lib: Introduce fs.IsParent (fixes #5324) (#5326)

lib/fs/util.go

@@ -77,3 +77,15 @@ func WindowsInvalidFilename(name string) bool {
 	// The path must not contain any disallowed characters
 	return strings.ContainsAny(name, windowsDisallowedCharacters)
 }
+
+func IsParent(path, parent string) bool {
+	if len(parent) == 0 {
+		// The empty string is the parent of everything except the empty
+		// string. (Avoids panic in the next step.)
+		return len(path) > 0
+	}
+	if parent[len(parent)-1] != PathSeparator {
+		parent += string(PathSeparator)
+	}
+	return strings.HasPrefix(path, parent)
+}