x/build: limit the number of auto-assigned reviewers

[josharian]

This is a follow-up issue to the discussion at https://groups.google.com/forum/#!topic/golang-dev/CdGrNJDcqec, to make sure it doesn't get lost.

In that thread @mvdan proposed:

If a CL were to ping more than 5 people, simply fallback to pinging someone on the core team (Andy? Ian? Brad?), who can then make a better call than a machine could

I think we should do this.

@dmitshur @andybons

[dmitshur]

I'll wait to hear @andybons thoughts on this, but I'd say if we're going to treat "would ping more than 5 people" specially, it should just avoid doing anything.

I don't think it's viable to pick only a single person on the core team to always get pinged, and it may not be a trivial effort to add a rotation going just for these kinds of CLs. The safest thing is for gopherbot to say "welp, this is too hard for me to deal with, I'll tag it as no-owners and back out". Then humans can pick it up, just like all previous CLs before gopherbot did the reviewer assignments.

About 'no-owners' Tag

The no-owners is applied by gopherbot when it can't find an owner for the affected code. See CL
121018 description:

If no owners can be found, the tag 'no-owners' is added to the
change to prevent gopherbot from making unneeded API calls.

[mvdan]

Using a tag instead of pinging someone from the core team seems fine to me. The big priority here is to keep the mass-CCing to a minimum.

Another major offender, which isn't a particularly interesting CL either: https://go-review.googlesource.com/c/go/+/140299