Function MountsToLayer

internal/erofsutils/mount.go:147–189

MountsToLayer returns the snapshot layer directory in order to generate EROFS-formatted blobs; The candidate will be checked with ".erofslayer" to make sure this active snapshot is really generated by the EROFS snapshotter instead of others.

(mounts []mount.Mount)

// The candidate will be checked with ".erofslayer" to make sure this active
// snapshot is really generated by the EROFS snapshotter instead of others.
func MountsToLayer(mounts []mount.Mount) (string, error) {
	var layer string

	// If mount[0].Type is prefixed with "mkfs/", it should be always the snapshot layer
	if strings.HasPrefix(mounts[0].Type, "mkfs/") {
		layer = filepath.Dir(mounts[0].Source)
	} else {
		// Otherwise, let's check the last mount entry
		mnt := mounts[len(mounts)-1]
		mt := strings.Split(mnt.Type, "/")

		switch mt[len(mt)-1] {
		case "bind", "erofs":
			layer = filepath.Dir(mnt.Source)
		case "overlay":
			var topLower string
			for _, o := range mnt.Options {
				if k, v, ok := strings.Cut(o, "="); ok {
					switch k {
					case "upperdir":
						layer = filepath.Dir(v)
					case "lowerdir":
						// Use the first mount source for the top lower layer
						topLower = filepath.Dir(mounts[0].Source)
					}
				}
			}
			if layer == "" {
				if topLower == "" {
					return "", fmt.Errorf("unsupported overlay layer for erofs differ: %w", errdefs.ErrNotImplemented)
				}
				layer = topLower
			}
		default:
			return "", fmt.Errorf("invalid filesystem type %q for erofs differ: %w", mnt.Type, errdefs.ErrNotImplemented)
		}
	}
	// If the layer is not prepared by the EROFS snapshotter, fall back to the next differ
	if _, err := os.Stat(filepath.Join(layer, ".erofslayer")); err != nil {
		return "", fmt.Errorf("mount layer type must be erofs-layer: %w", errdefs.ErrNotImplemented)
	}
	return layer, nil
}

// SupportGenerateFromTar checks if the installed version of mkfs.erofs supports
// the tar mode (--tar option).