Clear WebSocket event listeners on close.

[tretne]

Resolves #581

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[apollo-cla]

@tretne: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[hwillson]

Thanks @tretne!

[taras-slapdash]

Hi,

After this change we started getting the following error in our app:

[TypeError: Cannot set property 'onopen' of null]

e.close (webpack://[name]/node_modules/subscriptions-transport-ws/dist/client.js:126:33)
close (webpack://[name]/node_modules/subscriptions-transport-ws/dist/client.js:477:22)

I believe it is due to some race conditioning where two simultaneous calls are made to close().

Unfortunately, I don't have a repro for this bug so it's tricky to tell exactly what went wrong.

The call stack starts in the this.client.onclose handler (see https://github.com/apollographql/subscriptions-transport-ws/blob/v0.9.18/src/client.ts#L580-L584).

Any ideas?

[dko-slapdash]

@tretne I think this Cannot set property 'onopen' of null error happens when close() is called synchronously from within close() in a nested way (my guess - during this.unsubscribeAll() call - the subscription handlers may have some cleanups within themselves which close the the client).

The solution would be to either:

1. have one more check before using this.client on L169 and below, or
2. assign null to this.client early (if it's applicable).

[ibash]

Think @dko-slapdash is right -- I'm able to reproduce this pretty easily by loading my app with the network down.
e.g.

1. Host backend locally
2. Point ngrok at the backend
3. Point the client at ngrok

Refresh the app.

A solution is:

const client = this.client
this.client = null
// rest of cleanup ...

[ibash]

#829