fix: closure circular reference fix

[wxiaoyun]

No description provided.

[wxiaoyun]

I think Rc::Weak may not be the solution. Consider the following code:

fn fun() {
  return () => 12;
}

let foo = fun(); // line a
// line b
foo(); // line c

At line a, fun is called and the function environment extends the block environment, lets call them env_block and env_fun.
At this moment:

1. The runtime/thread struct has a strong reference to env_fun
2. The Closure object of foo has a weak reference to env_fun

As fun returns (RESET instruction is ran), the runtime/thread struct drops the reference to env_fun and points to env_block. Since env_fun strong reference count is zero, env_fun is freed.

At line c, foo is called we attempt to upgrade the weak reference to env_fun and fail. The program crashes.

Essentially, we need to deal with how to implement anonymous functions without it losing its environment. For now I think we need this, otherwise regular named function can create a circular reference. A potential workaround is that we let anonymous function be a special function that can hold a strong reference to its environment.

[leonidas1712]

Yeah there could be more bugs later when trying async too. Perhaps we can keep it as it is for now then come back to this later, and if we can't fix it we can mention it in the report as a limitation.

[wxiaoyun]

@leonidas1712 ok! Just to reiterate

• if we keep the current implementation, any named function will form a cycle with its environment and will not be freed
• if we merge this PR, we solve the cycle problem but we cannot use function as first class values