Support os.Chdir() like function for http.FileSystem

fs/chdir.go

@@ -0,0 +1,62 @@
+package fs
+
+import (
+	"net/http"
+	"os"
+	"path/filepath"
+)
+
+type chdirFileSystem struct {
+	prefix string
+	system http.FileSystem
+}
+
+func (c chdirFileSystem) Open(name string) (http.File, error) {
+	name = filepath.Clean(name)
+	return c.system.Open(filepath.Join(c.prefix, name))
+}
+
+func Chdir(httpFS http.FileSystem, dir string) (http.FileSystem, error) {
+	switch fs := httpFS.(type) {
+	case *chdirFileSystem:
+		return chdirOnChdirFS(fs, dir)
+	default:
+		return chdirOnOtherFS(fs, dir)
+	}
+}
+
+func chdirOnChdirFS(fs *chdirFileSystem, dir string) (*chdirFileSystem, error) {
+	if !isExistingDir(fs, dir) {
+		return nil, os.ErrNotExist
+	}
+	prefix := filepath.Join(fs.prefix, dir)
+	newFS := &chdirFileSystem{
+		prefix: filepath.Clean(prefix),
+		system: fs.system,
+	}
+	return newFS, nil
+}
+
+func chdirOnOtherFS(fs http.FileSystem, dir string) (http.FileSystem, error) {
+	if !isExistingDir(fs, dir) {
+		return nil, os.ErrNotExist
+	}
+	return &chdirFileSystem{
+		prefix: dir,
+		system: fs,
+	}, nil
+}
+
+func isExistingDir(fs http.FileSystem, dir string) bool {
+	f, err := fs.Open(dir)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+
+	info, err := f.Stat()
+	if err != nil {
+		return false
+	}
+	return info.IsDir()
+}

fs/fs_test.go

@@ -240,6 +240,24 @@ func TestWalk(t *testing.T) {
 	}
 }
 
+func TestChdir(t *testing.T) {
+	Register(mustZipTree("../testdata"))
+	fs, err := New()
+	if err != nil {
+		t.Errorf("New() = %v", err)
+		return
+	}
+	fs, err = Chdir(fs, "/file")
+	if err != nil {
+		t.Errorf("Chdir(fs, /file) = %v", err)
+		return
+	}
+	if _, err := fs.Open("/file.txt"); err != nil {
+		t.Errorf("fs.Open(/file.txt) = %v", err)
+		return
+	}
+}
+
 func TestHTTPFile_Readdir(t *testing.T) {
 	Register(mustZipTree("../testdata/readdir"))
 	fs, err := New()