avoid creating PyRef inside __traverse__ handler (#4479)

Cargo.toml

@@ -61,6 +61,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0.61"
 rayon = "1.6.1"
 futures = "0.3.28"
+static_assertions = "1.1.0"
 
 [build-dependencies]
 pyo3-build-config = { path = "pyo3-build-config", version = "=0.22.2", features = ["resolve-config"] }

newsfragments/4479.fixed.md

@@ -0,0 +1 @@
+Remove illegal reference counting op inside implementation of `__traverse__` handlers.

src/impl_/pymethods.rs

@@ -2,17 +2,20 @@ use crate::callback::IntoPyCallbackOutput;
 use crate::exceptions::PyStopAsyncIteration;
 use crate::gil::LockGIL;
 use crate::impl_::panic::PanicTrap;
+use crate::impl_::pycell::{PyClassObject, PyClassObjectLayout};
+use crate::pycell::impl_::PyClassBorrowChecker as _;
 use crate::pycell::{PyBorrowError, PyBorrowMutError};
 use crate::pyclass::boolean_struct::False;
 use crate::types::any::PyAnyMethods;
 #[cfg(feature = "gil-refs")]
 use crate::types::{PyModule, PyType};
 use crate::{
-    ffi, Borrowed, Bound, DowncastError, Py, PyAny, PyClass, PyClassInitializer, PyErr, PyObject,
-    PyRef, PyRefMut, PyResult, PyTraverseError, PyTypeCheck, PyVisit, Python,
+    ffi, Bound, DowncastError, Py, PyAny, PyClass, PyClassInitializer, PyErr, PyObject, PyRef,
+    PyRefMut, PyResult, PyTraverseError, PyTypeCheck, PyVisit, Python,
 };
 use std::ffi::CStr;
 use std::fmt;
+use std::marker::PhantomData;
 use std::os::raw::{c_int, c_void};
 use std::panic::{catch_unwind, AssertUnwindSafe};
 use std::ptr::null_mut;
@@ -234,6 +237,40 @@ impl PySetterDef {
 }
 
 /// Calls an implementation of __traverse__ for tp_traverse
+///
+/// NB cannot accept `'static` visitor, this is a sanity check below:
+///
+/// ```rust,compile_fail
+/// use pyo3::prelude::*;
+/// use pyo3::pyclass::{PyTraverseError, PyVisit};
+///
+/// #[pyclass]
+/// struct Foo;
+///
+/// #[pymethods]
+/// impl Foo {
+///     fn __traverse__(&self, _visit: PyVisit<'static>) -> Result<(), PyTraverseError> {
+///         Ok(())
+///     }
+/// }
+/// ```
+///
+/// Elided lifetime should compile ok:
+///
+/// ```rust
+/// use pyo3::prelude::*;
+/// use pyo3::pyclass::{PyTraverseError, PyVisit};
+///
+/// #[pyclass]
+/// struct Foo;
+///
+/// #[pymethods]
+/// impl Foo {
+///     fn __traverse__(&self, _visit: PyVisit<'_>) -> Result<(), PyTraverseError> {
+///         Ok(())
+///     }
+/// }
+/// ```
 #[doc(hidden)]
 pub unsafe fn _call_traverse<T>(
     slf: *mut ffi::PyObject,
@@ -252,25 +289,43 @@ where
     // Since we do not create a `GILPool` at all, it is important that our usage of the GIL
     // token does not produce any owned objects thereby calling into `register_owned`.
     let trap = PanicTrap::new("uncaught panic inside __traverse__ handler");
+    let lock = LockGIL::during_traverse();
+
+    // SAFETY: `slf` is a valid Python object pointer to a class object of type T, and
+    // traversal is running so no mutations can occur.
+    let class_object: &PyClassObject<T> = &*slf.cast();
+
+    let retval =
+    // `#[pyclass(unsendable)]` types can only be deallocated by their own thread, so
+    // do not traverse them if not on their owning thread :(
+    if class_object.check_threadsafe().is_ok()
+    // ... and we cannot traverse a type which might be being mutated by a Rust thread
+    && class_object.borrow_checker().try_borrow().is_ok() {
+        struct TraverseGuard<'a, T: PyClass>(&'a PyClassObject<T>);
+        impl<'a, T: PyClass> Drop for TraverseGuard<'a,  T> {
+            fn drop(&mut self) {
+                self.0.borrow_checker().release_borrow()
+            }
+        }
 
-    let py = Python::assume_gil_acquired();
-    let slf = Borrowed::from_ptr_unchecked(py, slf).downcast_unchecked::<T>();
-    let borrow = PyRef::try_borrow_threadsafe(&slf);
-    let visit = PyVisit::from_raw(visit, arg, py);
+        // `.try_borrow()` above created a borrow, we need to release it when we're done
+        // traversing the object. This allows us to read `instance` safely.
+        let _guard = TraverseGuard(class_object);
+        let instance = &*class_object.contents.value.get();
 
-    let retval = if let Ok(borrow) = borrow {
-        let _lock = LockGIL::during_traverse();
+        let visit = PyVisit { visit, arg, _guard: PhantomData };
 
-        match catch_unwind(AssertUnwindSafe(move || impl_(&*borrow, visit))) {
-            Ok(res) => match res {
-                Ok(()) => 0,
-                Err(PyTraverseError(value)) => value,
-            },
+        match catch_unwind(AssertUnwindSafe(move || impl_(instance, visit))) {
+            Ok(Ok(())) => 0,
+            Ok(Err(traverse_error)) => traverse_error.into_inner(),
             Err(_err) => -1,
         }
     } else {
         0
     };
+
+    // Drop lock before trap just in case dropping lock panics
+    drop(lock);
     trap.disarm();
     retval
 }

src/pycell.rs

@@ -673,14 +673,6 @@ impl<'py, T: PyClass> PyRef<'py, T> {
             .try_borrow()
             .map(|_| Self { inner: obj.clone() })
     }
-
-    pub(crate) fn try_borrow_threadsafe(obj: &Bound<'py, T>) -> Result<Self, PyBorrowError> {
-        let cell = obj.get_class_object();
-        cell.check_threadsafe()?;
-        cell.borrow_checker()
-            .try_borrow()
-            .map(|_| Self { inner: obj.clone() })
-    }
 }
 
 impl<'p, T, U> PyRef<'p, T>

src/pyclass.rs

@@ -9,6 +9,7 @@ mod create_type_object;
 mod gc;
 
 pub(crate) use self::create_type_object::{create_type_object, PyClassTypeObject};
+
 pub use self::gc::{PyTraverseError, PyVisit};
 
 /// Types that can be used as Python classes.

src/pyclass/gc.rs

@@ -1,48 +1,75 @@
-use std::os::raw::{c_int, c_void};
+use std::{
+    marker::PhantomData,
+    os::raw::{c_int, c_void},
+};
 
-use crate::{ffi, AsPyPointer, Python};
+use crate::{ffi, AsPyPointer};
 
 /// Error returned by a `__traverse__` visitor implementation.
 #[repr(transparent)]
-pub struct PyTraverseError(pub(crate) c_int);
+pub struct PyTraverseError(NonZeroCInt);
+
+impl PyTraverseError {
+    /// Returns the error code.
+    pub(crate) fn into_inner(self) -> c_int {
+        self.0.into()
+    }
+}
 
 /// Object visitor for GC.
 #[derive(Clone)]
-pub struct PyVisit<'p> {
+pub struct PyVisit<'a> {
     pub(crate) visit: ffi::visitproc,
     pub(crate) arg: *mut c_void,
-    /// VisitProc contains a Python instance to ensure that
-    /// 1) it is cannot be moved out of the traverse() call
-    /// 2) it cannot be sent to other threads
-    pub(crate) _py: Python<'p>,
+    /// Prevents the `PyVisit` from outliving the `__traverse__` call.
+    pub(crate) _guard: PhantomData<&'a ()>,
 }
 
-impl<'p> PyVisit<'p> {
+impl<'a> PyVisit<'a> {
     /// Visit `obj`.
     pub fn call<T>(&self, obj: &T) -> Result<(), PyTraverseError>
     where
         T: AsPyPointer,
     {
         let ptr = obj.as_ptr();
         if !ptr.is_null() {
-            let r = unsafe { (self.visit)(ptr, self.arg) };
-            if r == 0 {
-                Ok(())
-            } else {
-                Err(PyTraverseError(r))
+            match NonZeroCInt::new(unsafe { (self.visit)(ptr, self.arg) }) {
+                None => Ok(()),
+                Some(r) => Err(PyTraverseError(r)),
             }
         } else {
             Ok(())
         }
     }
+}
 
-    /// Creates the PyVisit from the arguments to tp_traverse
-    #[doc(hidden)]
-    pub unsafe fn from_raw(visit: ffi::visitproc, arg: *mut c_void, py: Python<'p>) -> Self {
-        Self {
-            visit,
-            arg,
-            _py: py,
-        }
+/// Workaround for `NonZero<c_int>` not being available until MSRV 1.79
+mod get_nonzero_c_int {
+    pub struct GetNonZeroCInt<const WIDTH: usize>();
+
+    pub trait NonZeroCIntType {
+        type Type;
+    }
+    impl NonZeroCIntType for GetNonZeroCInt<16> {
+        type Type = std::num::NonZeroI16;
+    }
+    impl NonZeroCIntType for GetNonZeroCInt<32> {
+        type Type = std::num::NonZeroI32;
+    }
+
+    pub type Type =
+        <GetNonZeroCInt<{ std::mem::size_of::<std::os::raw::c_int>() * 8 }> as NonZeroCIntType>::Type;
+}
+
+use get_nonzero_c_int::Type as NonZeroCInt;
+
+#[cfg(test)]
+mod tests {
+    use super::PyVisit;
+    use static_assertions::assert_not_impl_any;
+
+    #[test]
+    fn py_visit_not_send_sync() {
+        assert_not_impl_any!(PyVisit<'_>: Send, Sync);
     }
 }