Subscription callback is not nulled out when Subscription is unsubscribed

[felixfbecker]

Bug Report

Current Behavior
When instantiating a Subscription and passing a callback, that callback is saved as this._unsubscribe of the Subscription object:

rxjs/src/internal/Subscription.ts

Line 43 in d608cf3

(<any> this)._unsubscribe = unsubscribe;

When unsubscribing that Subscription, the function is called, but not nulled out afterwards:

rxjs/src/internal/Subscription.ts

Line 79 in d608cf3

_unsubscribe.call(this);

This means that if it did some manual resource cleanup like closing a MessagePort, that MessagePort still can't be garbage-collected, because the callback, and therefor the Subscription, still holds a reference to it.

This is different from Subscriptions added via add(), which are correctly de-referenced:

rxjs/src/internal/Subscription.ts

Line 66 in d608cf3

this._subscriptions = null;

Reproduction

• REPL or Repo link:
  (you can use https://stackblitz.com/ to create one to attach here)

const { port1 } = new MessageChannel()
const subscription = new Subscription(() => {
  port1.close()
})
subscription.unsubscribe()
console.log(subscription._unsubscribe)

Expected behavior
After calling the callback, this._unsubscribe should be nulled out so the callback and its references can be garbage-collected. Since a Subscription can only be unsubscribed once, there is no need to keep the reference.

Environment

• Runtime: All
• RxJS version: 6.5.5

Possible Solution
Set this._unsubscribe to null when unsubscribe() is called.