rebaseWhen=auto: disable auto rebasing when github merge queue is in use

[rarkins]

Tell us more.

GitHub's Merge queue already takes care of testing PRs against the head commit of the base branch, so Renovate's automatic rebasing when automerge=true is not necessary.

Merge queue is part of "branch protection" via the UA. e.g. for our main branch:

Yet through the branch protections REST API (e.g. https://api.github.com/repos/renovatebot/renovate/branches/main/protection) it's not shown.

I also can't find it listed in the REST API, e.g. https://docs.github.com/en/rest/branches/branch-protection?apiVersion=2022-11-28#get-branch-protection and it seems to be missing from GraphQL too: https://docs.github.com/en/graphql/reference/objects#branchprotectionrule

In GraphQL a PR does have a mergeQueue and isMergeQueueEnabled fields though, although currently we don't fetch PR lists with GraphQL (we generally prefer REST as if-modified-since caching works).

Here's a GraphQL query which seems to work:

query MyQuery {
  repository(name: "renovate", owner: "renovatebot") {
    mergeQueue(branch: "main") {
      id
    }
  }
}

Result:

{
  "data": {
    "repository": {
      "mergeQueue": {
        "id": "MQ_kwDOBJLedM0dmQ"
      }
    }
  }
}

The id returns null if I test with a dummy branch or on a repo with no queues.

It would be nice if we could get this info as part of the initRepo() graphql call but it seems like it can't be iterated on (i.e. "list all merge queues").

It seems then that this would result in one extra GraphQL API call per base branch per run, when rebaseWhen=auto. However we could potentially cache this information fairly aggressively as it's unlikely to change often and the impact of incorrectly cached info is low.

One other important point: we probably should know if a base branch has merge queue enabled (enforced) because Renovate-native automerging (including for branches) will then stop working.

[rarkins]

Related: #31525

[secustor]

Also something to consider is that there are not only BranchProtectionRules, but RepositoryRules too which can require MergeQueues