Problem

A program that fetches a maintner corpus and tries to use its data to make decisions may make a mistake, because the world view is inconsistent during short windows of time. Even though the windows are short, it's guaranteed to happen for any daemon that loops over doing corpus updates and making decisions immediately after.

The most visible high-level example of this is #21312.

Cause

This happens because there are effectively two GitHub data sources that are not synchronized:

1. changes to GitHub state (e.g., issue N now has labels X, Y, Z)
2. GitHub-generated events (e.g., issue N has had an "unlabeled" event)

To give a concrete example of an inconsistent state that maintner can report, consider when an issue has just been unlabeled. The first mutation received and processed by a corpus.Update call will be that the issue no longer has that label.

The mutation reporting that there has been an unlabeled event on the same issue may come in a few seconds later. Until it does, it will appear that the issue does not have said label and it has never been unlabeled (e.g., !gi.HasLabel("Documentation") && !gi.HasEvent("unlabeled") will be true). Which is not the reality (if one considers the reality to be one where the unlabeled event and its effect to happen simultaneously).

Details

These are two distinct mutations received and processed by corpus.Update method:

received mutation at time t0:
github_issue: <
  owner: "golang"
  repo: "go"
  number: 28103
  updated: <
    seconds: 1539629204
  >
  remove_label: 223401461
>

... (short window during which the issue doesn't have a label,
     but the accompanying "unlabeled" event hasn't been received yet;
     aka an inconsistent world state)

received mutation at time t1:
github_issue: <
  owner: "golang"
  repo: "go"
  number: 28103
  event: <
    id: 1904921842
    event_type: "unlabeled"
    actor_id: 1924134
    created: <
      seconds: 1539629204
    >
    label: <
      name: "Builders"
    >
  >
  event: <
    id: 1904921913
    event_type: "labeled"
    actor_id: 8566911
    created: <
      seconds: 1539629206
    >
    label: <
      name: "Builders"
    >
  >
  event_status: <
    server_date: <
      seconds: 1539629209
    >
  >
>

There is more relevant information in #21312 (comment).

/cc @bradfitz