Working with Merge Trains

What Is a Merge Train?

A merge train is an automated batching system (inspired by Rust rollups) that groups multiple PRs together for coordinated integration into the next branch. Instead of each PR going through the merge queue individually, teams push their PRs into a shared merge-train/* branch. Periodically, that branch is merged as a single unit into next.

Active Merge-Train Branches

How to Use a Merge Train

Targeting a Merge Train with Your PR

1. Create your feature branch off the appropriate merge-train branch (not our default branch next).
2. Open your PR targeting that merge-train branch (e.g., base: merge-train/barretenberg).
3. When your PR is approved and merged, it gets squashed into the merge-train branch.
4. The merge-train PR (which targets next) automatically accumulates your commit.

Key Rules for Contributors

• Base branch matters: Always branch from the branch specified in the CI_BASE_BRANCH environment variable. If it is not set, then ask the user their intent and offer to set CI_BASE_BRANCH in their shell's RC file.
• Your PR is squashed into the train: Individual PRs targeting a merge-train branch are squash-merged as usual. You should not use the merge commit merge method, but the squash method.
• The train itself is NOT squashed: The merge-train PR (e.g., merge-train/barretenberg -> next) is merged with a merge commit, preserving the individual squashed commits. This is why the ci-no-squash label is automatically applied.
• You generally don't need to worry about the train PR itself -- it is fully automated (creation, body updates, approval, merge, and recreation). You only need to pay attention to it if an alert is sent to your team channel.

CI Behavior for Merge Trains

• Specialized CI modes: PRs targeting merge-train/docs run docs-only CI. PRs targeting merge-train/barretenberg run barretenberg-only CI. This avoids running the full test suite for domain-specific changes.
• Merge-queue mode: When the merge-train PR enters GitHub's merge queue, it runs the full merge-queue CI mode (4 parallel grind runs on AMD64 + 1 ARM64). merge-train/spartan uses the heavier merge-queue-heavy mode (10 grind runs).
• Full concurrency: Merge-train PRs get unique CI concurrency groups (using github.run_id), so multiple CI runs can proceed in parallel without cancelling each other.
• Test history tracking: Test results are tracked for merge-train PRs, same as merge-queue runs.

Handling Merge-Train Failures

When CI Fails on the Merge-Train PR

Two options from the merge-train-readme.md:

Option 1: Direct Fix -- Push a fix directly to the merge-train branch. Use bypass merge to expedite (all users have this permission). You can use the ci-skip label to no-op CI if really necessary.

Option 2: Fix in Next -- Merge a revert or workaround into next. The fix will auto-propagate to the merge-train via the merge-train-next-to-branches workflow. Best when the root cause is in next or multiple trains are affected.

When Auto-Merge Is Blocked

The auto-merge script will not enable auto-merge if the last merge-queue CI run for the PR concluded with failure or cancelled. Someone needs to either fix the issue and push, or force-merge.

Merge Conflicts from Next

When merging next into a train branch causes conflicts, the merge-next.sh script:

• Aborts the merge
• Posts a comment on the latest next commit listing the conflicted files
• The team must manually resolve conflicts on their train branch

Bypassing Checks / Force-Merging

If the user needs to bypass CI checks for their merge-train PR (e.g., a known flaky failure, an urgent merge, or CI infrastructure issues):

1. Confirm intent: Always confirm with the user that they want to skip CI, since this merges untested code into next.
2. Add the ci-skip label: Apply the ci-skip label to the merge-train PR. This causes CI to skip entirely. Use: gh pr edit <PR_NUMBER> --add-label ci-skip
3. Force merge in the UI: The user can then use GitHub's "Merge without waiting for requirements to be met" button (bypass merge) in the PR UI. All users have this permission.

Important: Only do this when the user explicitly asks to bypass checks. Always confirm first since it skips all CI validation.

Backport Trains

A related system exists for backport branches (backport-to-*). These use the same auto-merge mechanism but with different settings:

• Branch pattern: backport-to-
• Inactivity threshold: 8 hours (instead of 4)
• Merge strategy: merge commit