Automatic mirroring from upstream (untouched upstream branches)

[dagood]

We plan to mirror branches from https://go.googlesource.com/go to this repo with their original names. Forked branches will begin with microsoft/ to avoid collisions.

This issue tracks creating automation to push branch updates to this repo from upstream.

[dagood]

I've been considering why we want to do this. The reasons I can think of:

• Convenience when making a fresh clone: you get some info about the upstream state (as of the last mirror).
  • Since we plan to have an infra branch that doesn't modify any of the Go source code, this doesn't seem like a huge value.
  • If you need a completely blank slate for upstreaming some changes, why not just
    git remote add golang https://go.googlesource.com/go?
• Detecting non-FF pushes that happen upstream.
  • Will upstream ever do non-FF pushes? (It's widely known that you should never do this on an active branch.)
  • Our auto-merge (Automatic branch merging from upstream #7) process would catch this anyway.

Setting it up securely could be an issue. I'm not sure if Fabric Bot is able to do mirror pushes without PRs based on https://docs.opensource.microsoft.com/tools/fabricbot/index.html (https://github.com/microsoftfabricbot). Making a new bot account could do it, but IIRC giving more bots write permission is discouraged. (Maybe more/less in microsoft than dotnet?)

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@microsoft/golang-compiler Does this reasoning seem good? Are there benefits to maintaining mirrored branches that I'm missing (or undervalued)?

I think we can deprioritize mirroring from upstream and consider/decide later. The auto-merges are the critical part. We could depend on auto-mirroring as the source of our merges, but I don't see a reason to.

[dagood]

With the switch to submodules, we now have a mirror for internal use (https://dev.azure.com/dnceng/internal/_git/microsoft-go-mirror), but no public mirroring is needed.