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// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package poll
import (
"internal/syscall/unix"
"sync"
"syscall"
)
var (
kernelVersion53Once sync.Once
kernelVersion53 bool
)
const maxCopyFileRangeRound = 1 << 30
// CopyFileRange copies at most remain bytes of data from src to dst, using
// the copy_file_range system call. dst and src must refer to regular files.
func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err error) {
kernelVersion53Once.Do(func() {
major, minor := unix.KernelVersion()
// copy_file_range(2) is broken in various ways on kernels older than 5.3,
// see issue #42400 and
// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#VERSIONS
if major > 5 || (major == 5 && minor >= 3) {
kernelVersion53 = true
}
})
if !kernelVersion53 {
return 0, false, nil
}
for remain > 0 {
max := remain
if max > maxCopyFileRangeRound {
max = maxCopyFileRangeRound
}
n, err := copyFileRange(dst, src, int(max))
switch err {
case syscall.EXDEV, syscall.EINVAL, syscall.EIO, syscall.EOPNOTSUPP, syscall.EPERM:
// Prior to Linux 5.3, it was not possible to
// copy_file_range across file systems. An attempt
// to do this will result in a EXDEV error.
//
// Even though we have checked the kernel version and blocked
// the attempts to copy_file_range(2) when the kernel version
// is older than 5.3, but until now the latest kernel (5.19.x)
// may still return EXDEV error in certain cases.
//
// If we see EXDEV, we have not transferred any data,
// and we can let the caller fall back to generic code.
//
// As for EINVAL, that is what we see if, for example,
// dst or src refer to a pipe rather than a regular
// file. This is another case where no data has been
// transferred, so we consider it unhandled.
//
// If src and dst are on CIFS, we can see EIO.
// See issue #42334.
//
// If the file is on NFS, we can see EOPNOTSUPP.
// See issue #40731.
//
// If the process is running inside a Docker container,
// we might see EPERM instead of ENOSYS. See issue
// #40893. Since EPERM might also be a legitimate error,
// don't mark copy_file_range(2) as unsupported.
return 0, false, nil
case nil:
if n == 0 {
// If we did not read any bytes at all,
// then this file may be in a file system
// where copy_file_range silently fails.
// https://lore.kernel.org/linux-fsdevel/20210126233840.GG4626@dread.disaster.area/T/#m05753578c7f7882f6e9ffe01f981bc223edef2b0
if written == 0 {
return 0, false, nil
}
// Otherwise src is at EOF, which means
// we are done.
return written, true, nil
}
remain -= n
written += n
default:
return written, true, err
}
}
return written, true, nil
}
// copyFileRange performs one round of copy_file_range(2).
func copyFileRange(dst, src *FD, max int) (written int64, err error) {
// The signature of copy_file_range(2) is:
//
// ssize_t copy_file_range(int fd_in, loff_t *off_in,
// int fd_out, loff_t *off_out,
// size_t len, unsigned int flags);
//
// Note that in the call to unix.CopyFileRange below, we use nil
// values for off_in and off_out. For the system call, this means
// "use and update the file offsets". That is why we must acquire
// locks for both file descriptors (and why this whole machinery is
// in the internal/poll package to begin with).
if err := dst.writeLock(); err != nil {
return 0, err
}
defer dst.writeUnlock()
if err := src.readLock(); err != nil {
return 0, err
}
defer src.readUnlock()
var n int
for {
n, err = unix.CopyFileRange(src.Sysfd, nil, dst.Sysfd, nil, max, 0)
if err != syscall.EINTR {
break
}
}
return int64(n), err
}