Fix data race for subscribers property

[tahirmt]

Fixes apollographql/apollo-ios#3512

This data race can happen if is a start/stop subscription at the same time as a message received on the websocket getting processed. The subscribers property is accessed from two threads at the same time causing a crash.

I am not really sure how I can add unit tests for this change. The data race type tests are really hard to write.

[netlify]

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🔨 Latest commit	2f48cfa

[svc-apollo-docs]

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File Changes

0 new, 14 changed, 0 removed

* (developer-tools)/ios/(latest)/tutorial/tutorial-add-graphql-schema.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-add-more-info-to-list.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-authenticate-operations.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-complete-details-view.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-configure-project.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-connect-queries-to-ui.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-define-additional-mutations.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-execute-first-query.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-first-mutation.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-introduction.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-paginate-results.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-running-code-generation.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-subscriptions.mdx
* (developer-tools)/ios/(latest)/tutorial/tutorial-write-your-first-query.mdx

Build ID: e09f794785939e4e7bc17390

URL: https://www.apollographql.com/docs/deploy-preview/e09f794785939e4e7bc17390

[BobaFetters]

Thanks for putting up a PR for this fix! Is there more work you are planning for this PR since its in a draft state still?

[tahirmt]

@BobaFetters no I just left it in draft because I wasn't sure of the feedback I would get regarding unit tests.

[BobaFetters]

@tahirmt Gotcha, yea like you said this type of data race is hard to really write a test for so I don't think thats necessary here

[tahirmt]

@BobaFetters any idea when this will be released?

[BobaFetters]

@tahirmt Not sure the exact day yet but we will likely do a release later this week once some other pending work is merged in as well.

[BobaFetters]

@tahirmt We got the other pending work merged so will be going ahead and pushing a new release today