io/fs: allow backslash in ValidPath, reject in os.DirFS.Open

Rejecting backslash introduces problems with presenting
underlying OS file systems that contain names with backslash.
Rejecting backslash also does not Windows-proof the syntax,
because colon can also be a path separator. And we are not
going to reject colon from all names. So don't reject backslash
either.

There is a similar problem on Windows with names containing
slashes, but those are more difficult (though not impossible)
to create.

Also document and enforce that paths must be UTF-8.

Fixes #44166.

Change-Id: Iac7a9a268025c1fd31010dbaf3f51e1660c7ae2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/290709
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/os/os_test.go b/src/os/os_test.go
index ee54b4aba1..a32e5fc11e 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -2719,6 +2719,40 @@ func TestDirFS(t *testing.T) {
 	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
 		t.Fatal(err)
 	}
+
+	// Test that Open does not accept backslash as separator.
+	d := DirFS(".")
+	_, err := d.Open(`testdata\dirfs`)
+	if err == nil {
+		t.Fatalf(`Open testdata\dirfs succeeded`)
+	}
+}
+
+func TestDirFSPathsValid(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skipf("skipping on Windows")
+	}
+
+	d := t.TempDir()
+	if err := os.WriteFile(filepath.Join(d, "control.txt"), []byte(string("Hello, world!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(d, `e:xperi\ment.txt`), []byte(string("Hello, colon and backslash!")), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	fsys := os.DirFS(d)
+	err := fs.WalkDir(fsys, ".", func(path string, e fs.DirEntry, err error) error {
+		if fs.ValidPath(e.Name()) {
+			t.Logf("%q ok", e.Name())
+		} else {
+			t.Errorf("%q INVALID", e.Name())
+		}
+		return nil
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
 }
 
 func TestReadFileProc(t *testing.T) {