Test each change on top of the latest main before merging

How to do it

Two changes can each pass alone and break main together, such as one renaming a function the other starts to call. A merge queue builds a temporary branch of main plus the changes ahead in the queue. It runs the full check on that branch and merges only when it passes. GitHub calls it a merge queue, and GitLab calls it a merge train. On GitHub, the CI workflow must also run on the merge_group event, or the required checks never report. A small team with few merges a day can instead require branches to be up to date before merging. Keep main green as Fix a broken main branch first says.

Examples

YAML example

# .github/workflows/check.yml: run on pull requests and on merge queue groups
on:
  pull_request:
  merge_group:

Shell example

# Check: main uses a merge queue
gh api repos/example/shop/rules/branches/main --jq '.[].type' | grep -qx merge_queue || { echo "FAIL: no merge queue on main"; exit 1; }

How to check

The main branch rules include a merge queue or require branches to be up to date. On GitHub, gh api repos/example/shop/rules/branches/main lists a merge_queue rule. The CI workflow lists merge_group among its triggers. In the CI history of the last 30 days, no run on main failed right after a merge whose own checks passed.

Sources

• GitHub: Managing a merge queue

Related rules

• ENG-15 Protect the main branch in Engineering
• ENG-17 Fix a broken main branch first in Engineering