Add String functions to filesystem implementations

pkg/apk/fs/memfs.go

@@ -54,6 +54,10 @@ func NewMemFS() FullFS {
 	}
 }
 
+func (m *memFS) String() string {
+	return "memfs"
+}
+
 // getNode returns the node for the given path. If the path is not found, it
 // returns an error.
 func (m *memFS) getNode(path string) (*node, error) {

pkg/apk/fs/rwosfs.go

@@ -207,6 +207,10 @@ type dirFS struct {
 	caseMapMutex sync.Mutex
 }
 
+func (f *dirFS) String() string {
+	return fmt.Sprintf("dirfs:%s", f.base)
+}
+
 func (f *dirFS) Readlink(name string) (string, error) {
 	// The underlying filesystem might not support symlinks, and it might be case-insensitive, so just
 	// use the one in memory.

pkg/apk/fs/sub.go

@@ -2,6 +2,7 @@ package fs
 
 import (
 	"errors"
+	"fmt"
 	"io/fs"
 	"path/filepath"
 	"time"
@@ -12,6 +13,10 @@ type SubFS struct {
 	Root string
 }
 
+func (s *SubFS) String() string {
+	return fmt.Sprintf("%s:%s", s.FS, s.Root)
+}
+
 func (s *SubFS) Open(path string) (fs.File, error) {
 	if !fs.ValidPath(path) {
 		return nil, &fs.PathError{Op: "open", Path: path, Err: fs.ErrInvalid}

pkg/tarfs/fs.go

@@ -104,6 +104,10 @@ func checksumFromHeader(header *tar.Header) ([]byte, error) {
 	return checksum, nil
 }
 
+func (m *memFS) String() string {
+	return "tarfs"
+}
+
 func (m *memFS) WriteHeader(hdr tar.Header, tfs fs.FS, pkg *apk.Package) (bool, error) {
 	switch hdr.Typeflag {
 	case tar.TypeDir: