Function compactLowerdirOption

core/mount/mount_linux.go:408–442

compactLowerdirOption updates overlay lowdir option and returns the common dir among all the lowdirs.

(opts []string)

// compactLowerdirOption updates overlay lowdir option and returns the common
// dir among all the lowdirs.
func compactLowerdirOption(opts []string) (string, []string) {
	idx, dirs := findOverlayLowerdirs(opts)
	if idx == -1 || len(dirs) == 1 {
		// no need to compact if there is only one lowerdir
		return "", opts
	}

	// find out common dir
	commondir := longestCommonPrefix(dirs)
	if commondir == "" {
		return "", opts
	}

	// NOTE: the snapshot id is based on digits.
	// in order to avoid to get snapshots/x, should be back to parent dir.
	// however, there is assumption that the common dir is ${root}/io.containerd.v1.overlayfs/snapshots.
	commondir = path.Dir(commondir)
	if commondir == "/" || commondir == "." {
		return "", opts
	}
	commondir = commondir + "/"

	newdirs := make([]string, 0, len(dirs))
	for _, dir := range dirs {
		if len(dir) <= len(commondir) {
			return "", opts
		}
		newdirs = append(newdirs, dir[len(commondir):])
	}

	newopts := copyOptions(opts)
	newopts = append(newopts[:idx], newopts[idx+1:]...)
	newopts = append(newopts, fmt.Sprintf("lowerdir=%s", strings.Join(newdirs, ":")))
	return commondir, newopts
}

// findOverlayLowerdirs returns the index of lowerdir in mount's options and
// all the lowerdir target.