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Rebasing a feature branch

A feature branch was pushed, so upstream has it as a PR. Then upstream's main moved on, and the monorepo's main pulled that in. The PR is now behind main: GitHub says "This branch is out-of-date with the base branch". To bring it up to date, feature is rebased onto main. m adds lock.txt:

The monorepo, a with two children, p on main and b on feature, which is H; upstream, k, which is U(H), with two children, m on main and b prime, which is T, R and feature

The rebase strategies​

git-splice syncs with the upstream branch of the same name. Rebasing feature onto main is a separate Git operation: do it in one repository only, then carry the result across with git-splice:

The two rebase strategies, each with the monorepo main and feature on the left and the upstream main and feature on the right. Rebase via upstream: 1, rebase or merge the upstream feature onto main, for example with Update branch; 2, on feature, git splice pull into the monorepo feature. Rebase via monorepo: 1, on main, git splice pull from the upstream main; 2, on feature, git rebase main; 3, git splice push --force to the upstream feature

StrategyHow to use itWhen to use it
Via upstreamClick "Update branch" on GitHub, or rebase in a clone of the upstream, then git splice pull on feature.The simple default.
Via monorepoOn feature, run git rebase main, then git splice push --force.When GitHub can't rebase, resolve conflicts with the whole monorepo to build and test. Rewrites the monorepo's feature branch and overwrites the PR; use only when nobody else pushes to them.

Don't mix the strategies: a rebase in the monorepo, then a pull of the PR that was never rebased, sends the old commits back to the PR.

Output​

rebase-setup.bash builds this state:

$ build_scenario scenario_feature_branch_behind_default_branch
$ git -C "$UPSTREAM" log --graph --format=%s main feature
* m: bump dep
| * b: feature change
|/  
* k: seed

git-splice compares feature with upstream's feature only, and those match. That the PR is behind main is a matter between the two branches of each repository:

$ git splice status
On branch feature

  up to date  vendor/a