feat: Add ExtendFS to take one FS and make a new one from it (#12)

* feat: Add ExtendFS to take one FS and make a new one from it

This is particularly dandy in situations when you have something that
implements fs.FS but you can't write to it and you want a memoryfs.FS to
work with and add stuff too.

fs.go

@@ -1,6 +1,7 @@
 package memoryfs
 
 import (
+	"io"
 	"io/fs"
 	"io/ioutil"
 	"strings"
@@ -29,6 +30,27 @@ func New() *FS {
 	}
 }
 
+// CloneFS allows you to take on fs.FS and wrap it in an fs that is writable
+func CloneFS(base fs.FS) *FS {
+	newFS := New()
+	fs.WalkDir(base, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if d.IsDir() {
+			return newFS.MkdirAll(path, d.Type().Perm())
+		}
+
+		// Lazy write the files, holding onto the base FS to read the content on demand
+		return newFS.WriteLazyFile(path, func() (io.Reader, error) {
+			return base.Open(path)
+		}, d.Type().Perm())
+	})
+
+	return newFS
+}
+
 // Stat returns a FileInfo describing the file.
 func (m *FS) Stat(name string) (fs.FileInfo, error) {
 	name = cleanse(name)

fs_test.go

@@ -270,6 +270,27 @@ func Test_WriteWhileOpen(t *testing.T) {
 	assert.Equal(t, "hello world", string(data))
 }
 
+func Test_CloneFS(t *testing.T) {
+	memfs := New()
+	require.NoError(t, memfs.WriteFile("file1.txt", []byte("This is the first file"), fs.ModePerm))
+	require.NoError(t, memfs.MkdirAll("/original/fs", fs.ModeDir))
+	require.NoError(t, memfs.WriteFile("original/fs/nested.txt", []byte("This is the nested file"), fs.ModePerm))
+
+	clone := CloneFS(memfs)
+	require.NoError(t, clone.WriteFile("second.txt", []byte("This is the second file"), fs.ModePerm))
+
+	for _, filename := range []string{"file1.txt", "/original/fs/nested.txt", "second.txt"} {
+		f, err := clone.Open(filename)
+		require.NoError(t, err)
+		defer func() { _ = f.Close() }()
+
+		data, err := ioutil.ReadAll(f)
+		require.NoError(t, err)
+		assert.NotNil(t, data)
+	}
+
+}
+
 func Test_MkdirAllRoot(t *testing.T) {
 	memfs := New()
 	err := memfs.MkdirAll(".", 0o644)