Warn PR author when eng/common is modified in consumer repo

[am11]

In arcade-powered repos, eng/common is logically a (readonly) mirror directory, which dotnet-maestro bot dutifully updates.

If a PR is submitted by a human being that updates a file under this directory, it is easy to misremember during the PR review. That causes a merge conflict in the next arcade update and mostly results in reverting the manual changes made earlier.

It would be nice if a bot post a comment to such PRs to help reminding reviewers about readonly-ness of eng/common, something to the effect:

Reminder: eng/common is a mirror directory. Please submit changes in dotnet/arcade repo to avoid conflicts during the subsequent arcade update.

ps: such reverts took place in dotnet/runtime repo few times, and currently eng/common in aspnetcore repo is diverged.

[missymessa]

There is currently a readme.md in eng/common that warns against making changes in that folder:

https://github.com/dotnet/arcade/blob/master/eng/common/README.md

[am11]

Great. I guess a common bot could point to this document in next line of that comment (See https://github.com/dotnet/arcade/blob/master/eng/common/README.md). If reviewers can forgot that eng/common is mirrored, it is likely that they will also forget to read the docs. :)

[garath]

We don't really have that kind of infrastructure to proactively detect and correctly act on changes to eng/common. Usually, (intentional or accidental) changes caught and managed in the PR.

Certainly, if we see more issues in this area changes here we'll look at investing more.

[danmoseley]

@akoeplinger I think you made the backport Github Action. do you know of any existing action that would do something like the above?

[am11]

In practice, only few people are aware of https://github.com/dotnet/arcade/blob/master/eng/common/README.md, so we end up sending three PRs:

• send PR to dotnet/{downstream repo} modifying something in eng/common. since 99% people don't remember this arbitrary detail that eng/common is a mirror directory, it gets merged.
  • i personally don't think it's anyone's fault, just the case of incomplete automation or opportunity to improve the PR experience.
• the next dotnet/arcade update PR overwrites the changes
• author sends PR to dotnet/arcade
• update downstream repo again (or wait for next infra update)

the fact is, this is not happening infrequently.

[akoeplinger]

@danmoseley This should be really easy to do with a GitHub Action that gets triggered by changes to eng/common, we'd just need to no-op for PRs from dotnet-bot i.e. legitimate updates. I'll take a stab at it.