go/src/os/readfrom_stub.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.

// +build !linux

package os

import "io"

func (f *File) readFrom(r io.Reader) (n int64, handled bool, err error) {
	return 0, false, nil
}
os, internal/poll, internal/syscall/unix: use copy_file_range on Linux Linux 4.5 introduced (and Linux 5.3 refined) the copy_file_range system call, which allows file systems the opportunity to implement copy acceleration techniques. This commit adds support for copy_file_range(2) to the os package. Introduce a new ReadFrom method on os.File, which makes os.File implement the io.ReaderFrom interface. If dst and src are both files, this enables io.Copy(dst, src) to call dst.ReadFrom(src), which, in turn, will call copy_file_range(2) if possible. If copy_file_range(2) is not supported by the host kernel, or if either of dst or src refers to a non-regular file, ReadFrom falls back to the regular io.Copy code path. Add internal/poll.CopyFileRange, which acquires locks on the appropriate poll.FDs and performs the actual work, as well as internal/syscall/unix.CopyFileRange, which wraps the copy_file_range system call itself at the lowest level. Rework file layout in internal/syscall/unix to accomodate the additional system call numbers needed for copy_file_range. Merge these definitions with the ones used by getrandom(2) into sysnum_linux_$GOARCH.go files. A note on additional optimizations: if dst and src both refer to pipes in the invocation dst.ReadFrom(src), we could, in theory, use the existing splice(2) code in package internal/poll to splice directly from src to dst. Attempting this runs into trouble with the poller, however. If we call splice(src, dst) and see EAGAIN, we cannot know if it came from src not being ready for reading or dst not being ready for writing. The write end of src and the read end of dst are not under our control, so we cannot reliably use the poller to wait for readiness. Therefore, it seems infeasible to use the new ReadFrom method to splice between pipes directly. In conclusion, for now, the only optimization enabled by the new ReadFrom method on *os.File is the copy_file_range optimization. Fixes #36817. Change-Id: I696372639fa0cdf704e3f65414f7321fc7d30adb Reviewed-on: https://go-review.googlesource.com/c/go/+/229101 Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> 2020-04-21 17:08:33 -06:00			`// 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.`

			`// +build !linux`

			`package os`

			`import "io"`

			`func (f *File) readFrom(r io.Reader) (n int64, handled bool, err error) {`
			`return 0, false, nil`
			`}`