Subtree Pulls

What is a Subtree

The most common way to include a repository inside another, git submodules, works by adding metadata on what repository and revision should be included as a submodule, along with the path the submodule should be cloned into. Git clients then can be instructed to clone all the submodules of a repository.

A big drawback of submodules when dealing with private repositories is that the git client must be authorized to access the repositories of all submodules, otherwise it will have an incomplete repository. This is problematic to setup securely on GitHub and the CI systems we use.

A different approach to including repositories inside another is subtrees. While with submodules each repository is still a fully separate git repository, subtrees merge the history of the included repository inside the parent repository’s history, resulting in a single bigger repository.

Subtrees provide builtin tooling to keep those inclusions up to date as the merged repository changes.

What is an Subtree Pull

Similar to upstream pulls, a subtree pull pulls in new commits from a corresponding subtree into the Ferrocene repository to keep them in sync. Likewise this procedure is usually performed by a periodic Github Action that opens a pull request automatically, only requiring sign off of a reviewer to get merged. Sometimes merge conflicts may occur in which case automation opens an issue describing the conflict and requiring the upstream pull to be done manually.

Manual intervention when the automation fails

Performing a manual pull works similar to doing a manual upstream pull. The following text will make use of the term <NEW_PULL_BRANCH> to refer to the new branch created by the pull initiator to perform the pull on.

Initiating the pull

Run the following git commands to set up a new local <NEW_PULL_BRANCH> branch to do the pull on:

git checkout origin/main # check out the ferrocene branch to do the pull for
git pull --rebase # make sure your local branch is up to date
git submodule update # update any submodules that might've changed
git checkout -b <NEW_PULL_BRANCH> # checkout a new branch to do the pull with

Performing the pull

Now run:

ferrocene/tools/pull-subtrees/ --target main

The script will attempt to update all the registered subtrees in order until all succeed or the first one fails.

If merging fails for one of the subtrees the script will stop and report which one failed as well as the conflicts that have occured.

After having manually fixed the conflicts, finish up the merge by doing the following:

git add . # add your changes to the staging area
git merge --continue # finish the merge
git push --set-upstream origin <NEW_PULL_BRANCH> # push out the pull request branch

Finally open up a pull request from the <NEW_PULL_BRANCH>.