Post error on stale rejected promise consumption #1
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In general, when we use a promise, we want to perform different UI actions depending on whether the promise is fulfilled or rejected. For instance, if a promise is rejected with an error from the server, we may want to show an alert to the user detailing the error, so they know whatever action they took wasn't successful. In a regular scenario (already working), the UI would be notified of the error like so: 1) user triggers some action which creates a handler and MLPromise 2) user puts the app into the background 3) server responds with an error - app extension is awoken 4) handler gets run in app extension, resolving the promise with error 5) user opens the app (restoring the state from before) 6) the promise is consumed, calling the AnyPromise that notifies the UI However, before this commit, it was possible for an error from the server to be silently dropped without alerting the user, if the user closed the app after the promise was resolved. Consider the following scenario (fixed by this commit): 1) user triggers some action which creates a handler and MLPromise 2) user puts the app into the background 3) server responds with an error - app extension is awoken 4) handler gets run in app extension, resolving the promise with error 5) user closes the app 6) user opens the app (fresh start: resolvers map is gone) Now, the promise created in 1) is stale (since the AnyPromise returned to the UI in 1) is not around anymore), so no popup is shown to the user. But, the action the user took was not successful, and without an error they may not be aware that the change did not go through, causing confusion when they realise this. Therefore: if we attempt to consume a stale, rejected promise, post an error so that a banner can be shown to the user with the error details.
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