testing/fstest: allow specifying file for "." in MapFS

Prior to this commit, specifying a file for "." in MapFS
created an invalid fs.FS and caused infinite recursion in fs.WalkDir.

Fixes #46776

Change-Id: Ia9e4ae1125355a74dba9ee6b36451b7fda75a862
Reviewed-on: https://go-review.googlesource.com/c/go/+/328409
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/fstest/mapfs.go

@@ -66,7 +66,9 @@ func (fsys MapFS) Open(name string) (fs.File, error) {
 		for fname, f := range fsys {
 			i := strings.Index(fname, "/")
 			if i < 0 {
+				if fname != "." {
 					list = append(list, mapFileInfo{fname, f})
+				}
 			} else {
 				need[fname[:i]] = true
 			}

src/testing/fstest/mapfs_test.go

@@ -5,6 +5,9 @@
 package fstest
 
 import (
+	"fmt"
+	"io/fs"
+	"strings"
 	"testing"
 )
 
@@ -17,3 +20,28 @@ func TestMapFS(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestMapFSChmodDot(t *testing.T) {
+	m := MapFS{
+		"a/b.txt": &MapFile{Mode: 0666},
+		".":       &MapFile{Mode: 0777 | fs.ModeDir},
+	}
+	buf := new(strings.Builder)
+	fs.WalkDir(m, ".", func(path string, d fs.DirEntry, err error) error {
+		fi, err := d.Info()
+		if err != nil {
+			return err
+		}
+		fmt.Fprintf(buf, "%s: %v\n", path, fi.Mode())
+		return nil
+	})
+	want := `
+.: drwxrwxrwx
+a: d---------
+a/b.txt: -rw-rw-rw-
+`[1:]
+	got := buf.String()
+	if want != got {
+		t.Errorf("MapFS modes want:\n%s\ngot:\n%s\n", want, got)
+	}
+}