Bors Merge Bot

Overview

To guard against merge skews we merge pull requests using a bot. Craig is that bot, an instance of bors-ng. It compiles approved PRs as if they were merged, and merges them only after the CI on the original PR passes and the merge commit compiles successfully (which is used as a rough proxy for there being no merge-skews). This is in contrast to the default GitHub workflow, which tests the PR itself, but doesn't test the merge to master until after merging.

Workflow

The normal workflow is pretty simple: once a PR has received an LGTM, add a comment saying bors r=<reviewer>[,<reviewer>]*. You can also use bors r+ which will make your name show up as the reviewer in the git log (See the Bors docs for a full list of commands). Bors will typically wait till the CI on your PR turns green before attempting to compile the merge commit. If you're amending + force pushing a commit and bors r+ immediately, you might see it not wait. What we think happens is that bors looks towards the previous commit status instead of the newest one (because on webhook race behavior - this is all speculation). If you don't see the following message, you've done it too soon, and risk bors failing after the fact.

Waiting for PR status (Github check) to be set, probably by CI. Bors will automatically try to run when all required PR statuses are set.

Craig will take the approved PR (along with any other PRs approved at around the same time), merge them to master (or whatever the PR's target branch is), and push that as the branch named staging. This will trigger the TeamCity step for the merge candidate. If everything goes right and the build compiles, Craig will fast-forward master to staging, and each of the PRs will be marked as merged. If at any time you need to cancel your in progress merge, type bors r-.

If the combined changes end up breaking the build, you've found a situation where Craig saved us from "merge skew". Craig will then proceed to bisect the PRs to identify which one(s) can merge safely and which one(s) caused the problem. If your PR merges successfully at this stage, great! Otherwise, Craig will comment with details about the failed build. At that point you'll need to investigate and resolve the issue. If the failure was due to merge skew, you'll need to resolve the merge.

If you'd like Craig to build your PR without merging to master, you can say bors try.

FAQ

  • How is Bors configured? There's not many configuration options, but what there is can be found in bors.toml in the .github directory.
  • When does Bors process the config file? At the time commands are issued, Bors looks for a bors.toml in the PR to be merged. This means if you haven't changed it, the config file from your PR's target branch is used.
  • Is there a log of what Bors does somewhere? Yep! For this repo, you can find it here.
  • How do I add Craig to other repos? Craig is already listening to all the repos under @cockroachdb. There are two steps to making Craig respond to commands somewhere else: part one is to add a bors.toml config file to the repo that tells Bors what CI status to wait on, and part two is adding everyone as reviewers on the repo's settings page on the Bors dashboard.
  • Craig told me "Permission denied"! You are not currently a reviewer, so just ask someone who is to add you to the list of reviewers on the settings page.
  • Should I use bors for backports? No, use the green merge button instead.
  • My question isn't answered here! Ask on the mailing list, or if you're an internal Cockroach dev, on Slack in the channel #bors.

Known Limitations

  • #163: currently Bors will not automatically close issues if they are listed only in commit messages, however they will be closed if they appear in the PR description.
  • #165: user management is currently relatively manual. If you need permissions to act as a reviewer, ask someone to add you.
  • See workflow race condition above. Maybe we could add an arbitrary sleep in bors code to prevent that from happening.
  • If bors fails for the above reason, it doesn't notify the PRs that it's done so, it fails silently. This is annoying.

More information