pyo3/sync.rs
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//! Synchronization mechanisms based on the Python GIL.
//!
//! With the acceptance of [PEP 703] (aka a "freethreaded Python") for Python 3.13, these
//! are likely to undergo significant developments in the future.
//!
//! [PEP 703]: https://peps.python.org/pep-703/
use crate::{
types::{any::PyAnyMethods, PyString, PyType},
Bound, Py, PyResult, PyVisit, Python,
};
use std::cell::UnsafeCell;
/// Value with concurrent access protected by the GIL.
///
/// This is a synchronization primitive based on Python's global interpreter lock (GIL).
/// It ensures that only one thread at a time can access the inner value via shared references.
/// It can be combined with interior mutability to obtain mutable references.
///
/// # Example
///
/// Combining `GILProtected` with `RefCell` enables mutable access to static data:
///
/// ```
/// # use pyo3::prelude::*;
/// use pyo3::sync::GILProtected;
/// use std::cell::RefCell;
///
/// static NUMBERS: GILProtected<RefCell<Vec<i32>>> = GILProtected::new(RefCell::new(Vec::new()));
///
/// Python::with_gil(|py| {
/// NUMBERS.get(py).borrow_mut().push(42);
/// });
/// ```
pub struct GILProtected<T> {
value: T,
}
impl<T> GILProtected<T> {
/// Place the given value under the protection of the GIL.
pub const fn new(value: T) -> Self {
Self { value }
}
/// Gain access to the inner value by giving proof of having acquired the GIL.
pub fn get<'py>(&'py self, _py: Python<'py>) -> &'py T {
&self.value
}
/// Gain access to the inner value by giving proof that garbage collection is happening.
pub fn traverse<'py>(&'py self, _visit: PyVisit<'py>) -> &'py T {
&self.value
}
}
unsafe impl<T> Sync for GILProtected<T> where T: Send {}
/// A write-once cell similar to [`once_cell::OnceCell`](https://docs.rs/once_cell/latest/once_cell/).
///
/// Unlike `once_cell::sync` which blocks threads to achieve thread safety, this implementation
/// uses the Python GIL to mediate concurrent access. This helps in cases where `once_cell` or
/// `lazy_static`'s synchronization strategy can lead to deadlocks when interacting with the Python
/// GIL. For an example, see
#[doc = concat!("[the FAQ section](https://pyo3.rs/v", env!("CARGO_PKG_VERSION"), "/faq.html)")]
/// of the guide.
///
/// Note that:
/// 1) `get_or_init` and `get_or_try_init` do not protect against infinite recursion
/// from reentrant initialization.
/// 2) If the initialization function `f` provided to `get_or_init` (or `get_or_try_init`)
/// temporarily releases the GIL (e.g. by calling `Python::import`) then it is possible
/// for a second thread to also begin initializing the `GITOnceCell`. Even when this
/// happens `GILOnceCell` guarantees that only **one** write to the cell ever occurs -
/// this is treated as a race, other threads will discard the value they compute and
/// return the result of the first complete computation.
///
/// # Examples
///
/// The following example shows how to use `GILOnceCell` to share a reference to a Python list
/// between threads:
///
/// ```
/// use pyo3::sync::GILOnceCell;
/// use pyo3::prelude::*;
/// use pyo3::types::PyList;
///
/// static LIST_CELL: GILOnceCell<Py<PyList>> = GILOnceCell::new();
///
/// pub fn get_shared_list(py: Python<'_>) -> &Bound<'_, PyList> {
/// LIST_CELL
/// .get_or_init(py, || PyList::empty_bound(py).unbind())
/// .bind(py)
/// }
/// # Python::with_gil(|py| assert_eq!(get_shared_list(py).len(), 0));
/// ```
#[derive(Default)]
pub struct GILOnceCell<T>(UnsafeCell<Option<T>>);
// T: Send is needed for Sync because the thread which drops the GILOnceCell can be different
// to the thread which fills it.
unsafe impl<T: Send + Sync> Sync for GILOnceCell<T> {}
unsafe impl<T: Send> Send for GILOnceCell<T> {}
impl<T> GILOnceCell<T> {
/// Create a `GILOnceCell` which does not yet contain a value.
pub const fn new() -> Self {
Self(UnsafeCell::new(None))
}
/// Get a reference to the contained value, or `None` if the cell has not yet been written.
#[inline]
pub fn get(&self, _py: Python<'_>) -> Option<&T> {
// Safe because if the cell has not yet been written, None is returned.
unsafe { &*self.0.get() }.as_ref()
}
/// Get a reference to the contained value, initializing it if needed using the provided
/// closure.
///
/// See the type-level documentation for detail on re-entrancy and concurrent initialization.
#[inline]
pub fn get_or_init<F>(&self, py: Python<'_>, f: F) -> &T
where
F: FnOnce() -> T,
{
if let Some(value) = self.get(py) {
return value;
}
match self.init(py, || Ok::<T, std::convert::Infallible>(f())) {
Ok(value) => value,
Err(void) => match void {},
}
}
/// Like `get_or_init`, but accepts a fallible initialization function. If it fails, the cell
/// is left uninitialized.
///
/// See the type-level documentation for detail on re-entrancy and concurrent initialization.
#[inline]
pub fn get_or_try_init<F, E>(&self, py: Python<'_>, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<T, E>,
{
if let Some(value) = self.get(py) {
return Ok(value);
}
self.init(py, f)
}
#[cold]
fn init<F, E>(&self, py: Python<'_>, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<T, E>,
{
// Note that f() could temporarily release the GIL, so it's possible that another thread
// writes to this GILOnceCell before f() finishes. That's fine; we'll just have to discard
// the value computed here and accept a bit of wasted computation.
let value = f()?;
let _ = self.set(py, value);
Ok(self.get(py).unwrap())
}
/// Get the contents of the cell mutably. This is only possible if the reference to the cell is
/// unique.
pub fn get_mut(&mut self) -> Option<&mut T> {
self.0.get_mut().as_mut()
}
/// Set the value in the cell.
///
/// If the cell has already been written, `Err(value)` will be returned containing the new
/// value which was not written.
pub fn set(&self, _py: Python<'_>, value: T) -> Result<(), T> {
// Safe because GIL is held, so no other thread can be writing to this cell concurrently.
let inner = unsafe { &mut *self.0.get() };
if inner.is_some() {
return Err(value);
}
*inner = Some(value);
Ok(())
}
/// Takes the value out of the cell, moving it back to an uninitialized state.
///
/// Has no effect and returns None if the cell has not yet been written.
pub fn take(&mut self) -> Option<T> {
self.0.get_mut().take()
}
/// Consumes the cell, returning the wrapped value.
///
/// Returns None if the cell has not yet been written.
pub fn into_inner(self) -> Option<T> {
self.0.into_inner()
}
}
impl GILOnceCell<Py<PyType>> {
/// Get a reference to the contained Python type, initializing it if needed.
///
/// This is a shorthand method for `get_or_init` which imports the type from Python on init.
pub(crate) fn get_or_try_init_type_ref<'py>(
&self,
py: Python<'py>,
module_name: &str,
attr_name: &str,
) -> PyResult<&Bound<'py, PyType>> {
self.get_or_try_init(py, || {
let type_object = py
.import_bound(module_name)?
.getattr(attr_name)?
.downcast_into()?;
Ok(type_object.unbind())
})
.map(|ty| ty.bind(py))
}
}
/// Interns `text` as a Python string and stores a reference to it in static storage.
///
/// A reference to the same Python string is returned on each invocation.
///
/// # Example: Using `intern!` to avoid needlessly recreating the same Python string
///
/// ```
/// use pyo3::intern;
/// # use pyo3::{prelude::*, types::PyDict};
///
/// #[pyfunction]
/// fn create_dict(py: Python<'_>) -> PyResult<Bound<'_, PyDict>> {
/// let dict = PyDict::new_bound(py);
/// // 👇 A new `PyString` is created
/// // for every call of this function.
/// dict.set_item("foo", 42)?;
/// Ok(dict)
/// }
///
/// #[pyfunction]
/// fn create_dict_faster(py: Python<'_>) -> PyResult<Bound<'_, PyDict>> {
/// let dict = PyDict::new_bound(py);
/// // 👇 A `PyString` is created once and reused
/// // for the lifetime of the program.
/// dict.set_item(intern!(py, "foo"), 42)?;
/// Ok(dict)
/// }
/// #
/// # Python::with_gil(|py| {
/// # let fun_slow = wrap_pyfunction_bound!(create_dict, py).unwrap();
/// # let dict = fun_slow.call0().unwrap();
/// # assert!(dict.contains("foo").unwrap());
/// # let fun = wrap_pyfunction_bound!(create_dict_faster, py).unwrap();
/// # let dict = fun.call0().unwrap();
/// # assert!(dict.contains("foo").unwrap());
/// # });
/// ```
#[macro_export]
macro_rules! intern {
($py: expr, $text: expr) => {{
static INTERNED: $crate::sync::Interned = $crate::sync::Interned::new($text);
INTERNED.get($py)
}};
}
/// Implementation detail for `intern!` macro.
#[doc(hidden)]
pub struct Interned(&'static str, GILOnceCell<Py<PyString>>);
impl Interned {
/// Creates an empty holder for an interned `str`.
pub const fn new(value: &'static str) -> Self {
Interned(value, GILOnceCell::new())
}
/// Gets or creates the interned `str` value.
#[inline]
pub fn get<'py>(&self, py: Python<'py>) -> &Bound<'py, PyString> {
self.1
.get_or_init(py, || PyString::intern_bound(py, self.0).into())
.bind(py)
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::types::{dict::PyDictMethods, PyDict};
#[test]
fn test_intern() {
Python::with_gil(|py| {
let foo1 = "foo";
let foo2 = intern!(py, "foo");
let foo3 = intern!(py, stringify!(foo));
let dict = PyDict::new_bound(py);
dict.set_item(foo1, 42_usize).unwrap();
assert!(dict.contains(foo2).unwrap());
assert_eq!(
dict.get_item(foo3)
.unwrap()
.unwrap()
.extract::<usize>()
.unwrap(),
42
);
});
}
#[test]
fn test_once_cell() {
Python::with_gil(|py| {
let mut cell = GILOnceCell::new();
assert!(cell.get(py).is_none());
assert_eq!(cell.get_or_try_init(py, || Err(5)), Err(5));
assert!(cell.get(py).is_none());
assert_eq!(cell.get_or_try_init(py, || Ok::<_, ()>(2)), Ok(&2));
assert_eq!(cell.get(py), Some(&2));
assert_eq!(cell.get_or_try_init(py, || Err(5)), Ok(&2));
assert_eq!(cell.take(), Some(2));
assert_eq!(cell.into_inner(), None)
})
}
}