Auto merge of #75272 - the8472:spec-copy, r=KodrAus

specialize io::copy to use copy_file_range, splice or sendfile

Fixes #74426.
Also covers #60689 but only as an optimization instead of an official API.

The specialization only covers std-owned structs so it should avoid the problems with #71091

Currently linux-only but it should be generalizable to other unix systems that have sendfile/sosplice and similar.

There is a bit of optimization potential around the syscall count. Right now it may end up doing more syscalls than the naive copy loop when doing short (<8KiB) copies between file descriptors.

The test case executes the following:

```
[pid 103776] statx(3, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0644, stx_size=17, ...}) = 0
[pid 103776] write(4, "wxyz", 4)        = 4
[pid 103776] write(4, "iklmn", 5)       = 5
[pid 103776] copy_file_range(3, NULL, 4, NULL, 5, 0) = 5

```

0-1 `stat` calls to identify the source file type. 0 if the type can be inferred from the struct from which the FD was extracted
𝖬 `write` to drain the `BufReader`/`BufWriter` wrappers. only happen when buffers are present. 𝖬 ≾ number of wrappers present. If there is a write buffer it may absorb the read buffer contents first so only result in a single write. Vectored writes would also be an option but that would require more invasive changes to `BufWriter`.
𝖭 `copy_file_range`/`splice`/`sendfile` until file size, EOF or the byte limit from `Take` is reached. This should generally be *much* more efficient than the read-write loop and also have other benefits such as DMA offload or extent sharing.

## Benchmarks

```

OLD

test io::tests::bench_file_to_file_copy         ... bench:      21,002 ns/iter (+/- 750) = 6240 MB/s    [ext4]
test io::tests::bench_file_to_file_copy         ... bench:      35,704 ns/iter (+/- 1,108) = 3671 MB/s  [btrfs]
test io::tests::bench_file_to_socket_copy       ... bench:      57,002 ns/iter (+/- 4,205) = 2299 MB/s
test io::tests::bench_socket_pipe_socket_copy   ... bench:     142,640 ns/iter (+/- 77,851) = 918 MB/s

NEW

test io::tests::bench_file_to_file_copy         ... bench:      14,745 ns/iter (+/- 519) = 8889 MB/s    [ext4]
test io::tests::bench_file_to_file_copy         ... bench:       6,128 ns/iter (+/- 227) = 21389 MB/s   [btrfs]
test io::tests::bench_file_to_socket_copy       ... bench:      13,767 ns/iter (+/- 3,767) = 9520 MB/s
test io::tests::bench_socket_pipe_socket_copy   ... bench:      26,471 ns/iter (+/- 6,412) = 4951 MB/s
```

library/std/src/fs.rs

@@ -1656,7 +1656,7 @@ pub fn rename<P: AsRef<Path>, Q: AsRef<Path>>(from: P, to: Q) -> io::Result<()>
 /// the length of the `to` file as reported by `metadata`.
 ///
 /// If you’re wanting to copy the contents of one file to another and you’re
-/// working with [`File`]s, see the [`io::copy`] function.
+/// working with [`File`]s, see the [`io::copy()`] function.
 ///
 /// # Platform-specific behavior
 ///

library/std/src/io/copy.rs

@@ -0,0 +1,88 @@
+use crate::io::{self, ErrorKind, Read, Write};
+use crate::mem::MaybeUninit;
+
+/// 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’re wanting to copy the contents of one file to another and you’re
+/// working with filesystem paths, see the [`fs::copy`] function.
+///
+/// [`fs::copy`]: crate::fs::copy
+///
+/// # 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
+///
+/// # 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(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    cfg_if::cfg_if! {
+        if #[cfg(any(target_os = "linux", target_os = "android"))] {
+            crate::sys::kernel_copy::copy_spec(reader, writer)
+        } else {
+            generic_copy(reader, writer)
+        }
+    }
+}
+
+/// The general read-write-loop implementation of
+/// `io::copy` that is used when specializations are not available or not applicable.
+pub(crate) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    let mut buf = MaybeUninit::<[u8; super::DEFAULT_BUF_SIZE]>::uninit();
+    // FIXME: #42788
+    //
+    //   - This creates a (mut) reference to a slice of
+    //     _uninitialized_ integers, which is **undefined behavior**
+    //
+    //   - Only the standard library gets to soundly "ignore" this,
+    //     based on its privileged knowledge of unstable rustc
+    //     internals;
+    unsafe {
+        reader.initializer().initialize(buf.assume_init_mut());
+    }
+
+    let mut written = 0;
+    loop {
+        let len = match reader.read(unsafe { buf.assume_init_mut() }) {
+            Ok(0) => return Ok(written),
+            Ok(len) => len,
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+            Err(e) => return Err(e),
+        };
+        writer.write_all(unsafe { &buf.assume_init_ref()[..len] })?;
+        written += len as u64;
+    }
+}

library/std/src/io/mod.rs

@@ -266,6 +266,8 @@ pub use self::buffered::IntoInnerError;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::buffered::{BufReader, BufWriter, LineWriter};
 #[stable(feature = "rust1", since = "1.0.0")]
+pub use self::copy::copy;
+#[stable(feature = "rust1", since = "1.0.0")]
 pub use self::cursor::Cursor;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::error::{Error, ErrorKind, Result};
@@ -279,11 +281,12 @@ pub use self::stdio::{_eprint, _print};
 #[doc(no_inline, hidden)]
 pub use self::stdio::{set_panic, set_print, LocalOutput};
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use self::util::{copy, empty, repeat, sink, Empty, Repeat, Sink};
+pub use self::util::{empty, repeat, sink, Empty, Repeat, Sink};
 
 pub(crate) use self::stdio::clone_io;
 
 mod buffered;
+pub(crate) mod copy;
 mod cursor;
 mod error;
 mod impls;

library/std/src/io/stdio.rs

@@ -409,6 +409,14 @@ impl Read for Stdin {
     }
 }
 
+// only used by platform-dependent io::copy specializations, i.e. unused on some platforms
+#[cfg(any(target_os = "linux", target_os = "android"))]
+impl StdinLock<'_> {
+    pub(crate) fn as_mut_buf(&mut self) -> &mut BufReader<impl Read> {
+        &mut self.inner
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl Read for StdinLock<'_> {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {

library/std/src/io/tests.rs

@@ -1,7 +1,7 @@
 use super::{repeat, Cursor, SeekFrom};
 use crate::cmp::{self, min};
-use crate::io::prelude::*;
 use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{BufRead, Read, Seek, Write};
 use crate::ops::Deref;
 
 #[test]

library/std/src/io/util.rs

@@ -4,78 +4,7 @@
 mod tests;
 
 use crate::fmt;
-use crate::io::{self, BufRead, ErrorKind, Initializer, IoSlice, IoSliceMut, Read, Write};
-use crate::mem::MaybeUninit;
-
-/// 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’re wanting to copy the contents of one file to another and you’re
-/// working with filesystem paths, see the [`fs::copy`] function.
-///
-/// [`fs::copy`]: crate::fs::copy
-///
-/// # 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
-///
-/// # 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(())
-/// }
-/// ```
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> io::Result<u64>
-where
-    R: Read,
-    W: Write,
-{
-    let mut buf = MaybeUninit::<[u8; super::DEFAULT_BUF_SIZE]>::uninit();
-    // FIXME: #42788
-    //
-    //   - This creates a (mut) reference to a slice of
-    //     _uninitialized_ integers, which is **undefined behavior**
-    //
-    //   - Only the standard library gets to soundly "ignore" this,
-    //     based on its privileged knowledge of unstable rustc
-    //     internals;
-    unsafe {
-        reader.initializer().initialize(buf.assume_init_mut());
-    }
-
-    let mut written = 0;
-    loop {
-        let len = match reader.read(unsafe { buf.assume_init_mut() }) {
-            Ok(0) => return Ok(written),
-            Ok(len) => len,
-            Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
-            Err(e) => return Err(e),
-        };
-        writer.write_all(unsafe { &buf.assume_init_ref()[..len] })?;
-        written += len as u64;
-    }
-}
+use crate::io::{self, BufRead, Initializer, IoSlice, IoSliceMut, Read, Write};
 
 /// A reader which is always at EOF.
 ///

library/std/src/lib.rs

@@ -317,6 +317,7 @@
 #![feature(toowned_clone_into)]
 #![feature(total_cmp)]
 #![feature(trace_macros)]
+#![feature(try_blocks)]
 #![feature(try_reserve)]
 #![feature(unboxed_closures)]
 #![feature(unsafe_block_in_unsafe_fn)]

library/std/src/sys/unix/fs.rs

@@ -1204,88 +1204,19 @@ pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
 
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
-    use crate::cmp;
-    use crate::sync::atomic::{AtomicBool, Ordering};
-
-    // Kernel prior to 4.5 don't have copy_file_range
-    // We store the availability in a global to avoid unnecessary syscalls
-    static HAS_COPY_FILE_RANGE: AtomicBool = AtomicBool::new(true);
-
-    unsafe fn copy_file_range(
-        fd_in: libc::c_int,
-        off_in: *mut libc::loff_t,
-        fd_out: libc::c_int,
-        off_out: *mut libc::loff_t,
-        len: libc::size_t,
-        flags: libc::c_uint,
-    ) -> libc::c_long {
-        libc::syscall(libc::SYS_copy_file_range, fd_in, off_in, fd_out, off_out, len, flags)
-    }
-
     let (mut reader, reader_metadata) = open_from(from)?;
     let max_len = u64::MAX;
     let (mut writer, _) = open_to_and_set_permissions(to, reader_metadata)?;
 
-    let has_copy_file_range = HAS_COPY_FILE_RANGE.load(Ordering::Relaxed);
-    let mut written = 0u64;
-    while written < max_len {
-        let copy_result = if has_copy_file_range {
-            let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64) as usize;
-            let copy_result = unsafe {
-                // We actually don't have to adjust the offsets,
-                // because copy_file_range adjusts the file offset automatically
-                cvt(copy_file_range(
-                    reader.as_raw_fd(),
-                    ptr::null_mut(),
-                    writer.as_raw_fd(),
-                    ptr::null_mut(),
-                    bytes_to_copy,
-                    0,
-                ))
-            };
-            if let Err(ref copy_err) = copy_result {
-                match copy_err.raw_os_error() {
-                    Some(libc::ENOSYS | libc::EPERM | libc::EOPNOTSUPP) => {
-                        HAS_COPY_FILE_RANGE.store(false, Ordering::Relaxed);
-                    }
-                    _ => {}
-                }
-            }
-            copy_result
-        } else {
-            Err(io::Error::from_raw_os_error(libc::ENOSYS))
-        };
-        match copy_result {
-            Ok(0) if written == 0 => {
-                // fallback to work around several kernel bugs where copy_file_range will fail to
-                // copy any bytes and return 0 instead of an error if
-                // - reading virtual files from the proc filesystem which appear to have 0 size
-                //   but are not empty. noted in coreutils to affect kernels at least up to 5.6.19.
-                // - copying from an overlay filesystem in docker. reported to occur on fedora 32.
-                return io::copy(&mut reader, &mut writer);
-            }
-            Ok(0) => return Ok(written), // reached EOF
-            Ok(ret) => written += ret as u64,
-            Err(err) => {
-                match err.raw_os_error() {
-                    Some(
-                        libc::ENOSYS | libc::EXDEV | libc::EINVAL | libc::EPERM | libc::EOPNOTSUPP,
-                    ) => {
-                        // Try fallback io::copy if either:
-                        // - Kernel version is < 4.5 (ENOSYS)
-                        // - Files are mounted on different fs (EXDEV)
-                        // - copy_file_range is broken in various ways on RHEL/CentOS 7 (EOPNOTSUPP)
-                        // - copy_file_range is disallowed, for example by seccomp (EPERM)
-                        // - copy_file_range cannot be used with pipes or device nodes (EINVAL)
-                        assert_eq!(written, 0);
-                        return io::copy(&mut reader, &mut writer);
-                    }
-                    _ => return Err(err),
-                }
-            }
-        }
+    use super::kernel_copy::{copy_regular_files, CopyResult};
+
+    match copy_regular_files(reader.as_raw_fd(), writer.as_raw_fd(), max_len) {
+        CopyResult::Ended(result) => result,
+        CopyResult::Fallback(written) => match io::copy::generic_copy(&mut reader, &mut writer) {
+            Ok(bytes) => Ok(bytes + written),
+            Err(e) => Err(e),
+        },
     }
-    Ok(written)
 }
 
 #[cfg(any(target_os = "macos", target_os = "ios"))]