Automate the r=me idiom

[Centril]

Oftentimes reviewers will write something like r=me with green travis.
It would be good to automate this sort of thing such that a reviewer can write:

@bors r=me

and then bors will r+ once travis is actually green.
That way, you don't need any other reviewer to intervene or to delegate bors privileges.

[kennytm]

r=me is explicitly blacklisted because it could also mean r=me after you have fixed this and that. I do not want to change that.

We could add a different opt-in command for like @bors r+ when-green (which could also allow @bors r=someone-else when-green).

We could add a forced pre-check after r+ before testing the PR:

• if the status+check is all green, homu will test it
• if the status+check has at least one red, homu will r- the PR (like a merge conflict)
• if the status+check has at least one yellow, homu will skip for the next PR; if the queue is exhausted homu will check again in 5 minutes (or after a GitHub event is received or other retry mechanism; whichever easier).

The reviewer can opt-out the Travis PR check by @bors trust-me-it-really-works.

One difficulty with automatic r+ is that a CI vendor may spuriously unable to update the GitHub status, keeping the PR forever in yellow.

Also, it's hard to verify the correctness without a test framework 🤷‍♀️

[Centril]

We could add a different opt-in command for like @bors r+ when-green (which could also allow @bors r=someone-else when-green).

Sure, that sounds good. The actual command isn't that important to me.

We could add a forced pre-check after r+ before testing the PR:

Maybe... that seems like a more invasive change that might be annoying when doing rollup ops or when accepting obviously passing PRs. I think when-green is good enough for now. :)

[camelid]

We talked about this again recently. @tmandry suggested @bors r+ await, which I think would be a good name for this.

[camsteffen]

There's a possibility that fixing CI leads to "oh this requires a lot more changes". So a final explicit approval should always be required IMO. r=me works well as "I trust you to approve given that additional changes are minor".

[camelid]

My idea of how this feature (@bors r+ await) would work is it would only approve the PR if CI passed. If CI failed, it would cancel the approval and the PR would have to be approved manually.

[camsteffen]

That makes more sense. Or it could be default behavior and opt-out with @bors r+ now.