Managing Git Submodules Branches When Switching Branches

Is This Written by ChatGPT?

Well, it's not not written by ChatGPT. But it's a collab.

Problem

Working with git submodules presents a unique challenge: when switching branches in the superproject (the main project), the submodule's branch doesn't automatically change. Instead, it stays on the previously checked-out branch. This lack of tracking can lead to inconsistencies between the superproject and the submodule, resulting in confusion and potential issues.

Note About Submodules

Before we delve into the solution, let's clarify the typical use of git submodules. Submodules are designed to allow a Git repository to be a subdirectory of another Git repository, maintaining their commits separately. In essence, submodules pin a specific commit, not a branch, from an external repository into your primary repository.

Therefore, if your submodule's branch needs to track the branch of your main project, it might indicate that these two components should not be separate repositories. However, in certain cases where you find tracking branches useful, the following solution could serve your needs.

Solution

To ensure that the submodule changes to a corresponding branch when switching branches in the superproject, we can create two scripts: one to set (record) the current branch of the submodule when switching branches, and another to restore the submodule to its recorded branch when switching back.

Implementation

In the implementation, we create two Python scripts: git-submodule-branch-set and git-submodule-branch-restore. Here's how they work:

1. When switching branches in the superproject, run git-submodule-branch-set. This script traverses all submodules, recording their current branch by creating a file in the superproject's .git directory. The filename is a combination of the branch name of the superproject and the path to the submodule, and the file's content is the name of the branch the submodule is currently on.

2. After switching back to a previous branch in the superproject, run git-submodule-branch-restore. This script goes through all submodules, looking for a corresponding file in the .git directory. If it finds one, it reads the submodule's branch from the file and checks out the submodule to that branch.

git-submodule-branch-set

This script records the current branch of the submodule when you switch branches in the superproject.

#!/usr/bin/env python3
# git-submodule-branch-set

import os
import subprocess

# Get the list of all submodules in the current repository
submodules_output = subprocess.check_output(['git', 'config', '--file', '.gitmodules', '--get-regexp', 'path']).strip().decode('utf8')
submodules = [line.split()[1] for line in submodules_output.split('\n')]

# Get the current branch of the superproject
branch = subprocess.check_output(['git', 'rev-parse', '--abbrev-ref', 'HEAD']).strip().decode('utf8')

for submodule in submodules:
    # Construct the filename where the submodule branch will be stored
    submodule_branch_file = os.path.join('.git', f'submodule_branch_{branch}_{submodule.replace("/", "_")}')

    # Record the current branch of the submodule
    submodule_branch = subprocess.check_output(['git', '-C', submodule, 'rev-parse', '--abbrev-ref', 'HEAD']).strip().decode('utf8')
    with open(submodule_branch_file, 'w') as file:
        file.write(submodule_branch)
    print(f"Submodule '{submodule}' set to branch '{submodule_branch}'")

git-submodule-branch-restore

This script restores the submodule to the recorded branch when you switch back to a previous branch.

#!/usr/bin/env python3
# git-submodule-branch-restore

import os
import subprocess

# Get the list of all submodules in the current repository
submodules_output = subprocess.check_output(['git', 'config', '--file', '.gitmodules', '--get-regexp', 'path']).strip().decode('utf8')
submodules = [line.split()[1] for line in submodules_output.split('\n')]

# Get the current branch of the superproject
branch = subprocess.check_output(['git', 'rev-parse', '--abbrev-ref', 'HEAD']).strip().decode('utf8')

for submodule in submodules:
    # Construct the filename where the submodule branch will be stored
    submodule_branch_file = os.path.join('.git', f'submodule_branch_{branch}_{submodule.replace("/", "_")}')

    # If a file exists that records the branch of the submodule, checkout to that branch in the submodule
    if os.path.isfile(submodule_branch_file):
        with open(submodule_branch_file, 'r') as file:
            submodule_branch = file.read().strip()
        subprocess.check_call(['git', '-C', submodule, 'checkout', submodule_branch])
        print(f"Submodule '{submodule}' restored to branch '{submodule_branch}'")