fuse integration: detect WinFsp mounts on Windows in mountedAt

A directory mount is a reparse point and a drive mount is a volume root; a plain directory is now refused instead of accepted.

test/fuse_integration/mounted_unsupported.go

@@ -1,4 +1,4 @@
-//go:build !unix
+//go:build !unix && !windows
 
 package fuse
 

test/fuse_integration/mounted_windows.go

@@ -0,0 +1,34 @@
+//go:build windows
+
+package fuse
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// deviceID has no portable meaning here: reporting one device keeps
+// waitForMount from mistaking the plain pre-mount directory for a live mount.
+func deviceID(path string) (uint64, error) {
+	return 0, nil
+}
+
+// mountedAt reports whether path is a WinFsp mount: a directory mount is a
+// reparse point and a drive mount is a volume root, while a plain directory
+// is neither.
+func mountedAt(path string) bool {
+	abs, err := filepath.Abs(path)
+	if err != nil {
+		return false
+	}
+	if filepath.Dir(abs) == abs {
+		return true
+	}
+	info, err := os.Lstat(abs)
+	if err != nil {
+		return false
+	}
+	data, ok := info.Sys().(*syscall.Win32FileAttributeData)
+	return ok && data.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT != 0
+}