Syncing Git Submodules Correctly

TODO: collapasable, copyable, download, etc

I am working in a monorepo with some git submodules. Sometimes when I check out main and do a git pull, it looks like my submodule that I don't work on gets out of sync and shows up as a note in my pull request.

SHould I use git pull --recurse-submodules or git submodule update --recursive... or something else?When working with a monorepo that includes Git submodules, it's common to encounter the issue of keeping submodules in sync, especially when switching branches or updating your repository. Both git pull --recurse-submodules and git submodule update --recursive are useful commands, but they serve slightly different purposes. Let's break down what each does:

1. git pull --recurse-submodules: This command is useful when you want to update your main repository and all of its submodules at the same time. It essentially performs a git pull in each submodule. This is handy when you've checked out the main branch (or any branch) and want to ensure that all submodules are updated to the commit specified in that branch of the main repository.

2. git submodule update --recursive: This command is used after you've already pulled changes into your main repository and want to update your submodules to match the commits referenced by the main repository. It doesn't fetch new changes from the remote submodules but updates the submodules to the commit currently pointed to by the main repository.

So, which one should you use?

• If your goal is to update both the main repository and all submodules with the latest changes from their respective remotes, then git pull --recurse-submodules is the right choice. This ensures everything is up-to-date with the remote branches.

• If you've already updated your main repository (e.g., switched branches in the main repo) and just need to get your submodules in sync with the main repository's current state, git submodule update --recursive is what you need.

In the context of your situation, where a submodule appears out of sync in your pull request, it seems more likely that you need to use git submodule update --recursive after pulling changes into your main repository. This will align your submodules with the state expected in the main repository, which should resolve the issue of the submodule showing up as a note in your pull request.

Remember, it's important to commit any changes to submodules as part of your workflow. If a submodule is at a different commit than expected, it should be updated and the new submodule state should be committed in the main repository. This ensures that anyone else working on the project will get the correct submodule state when they pull changes.Is it safe to run git submodule update --recursive? What could go wrong?Running git submodule update --recursive is generally safe, especially in the context of aligning your submodules with the state specified in the main repository. However, like any command, it has certain implications and potential issues you should be aware of:

1. Local Changes Overwritten: If you have local, uncommitted changes in your submodules, git submodule update --recursive can overwrite them. This command checks out the specific commit in the submodule that is referenced in the main repository, potentially discarding any local changes in the submodule.

2. Detached HEAD State: This command often leaves submodules in a detached HEAD state. This means that the submodule is not checked out to a specific branch; it's checked out to a specific commit. While this is not inherently problematic, it can be confusing if you're not expecting it. If you start making changes in this state and then commit, your changes won't be on any branch, which can complicate future merges.

3. Submodules Not Updated to Latest Remote Version: The command updates the submodules to the commit specified by the main repository, not necessarily to the latest commit available in the submodule's remote repository. If you need the latest changes from the submodule's remote, you'll need to manually fetch and merge those changes within the submodule.

To mitigate these potential issues, here are some best practices:

• Check for Local Changes: Before running the command, check for uncommitted changes in your submodules. If you have important changes, commit them first or stash them.

• Understand the Detached HEAD State: If you plan to work within a submodule, make sure you understand the implications of a detached HEAD state. If necessary, check out a specific branch within the submodule after updating.

• Communicate with Your Team: If you're working in a team environment, it's important to communicate about which submodule commits should be referenced in the main repository. Unexpected changes to submodule commits in the main repo can cause confusion.

• Regularly Fetch and Merge Submodule Changes: If keeping submodules up-to-date with their respective remotes is important for your workflow, regularly fetch and merge changes within each submodule, and then commit these submodule updates in the main repository.

So, while git submodule update --recursive is generally safe, being aware of these nuances and handling them appropriately will ensure a smooth experience when working with Git submodules.I ran git submodule update --recursive and there was no ouput. I ran git push and it said everything is up to date

The diff of my PR says "Submodule foo updated 4 files" then it lists file names and + - lines.

I don't work on foo and didnt' do anything. WHat is going on and how do I "clean up" my PR so foo doesn't appear?It sounds like the submodule foo was updated in the main repository to point to a new commit, and this update is being reflected in your pull request (PR). Here’s a breakdown of what’s happening and how you can resolve it:

1. No Output from git submodule update --recursive: If this command produced no output, it means your local copy of the submodule was already at the commit specified by the main repository. There were no changes to apply.

2. git push Showing Everything Up-to-Date: Since you haven't made any changes (or those changes are already pushed), Git correctly indicates that everything is up to date.

3. Submodule Changes in PR: The message "Submodule foo updated 4 files" in your PR indicates that the commit in the main repository now points to a different commit in the foo submodule, one which includes changes to 4 files. This can happen if someone else updated the submodule reference in the main repository and you pulled those changes.

To "clean up" your PR and exclude the unintended submodule changes, you need to reset the submodule reference in your branch to the state it was before these changes. Here's how you can do it:

1. Identify the Correct Commit for the Submodule: Find out which commit your branch is supposed to reference for the foo submodule. This could be the commit that was in use before the latest change.

2. Reset the Submodule to the Correct Commit:

   • Go into the submodule directory: cd path/to/foo
   • Check out the correct commit: git checkout [correct-commit-hash]
   • Go back to the main repository root: cd ../..

3. Commit and Push the Submodule Reference:

   • After checking out the correct commit in the submodule, the main repository will recognize this change. You can see this by running git status.
   • Commit this change: git commit -am "Revert submodule foo to [correct-commit-hash]"
   • Push this commit to your branch: git push

4. Verify Your PR: After pushing, check your PR to ensure that the unintended submodule changes are no longer part of it.

Remember, when dealing with submodules, a change in the main repository doesn't mean the content of the submodule has changed; it means the main repository is pointing to a different commit in the submodule. If you don't work on foo, it's likely someone else updated its reference in the main repository, and you just need to reset it to the previous state for your specific branch/PR.

Syncing Git Submodules Correctly created at Tue Jan 20 1970 18:04:21 GMT+0000 (Coordinated Universal Time)