@ -189,6 +189,8 @@ mod internal_tricks;
 pub mod marshal;
 mod object;
 mod objectprotocol;
+#[macro_use]
+pub mod panic;
 pub mod prelude;
 pub mod pycell;
 pub mod pyclass;

src/panic.rs

@@ -0,0 +1,36 @@
+use crate::exceptions::BaseException;
+use crate::Python;
+/// The exception raised when Rust code called from Python panics.
+///
+/// Like SystemExit, this exception is derived from BaseException so that
+/// it will typically propagate all the way through the stack and cause the
+/// Python interpreter to exit.
+pub struct PanicException {
+    _private: (),
+}
+
+pyo3_exception!(PanicException, BaseException);
+
+#[macro_export]
+macro_rules! catch_unwind {
+    ($py:ident, $block:expr) => {{
+        match std::panic::catch_unwind(|| -> $crate::PyResult<_> { $block }) {
+            Ok(result) => result,
+            Err(e) => {
+                // Try to format the error in the same way panic does
+                if let Some(string) = e.downcast_ref::<String>() {
+                    Err($crate::panic::PanicException::py_err((string.clone(),)))
+                } else if let Some(s) = e.downcast_ref::<&str>() {
+                    Err($crate::panic::PanicException::py_err((s.to_string(),)))
+                } else {
+                    Err($crate::panic::PanicException::py_err((
+                        "panic from Rust code",
+                    )))
+                }
+            }
+        }
+    }};
+}
+
+impl std::panic::UnwindSafe for Python<'_> {}
+impl std::panic::RefUnwindSafe for Python<'_> {}