hamlab/internal/files/move.go

package files

import (
	"context"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
	"syscall"
)

// IsCrossDevice reports a rename refused because the two ends are on different
// filesystems — on a NAS that is every move between datasets. The Sudo backend only has
// the helper's words for it.
func IsCrossDevice(err error) bool {
	return errors.Is(err, syscall.EXDEV) || (err != nil && strings.Contains(err.Error(), "cross-device"))
}

// Inside reports whether to lies within from — a folder cannot be moved into itself.
func Inside(from, to string) bool {
	return to == from || strings.HasPrefix(to, strings.TrimSuffix(from, "/")+"/")
}

// Move is Rename, and across filesystems a copy and a removal. The copy is built under a
// hidden temporary name beside where it will land and renamed into place only once it is
// complete, and the source is removed only after that: a failure part-way, or a cancel,
// leaves the source exactly as it was. Progress goes to w, a line at a time, for the task
// output that a long move is watched through.
func (Local) Move(ctx context.Context, from, to string, w io.Writer) error {
	if protected[from] {
		return errors.New("refusing to move " + from)
	}
	if Inside(from, to) {
		return errors.New("a folder can't be moved into itself")
	}
	if _, err := os.Lstat(to); err == nil {
		return errors.New("destination exists")
	}
	err := os.Rename(from, to)
	if err == nil || !IsCrossDevice(err) {
		return err
	}
	return moveByCopy(ctx, from, to, w)
}

// moveByCopy is the slow path: copy, rename into place, remove the original.
func moveByCopy(ctx context.Context, from, to string, w io.Writer) error {
	st, err := os.Lstat(from)
	if err != nil {
		return err
	}
	tmp := filepath.Join(filepath.Dir(to), "."+filepath.Base(to)+".hamlab-moving")
	_ = os.RemoveAll(tmp) // a previous attempt that died
	fmt.Fprintf(w, "%s is on another filesystem: copying, then removing the original\n", filepath.Dir(to))
	p := &progress{w: w}
	if err := copyPath(ctx, from, tmp, st, p); err != nil {
		fmt.Fprintln(w, "copy failed; removing the partial copy, the original is untouched")
		_ = os.RemoveAll(tmp)
		return err
	}
	if err := os.Rename(tmp, to); err != nil {
		_ = os.RemoveAll(tmp)
		return err
	}
	fmt.Fprintf(w, "copied %s; removing the original\n", p.String())
	if err := os.RemoveAll(from); err != nil {
		return fmt.Errorf("copied to %s, but the original could not be removed: %w", to, err)
	}
	fmt.Fprintf(w, "moved %s to %s\n", from, to)
	return nil
}

type progress struct {
	w          io.Writer
	files      int64
	bytes      int64
	shownFiles int64
	shownBytes int64
}

func (p *progress) add(files, bytes int64) {
	p.files += files
	p.bytes += bytes
	if p.files-p.shownFiles >= 500 || p.bytes-p.shownBytes >= 512<<20 {
		fmt.Fprintf(p.w, "copying… %s\n", p.String())
		p.shownFiles, p.shownBytes = p.files, p.bytes
	}
}

func (p *progress) String() string {
	return fmt.Sprintf("%d files, %s", p.files, fmtSize(p.bytes))
}

func fmtSize(b int64) string {
	const unit = 1024
	if b < unit {
		return fmt.Sprintf("%d B", b)
	}
	div, exp := int64(unit), 0
	for n := b / unit; n >= unit && exp < 4; n /= unit {
		div *= unit
		exp++
	}
	return fmt.Sprintf("%.1f %ciB", float64(b)/float64(div), "KMGTP"[exp])
}

// copyPath copies one entry of any kind, recursively for a directory, keeping mode, times
// and — when running as root — ownership. Devices, pipes and sockets are refused: a copy
// of one is not the thing itself.
func copyPath(ctx context.Context, src, dst string, st os.FileInfo, p *progress) error {
	if err := ctx.Err(); err != nil {
		return err
	}
	mode := st.Mode()
	switch {
	case mode&os.ModeSymlink != 0:
		target, err := os.Readlink(src)
		if err != nil {
			return err
		}
		if err := os.Symlink(target, dst); err != nil {
			return err
		}
		chown(dst, st)
		p.add(1, 0)
		return nil
	case mode.IsDir():
		if err := os.Mkdir(dst, mode.Perm()|0o700); err != nil { // writable while it fills, exact mode after
			return err
		}
		ents, err := os.ReadDir(src)
		if err != nil {
			return err
		}
		for _, e := range ents {
			lst, err := e.Info()
			if err != nil {
				return err
			}
			if err := copyPath(ctx, filepath.Join(src, e.Name()), filepath.Join(dst, e.Name()), lst, p); err != nil {
				return err
			}
		}
		chown(dst, st)
		if err := os.Chmod(dst, mode.Perm()); err != nil {
			return err
		}
		return os.Chtimes(dst, st.ModTime(), st.ModTime())
	case mode.IsRegular():
		n, err := copyFile(src, dst, st)
		if err != nil {
			return err
		}
		p.add(1, n)
		return nil
	default:
		return fmt.Errorf("%s: can't copy a device, pipe or socket", src)
	}
}

func copyFile(src, dst string, st os.FileInfo) (int64, error) {
	in, err := os.Open(src)
	if err != nil {
		return 0, err
	}
	defer in.Close()
	out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_EXCL, st.Mode().Perm())
	if err != nil {
		return 0, err
	}
	n, err := io.Copy(out, in)
	if err != nil {
		out.Close()
		return n, err
	}
	if err := out.Close(); err != nil {
		return n, err
	}
	chown(dst, st)
	if err := os.Chmod(dst, st.Mode().Perm()); err != nil { // umask has no say
		return n, err
	}
	return n, os.Chtimes(dst, st.ModTime(), st.ModTime())
}

// chown keeps the owner when this process may set one; an ordinary user's copy is the
// user's, which is what a rename would have left anyway.
func chown(dst string, st os.FileInfo) {
	if os.Geteuid() != 0 {
		return
	}
	if sys, ok := st.Sys().(*syscall.Stat_t); ok {
		_ = os.Lchown(dst, int(sys.Uid), int(sys.Gid))
	}
}