I would like to propose to replace the update-checkout script with the repo tool.
Motivation
From repo's Readme:
Repo is a tool built on top of Git. Repo helps manage many Git repositories, does the uploads to revision control systems, and automates parts of the development workflow. Repo is not meant to replace Git, only to make it easier to work with Git. The repo command is an executable Python script that you can put anywhere in your path.
Repo is actively maintained and addresses some of the issues which come up when using update-checkout, specifically on Windows:
it works reliably on windows
it can support exporting stable build points to XML.
it can work in quiet mode (1/4th of the CI logs are git commands executed by update-checkout.
I'm clearly biased, but I am strongly in favour of switching to repo
--github-comment is unrelated to the repository management and is used by the CI system to track the dependent changes for a cross-repo change. This can be a simple curl request.
--skip-repository is not a part of repo as it is meant to synchronize the full set to a known point. The --skip-repository allows the user to bypass that and get into potentially an invalid state. This is more about following POLA.
My primary use for --skip-repository is to prevent it from overwriting my local checkout when i've rebased my branch and i want to update dependencies. Since i have a Git alias set up to update a branch without checking it out, using --skip-repository means i also don't have to check out the base branch to update the dependencies.
We could remove most of it (I think that --skip-repository swift is needed for update-checkout to work?). The usage there is to avoid checking out macOS specific repositories to save time. This would be handled in repo with adding a group="notdefault,macOS" on the repositories.
The group arg looks great. It would allow to have an explicit list of repositories which are not required for Windows. I believe that this list only exists implicitly in the arguments of build-windows-toolchain.bat.
There is a palpable friction for swiftlang developers when working on patches that require corresponding changes across multiple repositories. Based on its command reference and sources, repo seems to have many great, optional features like cross-project branches, staging, rebasing, diffs, etc, that update-checkout lacks.
I'm definitely in favor of adding the needed manifests so people can choose to use repo's extra features, as that would provide a standard, streamlined way to juggle cross-project changes throughout the development cycle.
We don't have to immediately ditch update-checkout, as much of its existing functionality can probably be implemented by shelling out to repo. In a future where more components live in separate git repositories, like the standard library, tooling like repo may even become essential.
It would be easy to support both update-checkout and repo at the same time while we experiment/transition: comparing this repo manifest to our update-checkout-config.json it seems like we could easily add a flag in update-checkout to export the config to a repo manifest.
We could have a swift/utils/repo_manifests directory with the manifests that were exported from update-checkout-config.json.
