x/build: add x/tools trybot for main Go repository

[dmitshur]

As @jayconrod suggested in #34321 (comment), I think we should add an x/tools trybot builder for the main Go repository. It would check whether x/tools tests on master branch pass with the Go CL being tested. We can use linux-amd64 builder type since it's quick and least expensive.

The Go standard library ships with many packages such as go/parser, go/types, and others. The x/tools repository has many tests that provide additional test coverage.

Trybots for the main Go repository already take 5-10 minutes to run, while the x/tools tests take less than 5 minutes, so it wouldn't slow anything down, just provide more test coverage.

/cc @bradfitz @andybons @toothrot

[stamblerre]

Just wanted to chime in here to say that this would be extremely helpful for people developing in x/tools. go/packages and goimports depend on the behavior of go list, so if a breaking change is made in the go command, we may only notice the problem a few hours after the CL is submitted, and then someone gets interrupted figuring out the relevant CL and required fix. It would be really helpful to know about breaking changes upfront (even if they still get submitted). If there's anything I can do to help move this forward, I'd be happy to.

[bradfitz]

@stamblerre, look at x/build/cmd/coordinator and where the recent slowbots code hooks and modifies the try set. I suspect this would also live there. We'd just add a build that's for "tools" instead of "go". There's a func getRepoHead that'll tell you which git rev of x/tools to use.

While I agree we should include x/tools by default, I also think the slowbots should let us opt in to testing any x/ repo. Maybe TRY=x/foo, using x/foo for the opt-in TRY tokens.

[gopherbot]

Change https://golang.org/cl/203677 mentions this issue: cmd/coordinator: add TRY=x/foo syntax for testing subrepos

[stamblerre]

Thanks for the guidance, @bradfitz! Just mailed CL 203677, though I'm not sure about the correct way to test it.

[gopherbot]

Change https://golang.org/cl/203917 mentions this issue: cmd/coordinators: fix formatting for x/ repo TryBots

[stamblerre]

As of https://golang.org/cl/203917, you should now be able to comment "TRY=x/tools" on CLs that may be relevant to changes in x/tools (see https://golang.org/cl/201203 as an example).

@jayconrod and @bcmills: do you mind testing this out on your next few CLs to see if it works well for you? The next step would be to trigger it by default when certain directories or files are changed.