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merging-stacked-prs.md

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Use our how-to guide for instructions on merging stacked PRs. Learn about queueing actions, core merge behaviors for stacked PRs, and separating merge commits.

How to Merge Stacked PRs

Aviator offers an open-source CLI for managing stacked PRs within GitHub. When used alongside Aviator MergeQueue, it simplifies the process of validating and merging those stacked PRs (or subset of those stacked PRs) together.

When requested to merge stacked PRs, Aviator will validate all the stacked PRs as if they were a single PR, and then merge them together after validation. The merge behavior may be slightly different depending on the queue mode.

Queueing action

To merge a subset of the stack, request a merge for the topmost PR in that subset. So for instance, if your stack is:

master <- PR#1 <- PR#2 <- PR#3 <- PR#4

Requesting a stack-merge action for PR#3 will validate PR#1, PR#2 and PR#3 leaving PR#4 untouched. After merging, the stack would look like:

master <- PR#4

There are a few ways to request queueing a stack of PRs.

  • Chrome Extension (recommended): This is the simplest way to request way since the user experience for this is very similar to merging a regular PR. Go to PR#3 from above example in GitHub and click "Queue pull request" button:

Queue pull request action for stacked PRs

  • GitHub Label: You can also queue the stack the same way using GitHub label like you would a single PR. To do so, just apply the label at the top PR that would want to merge with the stack. In the example above, apply the label on PR#3 .
  • CLI: Another way to queue the PR is via the av command line. Simply checkout the branch associated with PR#3 and run the command:
av pr queue

Note that, using this command requires authenticating the CLI with your Aviator account.

  • Slash command: A stack or sub-stack can also be merged by commenting a slash command from the GitHub interface:
/aviator merge

Merge behavior

Depending on the queue mode being used, the merge behavior for stacked PRs can vary.

Sequential mode (default mode)

Taking the example above, in sequential mode:

  • Aviator will consider PR#1, PR#2 and PR#3 as a single PR in the queue.
  • When this PR reaches the top of the queue, Aviator only updates PR#3 with the target branch (mainline), and run the CI.
  • Once the CI passes, Aviator will change the base branch of PR#3 and merge it
  • Since PR#3 is stacked on top of PR#1 and PR#2, merging PR#3 also merges all the changes associated with PR#1 and PR#2.
  • Since GitHub does not recognize that PR#1 and PR#2 are already merged, Aviator automatically closes them and applies the GitHub label merged-by-mq to represent that the PRs are indeed merged.

See also Separating the merge commits.

Parallel mode

Similar to Sequential mode, parallel mode also considers the stack as a single PR in the queue. In parallel mode:

  • Aviator creates a batch using the head branch of PR#3 including all the changes of PR#1 and PR#2 as well.
  • After the batch passes CI, Aviator changes the base branch of PR#3 and merges it.
  • Since PR#3 is stacked on top of PR#1 and PR#2, merging PR#3 also merges all the changes associated with PR#1 and PR#2.
  • As in Sequential mode, Aviator closes PR#1 and PR#2, while applying the GitHub label merged-by-mq to represent that the PRs are indeed merged.

See also Separating the merge commits.

Fast forwarding

Fast forwarding mode provides a simpler experience for merging stacked PRs. Since in case of fast-forwarding, we fast-forward the mainline to commits in draft PRs, the CI is already passing. This allows Aviator to merge the PRs individually without GitHub blocking the commits.

In fast-forwarding mode:

  • When the stack with PR#3 is requested merging, Aviator creates one squash commit for each PR in the stack.
  • After CI passes, the mainline is fast-forwarded to the latest commit in the batch associated with PR#3, this includes separate commits associated with PR#2 and PR#1 in the linear history.

Separating the merge commits

By default the Sequential and Parallel modes create a single commit in the mainline for all of the stacked PRs that are queued together. This is because GitHub does not let an app account bypass the branch protection rules. Even when allowed to bypass the rules, GitHub will let the Aviator app only bypass the approval requirement and PR creation requirement, not the CI validation requirement.

Rulesets

This capability of bypassing the CI requirement is now available using GitHub’s Rulesets. Rulesets offer feature parity with the configurations you use in classic branch protection rules and you should be able to migrate all your existing configurations to using Rulesets.

Once migrated to Ruleset, you can add Aviator app in the bypass list.

Add Aviator GitHub app to Bypass list

Configuring separate commits

Once Rulesets are configured and other GitHub branch protection rules are disabled, you can configure Aviator to start creating separate commits for stacked PRs. To do so, set the following in the merge_strategy section of the configuration YAML.

merge_strategy:
  use_separate_commits_for_stack: true

For details, refer to the configuration reference.

If you need assistance in setting up the Rulesets, please contact [email protected].

Detecting stacked PRs without av CLI

If you are creating stacks manually or using a thirdparty tool, you can enable auto-detection of stacked PRs using the config:

merge_rules:
  auto_detect_stacks: true

Note that, auto-detection can have some side effects where Aviator might detect PRs in a stack even when they were not intentionally created as stacked. For instance, if you have a PR from a releasebranch to main to backport some changes, meanwhile have a PR from a featureAbranch to releasebranch, the auto-detect will detect this as a stack. To avoid this, you can add releasebranch as one of the base branches in the config.