Cosmos-sdk: Github Bot for companion backport PRs

[amaurymartiny]

Summary

Add a Github bot that creates backport PRs for all PRs merged to master that have the backport/{branch} label.

Problem Definition

Our current backport policy (not yet documented, afaik) is to:

• create a PR on master
• label it with backport/{branch}, where {branch} is the long-living release branch
• once that PR is merged in master, create a backport PR into the release/{branch} branch by cherry-picking the PR's squashed commit.
• the {branch} release manager approves/rejects the backport PR

The 3rd step is not ideal:

• we often forget to do it
• we shouldn't require external contributors to do it

This leads to days (like today) where we backport 10+ PRs at once, which is intensive and error-prone.

Proposal

A Github bot that would create companion backport PRs.

This bot would:

• watch for PRs with the backport/{branch} label
• for each of those PRs that gets merged into master:
  
  
  
  • create a PR on the release/{branch} branch by cherry-picking the parent PR's squashed commit
  
  
  • add the correct milestone to the backport PR (optional, only do it if its easy)
  
  

Advantages:

• save developers time by avoiding creating backport PRs
• we won't forget any backport PRs (so we would avoid backport-intensive days, or mega cherry-pick PRs)

Disadvantages:

• we might need to code this bot ourselves? Would be good to have a look at what exists out there.

cc @alessio @robert-zaremba

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For Admin Use

• [ ] Not duplicate issue
• [ ] Appropriate labels applied
• [ ] Appropriate contributors tagged
• [ ] Contributor assigned/self-assigned

[alessio]

CC'ing @marbar3778 @jgimeno @helder-moreira

[marbar3778]

yea this would be awsome. Mergify can do this for us https://docs.mergify.io/actions.html#backport

[aaronc]

Awesome! Also looks like there's a command for it that we could start using now I think https://docs.mergify.io/commands.html#backport. Although I'm not sure if this opens a PR or just cherry-picks and merges... Would be good to have a PR step in between.