Added Rust initialisation of Python-allocated bytes

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Add `Python::with_gil` for executing a closure with the Python GIL. [#1037](https://github.com/PyO3/pyo3/pull/1037)
 - Implement `Debug` for `PyIterator`. [#1051](https://github.com/PyO3/pyo3/pull/1051)
 - Implement type information for conversion failures. [#1050](https://github.com/PyO3/pyo3/pull/1050)
+- Add `PyBytes::new_with` and `PyBytes::new_with_uninit` for initialising Python allocated bytes inside Rust. [#1074](https://github.com/PyO3/pyo3/pull/1074)
 
 ### Changed
 - Exception types have been renamed from e.g. `RuntimeError` to `PyRuntimeError`, and are now only accessible by `&T` or `Py<T>` similar to other Python-native types. The old names continue to exist but are deprecated. [#1024](https://github.com/PyO3/pyo3/pull/1024)

src/types/bytes.rs

@@ -2,6 +2,7 @@ use crate::{
     ffi, AsPyPointer, FromPy, FromPyObject, PyAny, PyObject, PyResult, PyTryFrom, Python,
     ToPyObject,
 };
+use std::mem::MaybeUninit;
 use std::ops::Index;
 use std::os::raw::c_char;
 use std::slice::SliceIndex;
@@ -26,6 +27,87 @@ impl PyBytes {
         unsafe { py.from_owned_ptr(ffi::PyBytes_FromStringAndSize(ptr, len)) }
     }
 
+    /// Creates a new Python bytestring object and a mutable slice to its underlying buffer.
+    /// The bytestring is uninitialised and must not be read.
+    ///
+    /// Panics if out of memory.
+    #[inline]
+    unsafe fn new_with_uninit_impl(
+        py: Python<'_>,
+        len: usize,
+    ) -> (&mut [MaybeUninit<u8>], &PyBytes) {
+        let length = len as ffi::Py_ssize_t;
+        let pyptr = ffi::PyBytes_FromStringAndSize(std::ptr::null(), length);
+        // Iff pyptr is null, py.from_owned_ptr(pyptr) will panic
+        let pybytes = py.from_owned_ptr(pyptr);
+        let buffer = ffi::PyBytes_AsString(pyptr) as *mut MaybeUninit<u8>;
+        debug_assert!(!buffer.is_null());
+        let slice = std::slice::from_raw_parts_mut(buffer, len);
+        (slice, pybytes)
+    }
+
+    /// Creates a new Python bytestring object.
+    /// The bytestring is zero-initialised and can be read inside `init`.
+    /// The `init` closure can further initialise the bytestring.
+    ///
+    /// Panics if out of memory.
+    ///
+    /// # Example
+    /// ```
+    /// use pyo3::{prelude::*, types::PyBytes};
+    /// Python::with_gil(|py| -> PyResult<()> {
+    ///     let py_bytes = PyBytes::new_with(py, 10, |bytes: &mut [u8]| {
+    ///         bytes.copy_from_slice(b"Hello Rust");
+    ///     });
+    ///     let bytes: &[u8] = FromPyObject::extract(py_bytes)?;
+    ///     assert_eq!(bytes, b"Hello Rust");
+    ///     Ok(())
+    /// });
+    /// ```
+    pub fn new_with<F: Fn(&mut [u8])>(py: Python<'_>, len: usize, init: F) -> &PyBytes {
+        let (slice, pybytes) = unsafe { Self::new_with_uninit_impl(py, len) };
+        slice.iter_mut().for_each(|x| *x = MaybeUninit::zeroed());
+        let slice: &mut [u8] = unsafe { &mut *(slice as *mut [MaybeUninit<u8>] as *mut [u8]) };
+        init(slice);
+        pybytes
+    }
+
+    /// Creates a new Python bytestring object.
+    ///
+    /// # Safety
+    /// * The bytestring is uninitialised and must not be read inside `init`.
+    /// * The entire bytestring must be written to inside `init` such that any code that
+    /// follows does not read uninitialised memory.
+    ///
+    /// Panics if out of memory.
+    ///
+    /// # Example
+    /// ```
+    /// use pyo3::{prelude::*, types::PyBytes};
+    /// use std::mem::MaybeUninit;
+    /// Python::with_gil(|py| -> PyResult<()> {
+    ///     let py_bytes =
+    ///         unsafe { PyBytes::new_with_uninit(py, 10, |uninit_bytes: &mut [MaybeUninit<u8>]| {
+    ///             uninit_bytes
+    ///                 .iter_mut()
+    ///                 .zip(b"Hello Rust".iter())
+    ///                 .for_each(|(ub, b)| *ub = MaybeUninit::new(*b));
+    ///         }) };
+    ///     let bytes: &[u8] = FromPyObject::extract(py_bytes)?;
+    ///     assert_eq!(bytes, b"Hello Rust");
+    ///     Ok(())
+    /// });
+    /// ```
+    pub unsafe fn new_with_uninit<F: Fn(&mut [MaybeUninit<u8>])>(
+        py: Python<'_>,
+        len: usize,
+        init: F,
+    ) -> &PyBytes {
+        let (slice, pybytes) = Self::new_with_uninit_impl(py, len);
+        init(slice);
+        pybytes
+    }
+
     /// Creates a new Python byte string object from a raw pointer and length.
     ///
     /// Panics if out of memory.
@@ -91,4 +173,42 @@ mod test {
         let bytes = PyBytes::new(py, b"Hello World");
         assert_eq!(bytes[1], b'e');
     }
+
+    #[test]
+    fn test_bytes_new_with() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let py_bytes = PyBytes::new_with(py, 10, |b: &mut [u8]| {
+            b.copy_from_slice(b"Hello Rust");
+        });
+        let bytes: &[u8] = FromPyObject::extract(py_bytes).unwrap();
+        assert_eq!(bytes, b"Hello Rust");
+    }
+
+    #[test]
+    fn test_bytes_new_with_zero_initialised() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let py_bytes = PyBytes::new_with(py, 10, |_b: &mut [u8]| ());
+        let bytes: &[u8] = FromPyObject::extract(py_bytes).unwrap();
+        assert_eq!(bytes, &[0; 10]);
+    }
+
+    #[test]
+    fn test_bytes_new_uninit() {
+        use std::mem::MaybeUninit;
+
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let py_bytes = unsafe {
+            PyBytes::new_with_uninit(py, 10, |uninit_bytes: &mut [MaybeUninit<u8>]| {
+                uninit_bytes
+                    .iter_mut()
+                    .zip(b"Hello Rust".iter())
+                    .for_each(|(ub, b)| *ub = MaybeUninit::new(*b));
+            })
+        };
+        let bytes: &[u8] = FromPyObject::extract(py_bytes).unwrap();
+        assert_eq!(bytes, b"Hello Rust");
+    }
 }