golang copy_file_range_linux 代码
文件路径：/src/internal/poll/copy_file_range_linux.go
/ 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/atomic"
"syscall"
)
var copyFileRangeSupported int32 = -1 / accessed atomically
const maxCopyFileRangeRound = 1 << 30
func kernelVersion() (major int, minor int) {
var uname syscall.Utsname
if err := syscall.Uname(&uname); err != nil {
return
}
rl := uname.Release
var values [2]int
vi := 0
value := 0
for _, c := range rl {
if '0' <= c && c <= '9' {
value = (value * 10) + int(c-'0')
} else {
/ Note that we're assuming N.N.N here. If we see anything else we are likely to
/ mis-parse it.
values[vi] = value
vi++
if vi >= len(values) {
break
}
value = 0
}
}
switch vi {
case 0:
return 0, 0
case 1:
return values[0], 0
case 2:
return values[0], values[1]
}
return
}
/ 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) {
if supported := atomic.LoadInt32(©FileRangeSupported); supported == 0 {
return 0, false, nil
} else if supported == -1 {
major, minor := kernelVersion()
if major > 5 || (major == 5 && minor >= 3) {
atomic.StoreInt32(©FileRangeSupported, 1)
} else {
/ 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
atomic.StoreInt32(©FileRangeSupported, 0)
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.ENOSYS:
/ copy_file_range(2) was introduced in Linux 4.5.
/ Go supports Linux >= 2.6.33, so the system call
/ may not be present.
/
/ If we see ENOSYS, we have certainly not transferred
/ any data, so we can tell the caller that we
/ couldn't handle the transfer and let them fall
/ back to more generic code.
/
/ Seeing ENOSYS also means that we will not try to
/ use copy_file_range(2) again.
atomic.StoreInt32(©FileRangeSupported, 0)
return 0, false, nil
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. Similarly to
/ the ENOSYS case above, 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
}