syncthing/lib/fs/basicfs_copy_range_ioctl.go

// Copyright (C) 2019 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.

// +build linux,!ppc,!ppc64,!ppc64le

package fs

import (
	"io"
	"syscall"
	"unsafe"
)

func init() {
	registerCopyRangeImplementation(CopyRangeMethodIoctl, copyRangeImplementationForBasicFile(copyRangeIoctl))
}

const FICLONE = 0x40049409
const FICLONERANGE = 0x4020940d

/*
http://man7.org/linux/man-pages/man2/ioctl_ficlonerange.2.html

struct file_clone_range {
   __s64 src_fd;
   __u64 src_offset;
   __u64 src_length;
   __u64 dest_offset;
};
*/
type fileCloneRange struct {
	srcFd     int64
	srcOffset uint64
	srcLength uint64
	dstOffset uint64
}

func copyRangeIoctl(src, dst basicFile, srcOffset, dstOffset, size int64) error {
	fi, err := src.Stat()
	if err != nil {
		return err
	}

	if srcOffset+size > fi.Size() {
		return io.ErrUnexpectedEOF
	}

	// https://www.man7.org/linux/man-pages/man2/ioctl_ficlonerange.2.html
	// If src_length is zero, the ioctl reflinks to the end of the source file.
	if srcOffset+size == fi.Size() {
		size = 0
	}

	if srcOffset == 0 && dstOffset == 0 && size == 0 {
		// Optimization for whole file copies.
		var errNo syscall.Errno
		_, err := withFileDescriptors(src, dst, func(srcFd, dstFd uintptr) (int, error) {
			_, _, errNo = syscall.Syscall(syscall.SYS_IOCTL, dstFd, FICLONE, srcFd)
			return 0, nil
		})
		// Failure in withFileDescriptors
		if err != nil {
			return err
		}
		if errNo != 0 {
			return errNo
		}
		return nil
	}

	var errNo syscall.Errno
	_, err = withFileDescriptors(src, dst, func(srcFd, dstFd uintptr) (int, error) {
		params := fileCloneRange{
			srcFd:     int64(srcFd),
			srcOffset: uint64(srcOffset),
			srcLength: uint64(size),
			dstOffset: uint64(dstOffset),
		}
		_, _, errNo = syscall.Syscall(syscall.SYS_IOCTL, dstFd, FICLONERANGE, uintptr(unsafe.Pointer(&params)))
		return 0, nil
	})
	// Failure in withFileDescriptors
	if err != nil {
		return err
	}
	if errNo != 0 {
		return errNo
	}
	return nil
}