os: force consistent mtime before running fstest on directory on Windows

FindFileNext sometimes returns a different mtime than looking at the
file directly, because the MFT on NTFS is written to lazily. In order to
keep these in sync, we use GetFileInformationByHandle to get the actual
mtime, and then write it back to the file explicitly.

Fixes #42637.

Change-Id: I774016d3ac55d0dc9b0f9c1b681516c33ba0d28a
Reviewed-on: https://go-review.googlesource.com/c/go/+/285720
Run-TryBot: Jason A. Donenfeld <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Trust: Jason A. Donenfeld <[email protected]>

src/os/os_test.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"internal/testenv"
 	"io"
+	"io/fs"
 	"os"
 	. "os"
 	osexec "os/exec"
@@ -2689,6 +2690,32 @@ func TestOpenFileKeepsPermissions(t *testing.T) {
 }
 
 func TestDirFS(t *testing.T) {
+	// On Windows, we force the MFT to update by reading the actual metadata from GetFileInformationByHandle and then
+	// explicitly setting that. Otherwise it might get out of sync with FindFirstFile. See golang.org/issues/42637.
+	if runtime.GOOS == "windows" {
+		if err := filepath.WalkDir("./testdata/dirfs", func(path string, d fs.DirEntry, err error) error {
+			if err != nil {
+				t.Fatal(err)
+			}
+			info, err := d.Info()
+			if err != nil {
+				t.Fatal(err)
+			}
+			stat, err := Stat(path) // This uses GetFileInformationByHandle internally.
+			if err != nil {
+				t.Fatal(err)
+			}
+			if stat.ModTime() == info.ModTime() {
+				return nil
+			}
+			if err := Chtimes(path, stat.ModTime(), stat.ModTime()); err != nil {
+				t.Log(err) // We only log, not die, in case the test directory is not writable.
+			}
+			return nil
+		}); err != nil {
+			t.Fatal(err)
+		}
+	}
 	if err := fstest.TestFS(DirFS("./testdata/dirfs"), "a", "b", "dir/x"); err != nil {
 		t.Fatal(err)
 	}