finally.hh: delete copy constructor which is a bad idea

Change-Id: I6d0b5736893c44bddc6f5789b452b434f8671b9b

src/libutil/finally.hh

@@ -9,8 +9,15 @@ class Finally
 {
 private:
     Fn fun;
+    bool movedFrom = false;
 
 public:
     Finally(Fn fun) : fun(std::move(fun)) { }
-    ~Finally() { fun(); }
+    // Copying Finallys is definitely not a good idea and will cause them to be
+    // called twice.
+    Finally(Finally &other) = delete;
+    Finally(Finally &&other) : fun(std::move(other.fun)) {
+        other.movedFrom = true;
+    }
+    ~Finally() { if (!movedFrom) fun(); }
 };