Move std::io::copy to alloc::io

library/alloc/src/io/copy.rs

@@ -0,0 +1,90 @@
+mod generic;
+mod specialization;
+
+use self::generic::generic_copy;
+#[doc(hidden)]
+#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
+pub use self::specialization::SpecCopy;
+use self::specialization::specialized_copy;
+use crate::io::{Read, Result, Write};
+
+/// Used as a part of [copy specialization](SpecCopy) to communicate how many bytes
+/// were copied, and whether copying is done.
+///
+/// * [`Ended(n)`](CopyState::Ended) indicates copying completed, moving a total
+///   of `n` bytes.
+/// * [`Fallback(n)`](CopyState::Fallback) indicates copying is _might_ not be
+///   complete, and so far `n` bytes have been copied using specialization.
+///   The remaining must be copied using a fallback implementation.
+///
+/// If a particular `Read` and `Write` combination do not implement a specialized
+/// copy routine, the specialized function will return `Fallback(0)`.
+#[derive(Debug)]
+#[doc(hidden)]
+#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
+pub enum CopyState {
+    Ended(u64),
+    Fallback(u64),
+}
+
+/// Copies the entire contents of a reader into a writer.
+///
+/// This function will continuously read data from `reader` and then
+/// write it into `writer` in a streaming fashion until `reader`
+/// returns EOF.
+///
+/// On success, the total number of bytes that were copied from
+/// `reader` to `writer` is returned.
+///
+/// If you want to copy the contents of one file to another and you’re
+/// working with filesystem paths, see the [`fs::copy`] function.
+///
+// FIXME(#74481): Hard-links required to link from `alloc` to `std`
+/// [`fs::copy`]: ../../std/fs/fn.copy.html
+///
+/// # Errors
+///
+/// This function will return an error immediately if any call to [`read`] or
+/// [`write`] returns an error. All instances of [`ErrorKind::Interrupted`] are
+/// handled by this function and the underlying operation is retried.
+///
+/// [`read`]: Read::read
+/// [`write`]: Write::write
+/// [`ErrorKind::Interrupted`]: crate::io::ErrorKind::Interrupted
+///
+/// # Examples
+///
+/// ```
+/// use std::io;
+///
+/// fn main() -> io::Result<()> {
+///     let mut reader: &[u8] = b"hello";
+///     let mut writer: Vec<u8> = vec![];
+///
+///     io::copy(&mut reader, &mut writer)?;
+///
+///     assert_eq!(&b"hello"[..], &writer[..]);
+///     Ok(())
+/// }
+/// ```
+///
+/// # Platform-specific behavior
+///
+/// On Linux (including Android), this function uses `copy_file_range(2)`,
+/// `sendfile(2)` or `splice(2)` syscalls to move data directly between file
+/// descriptors if possible.
+///
+/// Note that platform-specific behavior may change in the future.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    match specialized_copy(reader, writer)? {
+        CopyState::Ended(copied) => Ok(copied),
+        CopyState::Fallback(copied) => {
+            generic_copy(reader, writer).map(|additional| copied + additional)
+        }
+    }
+}

library/alloc/src/io/copy/generic.rs

@@ -0,0 +1,256 @@
+use core::mem::MaybeUninit;
+
+#[cfg(not(no_global_oom_handling))]
+use crate::collections::VecDeque;
+use crate::io::{BorrowedBuf, BufReader, BufWriter, DEFAULT_BUF_SIZE, Read, Result, Write};
+use crate::vec::Vec;
+#[cfg_attr(
+    no_global_oom_handling,
+    expect(unused_imports, reason = "only required for VecDeque specialization")
+)]
+use crate::{alloc::Allocator, io::IoSlice};
+
+/// The userspace read-write-loop implementation of `io::copy` that is used when
+/// OS-specific specializations for copy offloading are not available or not applicable.
+///
+/// This function is able to perform a mild amount of specialization based on
+/// the size of the `reader` and `writer` buffers, if they have any.
+///
+/// * If `reader`'s buffer is large enough ([`>=DEFAULT_BUF_SIZE`](DEFAULT_BUF_SIZE)),
+///   _and_ it is larger than `writer`'s buffer, copying will be controlled by `R`.
+/// * Otherwise, copying will be controlled by `writer`.
+///
+/// Currently, `[u8]`, `Vec<u8>`, `VecDeque<u8>`, `BufReader<T>` and `BufWriter<T>`
+/// are specialized with.
+pub(super) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    let read_priority = BufferedReaderSpec::buffer_priority(reader);
+    let write_priority = BufferedWriterSpec::buffer_priority(writer);
+
+    if read_priority >= DEFAULT_BUF_SIZE && read_priority >= write_priority {
+        return BufferedReaderSpec::copy_to(reader, writer);
+    }
+
+    BufferedWriterSpec::copy_from(writer, reader)
+}
+
+/// This is used by [`generic_copy`] to decide whether to use [`BufferedReaderSpec::copy_to`]
+/// or [`BufferedWriterSpec::copy_from`].
+type BufferPriority = usize;
+
+/// Unbuffered readers and writers have the lowest priority.
+const UNBUFFERED: BufferPriority = BufferPriority::MIN;
+
+/// Readers and writers with their entire contents have the highest priority.
+const IN_MEMORY: BufferPriority = BufferPriority::MAX;
+
+/// Specialization of the read-write loop in [`generic_copy`] that reuses the
+/// internal buffer of a [`BufReader`]. If there's no buffer then the writer side
+/// should be used instead.
+trait BufferedReaderSpec {
+    fn buffer_priority(&self) -> BufferPriority;
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64>;
+}
+
+impl<T> BufferedReaderSpec for T
+where
+    Self: Read,
+    T: ?Sized,
+{
+    #[inline]
+    default fn buffer_priority(&self) -> BufferPriority {
+        UNBUFFERED
+    }
+
+    default fn copy_to(&mut self, _to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        unreachable!("only called from specializations")
+    }
+}
+
+impl BufferedReaderSpec for &[u8] {
+    fn buffer_priority(&self) -> BufferPriority {
+        IN_MEMORY
+    }
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        let len = self.len();
+        to.write_all(self)?;
+        *self = &self[len..];
+        Ok(len as u64)
+    }
+}
+
+#[cfg(not(no_global_oom_handling))]
+impl<A: Allocator> BufferedReaderSpec for VecDeque<u8, A> {
+    fn buffer_priority(&self) -> BufferPriority {
+        IN_MEMORY
+    }
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        let len = self.len();
+        let (front, back) = self.as_slices();
+        let bufs = &mut [IoSlice::new(front), IoSlice::new(back)];
+        to.write_all_vectored(bufs)?;
+        self.clear();
+        Ok(len as u64)
+    }
+}
+
+impl<I> BufferedReaderSpec for BufReader<I>
+where
+    Self: Read,
+    I: ?Sized,
+{
+    fn buffer_priority(&self) -> BufferPriority {
+        self.capacity()
+    }
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        let mut len = 0;
+
+        loop {
+            // Hack: this relies on `impl Read for BufReader` always calling fill_buf
+            // if the buffer is empty, even for empty slices.
+            // It can't be called directly here since specialization prevents us
+            // from adding I: Read
+            match self.read(&mut []) {
+                Ok(_) => {}
+                Err(e) if e.is_interrupted() => continue,
+                Err(e) => return Err(e),
+            }
+            let buf = self.buffer();
+            if self.buffer().len() == 0 {
+                return Ok(len);
+            }
+
+            // In case the writer side is a BufWriter then its write_all
+            // implements an optimization that passes through large
+            // buffers to the underlying writer. That code path is #[cold]
+            // but we're still avoiding redundant memcopies when doing
+            // a copy between buffered inputs and outputs.
+            to.write_all(buf)?;
+            len += buf.len() as u64;
+            self.discard_buffer();
+        }
+    }
+}
+
+/// Specialization of the read-write loop in `generic_copy` that either uses a
+/// stack buffer or reuses the internal buffer of a [`BufWriter`].
+trait BufferedWriterSpec: Write {
+    fn buffer_priority(&self) -> BufferPriority;
+
+    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64>;
+}
+
+impl<W: Write + ?Sized> BufferedWriterSpec for W {
+    #[inline]
+    default fn buffer_priority(&self) -> BufferPriority {
+        UNBUFFERED
+    }
+
+    default fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
+        // Unlike `BufferedReaderSpec::copy_to`, this _will_ be called as the fallback
+        // when both the reader and writer provide no specialization.
+        stack_buffer_copy(reader, self)
+    }
+}
+
+impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
+    fn buffer_priority(&self) -> BufferPriority {
+        self.capacity()
+    }
+
+    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
+        if self.capacity() < DEFAULT_BUF_SIZE {
+            // Since neither this buffer nor the reader's buffer are large enough,
+            // fall back to the unspecialized implementation.
+            return stack_buffer_copy(reader, self);
+        }
+
+        let mut len = 0;
+        let mut init = false;
+
+        loop {
+            let buf = self.buffer_mut();
+            let mut read_buf: BorrowedBuf<'_, u8> = buf.spare_capacity_mut().into();
+
+            if init {
+                // SAFETY: `init` is only true after `reader` initializes
+                // `read_buf`. See the comment about `flush_buf` below.
+                unsafe { read_buf.set_init() };
+            }
+
+            if read_buf.capacity() >= DEFAULT_BUF_SIZE {
+                let mut cursor = read_buf.unfilled();
+                match reader.read_buf(cursor.reborrow()) {
+                    Ok(()) => {
+                        let bytes_read = cursor.written();
+
+                        if bytes_read == 0 {
+                            return Ok(len);
+                        }
+
+                        init = read_buf.is_init();
+                        len += bytes_read as u64;
+
+                        // SAFETY: BorrowedBuf guarantees all of its filled bytes are init
+                        unsafe { buf.set_len(buf.len() + bytes_read) };
+
+                        // Read again if the buffer still has enough capacity, as BufWriter itself would do
+                        // This will occur if the reader returns short reads
+                    }
+                    Err(ref e) if e.is_interrupted() => {}
+                    Err(e) => return Err(e),
+                }
+            } else {
+                // SAFETY: `flush_buf` will not de-initialize any elements of
+                // the spare capacity so we can remember `init` across this.
+                self.flush_buf()?;
+            }
+        }
+    }
+}
+
+impl BufferedWriterSpec for Vec<u8> {
+    fn buffer_priority(&self) -> BufferPriority {
+        self.capacity() - self.len()
+    }
+
+    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
+        reader.read_to_end(self).map(|bytes| u64::try_from(bytes).expect("usize overflowed u64"))
+    }
+}
+
+/// Copies from `reader` to `writer` using a stack-allocated buffer (a fixed sized array).
+fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
+    reader: &mut R,
+    writer: &mut W,
+) -> Result<u64> {
+    let buf: &mut [_] = &mut [MaybeUninit::uninit(); DEFAULT_BUF_SIZE];
+    let mut buf: BorrowedBuf<'_, u8> = buf.into();
+
+    let mut len = 0;
+
+    loop {
+        match reader.read_buf(buf.unfilled()) {
+            Ok(()) => {}
+            Err(e) if e.is_interrupted() => continue,
+            Err(e) => return Err(e),
+        };
+
+        if buf.filled().is_empty() {
+            break;
+        }
+
+        len += buf.filled().len() as u64;
+        writer.write_all(buf.filled())?;
+        buf.clear();
+    }
+
+    Ok(len)
+}