Return Error Status when attempting Datastore mutation, CognitoUserPools logged out state

[wooj2]

Is your feature request related to a problem? Please describe.
Originally reported here #395
There is a case if you configure an iOS app with the following:

1. Authentication: cognito user pools
2. Datastore w/ syncing capability via API
3. A basic schema without @auth, for example, the one we have in our documentation:

type Post @model {
  id: ID!
  title: String!
  status: PostStatus!
  rating: Int
  content: String
}
enum PostStatus {
  DRAFT
  PUBLISHED
}

Given that when calling Amplify.Auth.fetchAuthSession returns false, attempt to make a mutation via Amplify.DataStore.save(), data is saved locally but fails to be synchronized to the backend.

Describe the solution you'd like
At a high level, the customer would like to know if something failed in the sync engine process, and perhaps if the mutation can never make it to the backend, then the mutation should not be stored locally, and an error should be presented to the user. This can be implemented in a variety of ways, but we have not done the work to come up with a design. Potential approaches to explore:

• callback
• hub notification
• event on the publisher

Additional context

• We will need to make sure that the behavior on iOS is also consistent on Android and JS.

[drochetti]

At a high level, the customer would like to know if something failed in the sync engine process, and perhaps if the mutation can never make it to the backend

That expectation is correct and the DataStore configuration has an errorHandler that should be triggered in that scenario.

[wooj2]

@drochetti Great call out.

I tested a custom errorHandler and the errorHandler does get called, but the locally stored item doesn't seem to get sync'ed to the backend (even when you go back into an authenticated state and restart the sync engine). Looks like we'll need to dig into this.

[drochetti]

@wooj2 @diegocstn is this one possibly related to the sync engine start/stop/restart logic being currently reviewed?

[diegocstn]

I believe with the current state of DataStore, per #815 and #1299, the sync engine should not halt in case of authentication errors and retry the mutation.