Fixed buffers to support ReadFixed and WriteFixed ops (#54)

Add infrastructure to manage pre-registered buffers, and operation
methods `File::read_fixed_at` and `File::write_fixed_at` to make use of
them. Exclusive access to buffer data between the application and
in-flight ops is controlled at runtime.

This is initial API to enable fixed buffers. Future developments may
include:

- ✅ Improved parameter polymorphism with ~#53~ #172;
- An internal linked list of free buffers, to be able to check out the
next available buffer from `FixedBufRegistry` without enumerating or
keeping track of indices;
- Support `IORING_REGISTER_BUFFERS2`/`IORING_REGISTER_BUFFERS_UPDATE`.

src/buf/fixed/buffers.rs

@@ -0,0 +1,113 @@
+use libc::{iovec, UIO_MAXIOV};
+use std::cmp;
+use std::mem;
+use std::ptr;
+use std::slice;
+
+// Internal state shared by FixedBufRegistry and FixedBuf handles.
+pub(crate) struct FixedBuffers {
+    // Pointer to an allocated array of iovec records referencing
+    // the allocated buffers. The number of initialized records is the
+    // same as the length of the states array.
+    raw_bufs: ptr::NonNull<iovec>,
+    // State information on the buffers. Indices in this array correspond to
+    // the indices in the array at raw_bufs.
+    states: Vec<BufState>,
+    // Original capacity of raw_bufs as a Vec.
+    orig_cap: usize,
+}
+
+// State information of a buffer in the registry,
+enum BufState {
+    // The buffer is not in use.
+    // The field records the length of the initialized part.
+    Free { init_len: usize },
+    // The buffer is checked out.
+    // Its data are logically owned by the FixedBuf handle,
+    // which also keeps track of the length of the initialized part.
+    CheckedOut,
+}
+
+impl FixedBuffers {
+    pub(crate) fn new(bufs: impl Iterator<Item = Vec<u8>>) -> Self {
+        let bufs = bufs.take(cmp::min(UIO_MAXIOV as usize, 65_536));
+        let (size_hint, _) = bufs.size_hint();
+        let mut iovecs = Vec::with_capacity(size_hint);
+        let mut states = Vec::with_capacity(size_hint);
+        for mut buf in bufs {
+            iovecs.push(iovec {
+                iov_base: buf.as_mut_ptr() as *mut _,
+                iov_len: buf.capacity(),
+            });
+            states.push(BufState::Free {
+                init_len: buf.len(),
+            });
+            mem::forget(buf);
+        }
+        debug_assert_eq!(iovecs.len(), states.len());
+        // Safety: Vec::as_mut_ptr never returns null
+        let raw_bufs = unsafe { ptr::NonNull::new_unchecked(iovecs.as_mut_ptr()) };
+        let orig_cap = iovecs.capacity();
+        mem::forget(iovecs);
+        FixedBuffers {
+            raw_bufs,
+            states,
+            orig_cap,
+        }
+    }
+
+    // If the indexed buffer is free, changes its state to checked out and
+    // returns its data. If the buffer is already checked out, returns None.
+    pub(crate) fn check_out(&mut self, index: usize) -> Option<(iovec, usize)> {
+        let iovecs_ptr = self.raw_bufs;
+        self.states.get_mut(index).and_then(|state| match *state {
+            BufState::Free { init_len } => {
+                *state = BufState::CheckedOut;
+                // Safety: the allocated array under the pointer is valid
+                // for the lifetime of self, the index is inside the array
+                // as checked by Vec::get_mut above, called on the array of
+                // states that has the same length.
+                let iovec = unsafe { iovecs_ptr.as_ptr().add(index).read() };
+                Some((iovec, init_len))
+            }
+            BufState::CheckedOut => None,
+        })
+    }
+
+    // Sets the indexed buffer's state to free and records the updated length
+    // of its initialized part. The buffer addressed must be in the checked out
+    // state, otherwise this function may panic.
+    pub(crate) fn check_in(&mut self, index: usize, init_len: usize) {
+        let state = self.states.get_mut(index).expect("invalid buffer index");
+        debug_assert!(
+            matches!(state, BufState::CheckedOut),
+            "the buffer must be checked out"
+        );
+        *state = BufState::Free { init_len };
+    }
+
+    pub(crate) fn iovecs(&self) -> &[iovec] {
+        // Safety: the raw_bufs pointer is valid for the lifetime of self,
+        // the slice length is valid by construction.
+        unsafe { slice::from_raw_parts(self.raw_bufs.as_ptr(), self.states.len()) }
+    }
+}
+
+impl Drop for FixedBuffers {
+    fn drop(&mut self) {
+        let iovecs = unsafe {
+            Vec::from_raw_parts(self.raw_bufs.as_ptr(), self.states.len(), self.orig_cap)
+        };
+        for (i, iovec) in iovecs.iter().enumerate() {
+            match self.states[i] {
+                BufState::Free { init_len } => {
+                    let ptr = iovec.iov_base as *mut u8;
+                    let cap = iovec.iov_len;
+                    let v = unsafe { Vec::from_raw_parts(ptr, init_len, cap) };
+                    mem::drop(v);
+                }
+                BufState::CheckedOut => unreachable!("all buffers must be checked in"),
+            }
+        }
+    }
+}

src/buf/fixed/handle.rs

@@ -0,0 +1,110 @@
+use super::FixedBuffers;
+use crate::buf::{IoBuf, IoBufMut};
+
+use libc::iovec;
+use std::cell::RefCell;
+use std::fmt::{self, Debug};
+use std::mem::ManuallyDrop;
+use std::ops::{Deref, DerefMut};
+use std::rc::Rc;
+
+/// A unique handle to a memory buffer that can be pre-registered with
+/// the kernel for `io-uring` operations.
+///
+/// `FixedBuf` handles can be obtained from a [`FixedBufRegistry`] collection.
+/// For each buffer, only a single `FixedBuf` handle can be either used by the
+/// application code or owned by an I/O operation at any given time,
+/// thus avoiding data races between `io-uring` operations in flight and
+/// the application accessing buffer data.
+///
+/// [`FixedBufRegistry`]: super::FixedBufRegistry
+///
+pub struct FixedBuf {
+    registry: Rc<RefCell<FixedBuffers>>,
+    buf: ManuallyDrop<Vec<u8>>,
+    index: u16,
+}
+
+impl Drop for FixedBuf {
+    fn drop(&mut self) {
+        let mut registry = self.registry.borrow_mut();
+        debug_assert_eq!(
+            registry.iovecs()[self.index as usize].iov_base as *const u8,
+            self.buf.as_ptr()
+        );
+        debug_assert_eq!(
+            registry.iovecs()[self.index as usize].iov_len,
+            self.buf.capacity()
+        );
+        registry.check_in(self.index as usize, self.buf.len());
+    }
+}
+
+impl FixedBuf {
+    pub(super) unsafe fn new(
+        registry: Rc<RefCell<FixedBuffers>>,
+        iovec: iovec,
+        init_len: usize,
+        index: u16,
+    ) -> Self {
+        let buf = Vec::from_raw_parts(iovec.iov_base as _, init_len, iovec.iov_len);
+        FixedBuf {
+            registry,
+            buf: ManuallyDrop::new(buf),
+            index,
+        }
+    }
+
+    pub(crate) fn buf_index(&self) -> u16 {
+        self.index
+    }
+}
+
+unsafe impl IoBuf for FixedBuf {
+    fn stable_ptr(&self) -> *const u8 {
+        self.buf.as_ptr()
+    }
+
+    fn bytes_init(&self) -> usize {
+        self.buf.len()
+    }
+
+    fn bytes_total(&self) -> usize {
+        self.buf.capacity()
+    }
+}
+
+unsafe impl IoBufMut for FixedBuf {
+    fn stable_mut_ptr(&mut self) -> *mut u8 {
+        self.buf.as_mut_ptr()
+    }
+
+    unsafe fn set_init(&mut self, pos: usize) {
+        if self.buf.len() < pos {
+            self.buf.set_len(pos)
+        }
+    }
+}
+
+impl Deref for FixedBuf {
+    type Target = [u8];
+
+    fn deref(&self) -> &[u8] {
+        &self.buf
+    }
+}
+
+impl DerefMut for FixedBuf {
+    fn deref_mut(&mut self) -> &mut [u8] {
+        &mut self.buf
+    }
+}
+
+impl Debug for FixedBuf {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("FixedBuf")
+            .field("buf", &*self.buf) // deref ManuallyDrop
+            .field("index", &self.index)
+            .finish_non_exhaustive()
+    }
+}

src/buf/fixed/mod.rs

@@ -0,0 +1,21 @@
+//! Buffers pre-registered with the kernel.
+//!
+//! This module provides facilities for registering in-memory buffers with
+//! the `tokio-uring` runtime. Operations like [`File::read_fixed_at`][rfa] and
+//! [`File::write_fixed_at`][wfa] make use of buffers pre-mapped by
+//! the kernel to reduce per-I/O overhead.
+//! The [`FixedBufRegistry::register`] method is used to register a collection of
+//! buffers with the kernel; it must be called before any of the [`FixedBuf`]
+//! handles to the collection's buffers can be used with I/O operations.
+//!
+//! [rfa]: crate::fs::File::read_fixed_at
+//! [wfa]: crate::fs::File::write_fixed_at
+
+mod buffers;
+pub(crate) use self::buffers::FixedBuffers;
+
+mod handle;
+pub use handle::FixedBuf;
+
+mod registry;
+pub use registry::FixedBufRegistry;

src/buf/fixed/registry.rs

@@ -0,0 +1,114 @@
+use super::{buffers::FixedBuffers, FixedBuf};
+
+use std::cell::RefCell;
+use std::io;
+use std::rc::Rc;
+
+/// An indexed collection of I/O buffers pre-registered with the kernel.
+///
+/// `FixedBufRegistry` allows the application to manage a collection of buffers
+/// allocated in memory, that can be registered in the current `tokio-uring`
+/// context using the [`register`] method.
+///
+/// A `FixedBufRegistry` value is a lightweight handle for a collection of
+/// allocated buffers. Cloning of a `FixedBufRegistry` creates a new reference to
+/// the same collection of buffers.
+///
+/// The buffers of the collection are not deallocated until:
+/// - all `FixedBufRegistry` references to the collection have been dropped;
+/// - all [`FixedBuf`] handles to individual buffers in the collection have
+///   been dropped, including the buffer handles owned by any I/O operations
+///   in flight;
+/// - The `tokio-uring` [`Runtime`] the buffers are registered with
+///   has been dropped.
+///
+/// [`register`]: Self::register
+/// [`Runtime`]: crate::Runtime
+#[derive(Clone)]
+pub struct FixedBufRegistry {
+    inner: Rc<RefCell<FixedBuffers>>,
+}
+
+impl FixedBufRegistry {
+    /// Creates a new collection of buffers from the provided allocated vectors.
+    ///
+    /// The buffers are assigned 0-based indices in the order of the iterable
+    /// input parameter. The returned collection takes up to [`UIO_MAXIOV`]
+    /// buffers from the input. Any items in excess of that amount are silently
+    /// dropped, unless the input iterator produces the vectors lazily.
+    ///
+    /// [`UIO_MAXIOV`]: libc::UIO_MAXIOV
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio_uring::buf::fixed::FixedBufRegistry;
+    /// use std::iter;
+    ///
+    /// let registry = FixedBufRegistry::new(iter::repeat(vec![0; 4096]).take(10));
+    /// ```
+    pub fn new(bufs: impl IntoIterator<Item = Vec<u8>>) -> Self {
+        FixedBufRegistry {
+            inner: Rc::new(RefCell::new(FixedBuffers::new(bufs.into_iter()))),
+        }
+    }
+
+    /// Registers the buffers with the kernel.
+    ///
+    /// This method must be called in the context of a `tokio-uring` runtime.
+    /// The registration persists for the lifetime of the runtime, unless
+    /// revoked by the [`unregister`] method. Dropping the
+    /// `FixedBufRegistry` instance this method has been called on does not revoke
+    /// the registration or deallocate the buffers.
+    ///
+    /// [`unregister`]: Self::unregister
+    ///
+    /// This call can be blocked in the kernel to complete any operations
+    /// in-flight on the same `io-uring` instance. The application is
+    /// recommended to register buffers before starting any I/O operations.
+    ///
+    /// # Errors
+    ///
+    /// If a collection of buffers is currently registered in the context
+    /// of the `tokio-uring` runtime this call is made in, the function returns
+    /// an error.
+    pub fn register(&self) -> io::Result<()> {
+        crate::io::register_buffers(&self.inner)
+    }
+
+    /// Unregisters this collection of buffers.
+    ///
+    /// This method must be called in the context of a `tokio-uring` runtime,
+    /// where the buffers should have been previously registered.
+    ///
+    /// This operation invalidates any `FixedBuf` handles checked out from
+    /// this registry instance. Continued use of such handles in I/O
+    /// operations may result in an error.
+    ///
+    /// # Errors
+    ///
+    /// If another collection of buffers is currently registered in the context
+    /// of the `tokio-uring` runtime this call is made in, the function returns
+    /// an error. Calling `unregister` when no `FixedBufRegistry` is currently
+    /// registered on this runtime also returns an error.
+    pub fn unregister(&self) -> io::Result<()> {
+        crate::io::unregister_buffers(&self.inner)
+    }
+
+    /// Returns a buffer identified by the specified index for use by the
+    /// application, unless the buffer is already in use.
+    ///
+    /// The buffer is released to be available again once the
+    /// returned `FixedBuf` handle has been dropped. An I/O operation
+    /// using the buffer takes ownership of it and returns it once completed,
+    /// preventing shared use of the buffer while the operation is in flight.
+    pub fn check_out(&self, index: usize) -> Option<FixedBuf> {
+        let mut inner = self.inner.borrow_mut();
+        inner.check_out(index).map(|(iovec, init_len)| {
+            debug_assert!(index <= u16::MAX as usize);
+            // Safety: the validity of iovec and init_len is ensured by
+            // FixedBuffers::check_out
+            unsafe { FixedBuf::new(Rc::clone(&self.inner), iovec, init_len, index as u16) }
+        })
+    }
+}

src/buf/mod.rs

@@ -4,6 +4,8 @@
 //! crate defines [`IoBuf`] and [`IoBufMut`] traits which are implemented by buffer
 //! types that respect the `io-uring` contract.
 
+pub mod fixed;
+
 mod io_buf;
 pub use io_buf::IoBuf;