Gil (git links) tool

Gil is a git links tool to manage complex recursive repositories dependencies with cross references and cycles.

This tool provides a solution to the git recursive submodules dependency problem.

Contents

• Requirements
• Setup
• Sample
• Usage
• Motivation: git submodules vs git links
  • Git submodules management
  • Recursive submodules problem
• Motivation: git subtrees vs git links
  • Git subtrees management
  • Subtrees manual management problem

Requirements

• Linux
• MacOS
• Windows
• git
• python3

Setup

sudo pip3 install --prefix /usr/local gil

Sample

Consider we have the following git repository dependency graph:

Where:

• Blue nodes are third-party components
• Green nodes are components that we develop
• Edges show components dependency with a branch

Root .gitlinks file

In order to describe the sample model with git links we have to define root .gitlinks file in your sample repository with a following content:

# Projects
CppBenchmark CppBenchmark https://github.com/chronoxor/CppBenchmark.git master
CppCommon CppCommon https://github.com/chronoxor/CppCommon.git master
CppLogging CppLogging https://github.com/chronoxor/CppLogging.git master

# Modules
Catch2 modules/Catch2 https://github.com/catchorg/Catch2.git master
cpp-optparse modules/cpp-optparse https://github.com/weisslj/cpp-optparse.git master
fmt modules/fmt https://github.com/fmtlib/fmt.git master
HdrHistogram modules/HdrHistogram https://github.com/HdrHistogram/HdrHistogram_c.git master
zlib modules/zlib https://github.com/madler/zlib.git master

# Scripts
build scripts/build https://github.com/chronoxor/CppBuildScripts.git master
cmake scripts/cmake https://github.com/chronoxor/CppCMakeScripts.git master

Each line describe git link in the following format:

1. Unique name of the repository
2. Relative path of the repository (started from the path of .gitlinks file)
3. Git repository which will be used in git clone command
4. Repository branch to checkout
5. (Optional) Relative path in the base repository
6. (Optional) Relative path in the target repository

Empty line or line started with # are not parsed (treated as comment).

CppBenchmark .gitlinks file

CppBenchmark .gitlinks file should be committed into the CppBenchmark project and have the following content:

# Modules
Catch2 modules/Catch2 https://github.com/catchorg/Catch2.git master
cpp-optparse modules/cpp-optparse https://github.com/weisslj/cpp-optparse.git master
HdrHistogram modules/HdrHistogram https://github.com/HdrHistogram/HdrHistogram_c.git master
zlib modules/zlib https://github.com/madler/zlib.git master

# Scripts
build build https://github.com/chronoxor/CppBuildScripts.git master
cmake cmake https://github.com/chronoxor/CppCMakeScripts.git master

CppCommon .gitlinks file

CppCommon .gitlinks file should be committed into the CppCommon project and have the following content:

# Modules
Catch2 modules/Catch2 https://github.com/catchorg/Catch2.git master
CppBenchmark modules/CppBenchmark https://github.com/chronoxor/CppBenchmark.git master
fmt modules/fmt https://github.com/fmtlib/fmt.git master

# Scripts
build build https://github.com/chronoxor/CppBuildScripts.git master
cmake cmake https://github.com/chronoxor/CppCMakeScripts.git master

CppLogging .gitlinks file

CppLogging .gitlinks file should be committed into the CppLogging project and have the following content:

# Modules
Catch2 modules/Catch2 https://github.com/catchorg/Catch2.git master
cpp-optparse modules/cpp-optparse https://github.com/weisslj/cpp-optparse.git master
CppBenchmark modules/CppBenchmark https://github.com/chronoxor/CppBenchmark.git master
CppCommon modules/CppCommon https://github.com/chronoxor/CppCommon.git master
zlib modules/zlib https://github.com/madler/zlib.git master

# Scripts
build build https://github.com/chronoxor/CppBuildScripts.git master
cmake cmake https://github.com/chronoxor/CppCMakeScripts.git master

Update the repository

Finally you have to update your root sample repository:

# Clone and link all git links dependencies from .gitlinks file
gil clone
gil link

# The same result with a single command
gil update

As the result you'll clone all required projects and link them to each other in a proper way:

Working path: ~/gil/sample
...
Updating git links: ~/gil/sample/.gitlinks
Updating git links: ~/gil/sample/CppBenchmark/.gitlinks
Update git link: ~/gil/sample/modules/Catch2 -> ~/gil/sample/CppBenchmark/modules/Catch2
Update git link: ~/gil/sample/modules/cpp-optparse -> ~/gil/sample/CppBenchmark/modules/cpp-optparse
Update git link: ~/gil/sample/modules/HdrHistogram -> ~/gil/sample/CppBenchmark/modules/HdrHistogram
Update git link: ~/gil/sample/modules/zlib -> ~/gil/sample/CppBenchmark/modules/zlib
Update git link: ~/gil/sample/scripts/build -> ~/gil/sample/CppBenchmark/build
Update git link: ~/gil/sample/scripts/cmake -> ~/gil/sample/CppBenchmark/cmake
Updating git links: ~/gil/sample/CppCommon/.gitlinks
Update git link: ~/gil/sample/modules/Catch2 -> ~/gil/sample/CppCommon/modules/Catch2
Update git link: ~/gil/sample/CppBenchmark -> ~/gil/sample/CppCommon/modules/CppBenchmark
Update git link: ~/gil/sample/modules/fmt -> ~/gil/sample/CppCommon/modules/fmt
Update git link: ~/gil/sample/scripts/build -> ~/gil/sample/CppCommon/build
Update git link: ~/gil/sample/scripts/cmake -> ~/gil/sample/CppCommon/cmake
Updating git links: ~/gil/sample/CppLogging/.gitlinks
Update git link: ~/gil/sample/modules/Catch2 -> ~/gil/sample/CppLogging/modules/Catch2
Update git link: ~/gil/sample/modules/cpp-optparse -> ~/gil/sample/CppLogging/modules/cpp-optparse
Update git link: ~/gil/sample/CppBenchmark -> ~/gil/sample/CppLogging/modules/CppBenchmark
Update git link: ~/gil/sample/CppCommon -> ~/gil/sample/CppLogging/modules/CppCommon
Update git link: ~/gil/sample/modules/zlib -> ~/gil/sample/CppLogging/modules/zlib
Update git link: ~/gil/sample/scripts/build -> ~/gil/sample/CppLogging/build
Update git link: ~/gil/sample/scripts/cmake -> ~/gil/sample/CppLogging/cmake

All repositories will be checkout to required branches.

If content of any .gitlinks changes it is required to run gil update command to re-update git links - new repositories will be cloned and linked properly!

Commit, push, pull

You can work and change any files in your repositories and perform all git operations for any repository to commit and contribute your changes.

If you want to commit all changes in some repository with all changes in child linked repositories you can do it with a single command:

gil commit -a -m "Some big update"

This command will visit the current and all child repositories and perform the corresponding git commit command with provided arguments.

Please note that gil command visit only child repositories that are described in .gitlinks file no parent dependencies will be processed. For this reason if you want to commit, pull or push all repositories the corresponding command should be run in the root directory where root .gitlinks is placed.

Pull, push commands works in a similar way:

gil pull
gil push

Build

Please investigate and follow links in the sample repository in order to understand the logic how gil (git links) tool manages dependencies.

Finally you can build sample projects with provided build scripts:

• ~/gil/sample/CppBenchmark/build
• ~/gil/sample/CppCommon/build
• ~/gil/sample/CppLogging/build

Additionally each module can be cloned and successfully build without root repository. In this case local .gitlinks file will be used to resolve all dependencies!

Usage

Gil (git links) tool supports the following commands:

usage: gil command arguments
Supported commands:
        help - show this help
        version - show version
        context - show git links context
        clone [args] - clone git repositories
        link - link git repositories
        update - update git repositories (clone & link)
        pull [args] - pull git repositories
        push [args] - push git repositories
        commit [args] - commit git repositories
        status [args] - show status of git repositories

• gil context - command will show the current git link context of the current directory;
• gil clone [args] - clone all repositories that are missed in the current context;
• gil link - link all repositories that are missed in the current context;
• gil update - clone and link in a single operation;
• gil pull [args] - pull all repositories in the current directory;
• gil push [args] - push all repositories in the current directory;
• gil commit [args] - commit all repositories in the current directory;
• gil status [args] - show status of all repositories in the current directory;

Motivation: git submodules vs git links

Git submodules allow to get the same idea of repositories dependency, but with a serious limitation - git submodules degrade to tree-like recursive structure with repositories duplication.

Git submodules management

Git submodules provides several operations to manage repositories dependency.

Add submodule dependency

# Add git submodule of 'master' branch into the relative directory 'modules/zlib'
git submodule add -b master https://github.com/madler/zlib.git modules/zlib

Initialize submodules

# Update all submodules in the current repository
git submodule update --init --recursive --remote

Refresh submodules from remote repositories

git pull
git submodule update --init --recursive --remote
git pull --recurse-submodules
git submodule foreach "(git checkout master; git pull)"

Synchronize submodules with remote repositories

git pull
git submodule update --init --recursive --remote
git pull --recurse-submodules
git submodule foreach "(git checkout master; git pull)"
git add --all
git commit -m "Submodule Sync"
git push

Remove submodule dependency

# Remove git submodule from the relative directory 'modules/zlib'
git submodule deinit -f -- modules/zlib
rm -rf .git/modules/modules/zlib
git rm -f modules/zlib

Recursive submodules problem

Git submodules representation of the described sample repository is the following:

~/gil/sample/CppBenchmark
~/gil/sample/CppBenchmark/build
~/gil/sample/CppBenchmark/cmake
~/gil/sample/CppBenchmark/modules/Catch2
~/gil/sample/CppBenchmark/modules/cpp-optparse
~/gil/sample/CppBenchmark/modules/HdrHistogram
~/gil/sample/CppBenchmark/modules/zlib
~/gil/sample/CppCommon
~/gil/sample/CppCommon/build - DUPLICATE!
~/gil/sample/CppCommon/cmake - DUPLICATE!
~/gil/sample/CppCommon/modules/Catch2 - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/build - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/cmake - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/modules/Catch2 - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/modules/cpp-optparse - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/modules/HdrHistogram - DUPLICATE!
~/gil/sample/CppCommon/modules/CppBenchmark/modules/zlib - DUPLICATE!
~/gil/sample/CppCommon/modules/fmt
~/gil/sample/CppLogging
~/gil/sample/CppLogging/build - DUPLICATE!
~/gil/sample/CppLogging/cmake - DUPLICATE!
~/gil/sample/CppLogging/modules/Catch2 - DUPLICATE!
~/gil/sample/CppLogging/modules/cpp-optparse - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/build - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/cmake - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/modules/Catch2 - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/modules/cpp-optparse - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/modules/HdrHistogram - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark/modules/zlib - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/build - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/cmake - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/Catch2 - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/build - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/cmake - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/modules/Catch2 - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/modules/cpp-optparse - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/modules/HdrHistogram - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark/modules/zlib - DUPLICATE!
~/gil/sample/CppLogging/modules/zlib - DUPLICATE!

Only 10 of 41 repositories are unique. Other duplicates each other multiple times. Moreover some nested repositories included recursively! CppLogging project has the third level of nesting (CppLogging -> CppCommon -> CppBenchmark).

Each new level heavily increases the rate of duplication. As the result all submodule management operations becomes slow and requires multiple executions of synchronize script to pass changes from top level to the bottom. The count of synchronizations in the worst case equals to recursive levels count.

All this issues are solved with gil (git links) tool which allows to link all required repositories together avoiding duplication. The tool also provides operations to manage changes in all repositories with a simple easy to use commands.

Motivation: git subtrees vs git links

Git subtrees is an easy to use alternative to git submodules. It allows to nest one repository inside another as a sub-directory.

From the one side git subtrees are very lightweight with simple workflow. They don't add any metadata files like git submodule does (i.e., .gitmodule) and sub-project's code is available right after the clone of the super project is done. Contents of the module can be modified without having a separate repository copy of the dependency somewhere else.

However they have several drawbacks. Contributing code back upstream for the sub-projects is slightly more complicated. And developer should be very careful of not mixing super and sub-project code in commits.

Git subtrees requires precise manual management in case use have complicated projects structure and lots of upstream commits to dependent projects.

Git subtrees management

Git subtrees provides several operations to manage repositories dependency.

Add subtree dependency

# Add git subtree of 'master' branch into the relative directory 'modules/zlib'
git subtree add -P modules/zlib https://github.com/madler/zlib.git master --squash

Update subtree from the upstream remote repository

git subtree pull -P modules/zlib https://github.com/madler/zlib.git master --squash

Contribute changes from subtree back to the upstream remote repository

git subtree push -P modules/zlib https://github.com/madler/zlib.git master

Remove subtree dependency

# Remove git subtree from the relative directory 'modules/zlib'
git rm -f modules/zlib

Subtrees manual management problem

Initially git subtrees lack of the same repository duplicate problem which git submodules have. Therefore all repositories with multiple dependencies will be duplicated and require manual and careful management.

For instance CppBenchmark repository presented four times in our sample:

~/gil/sample/CppBenchmark
~/gil/sample/CppCommon/modules/CppBenchmark - DUPLICATE!
~/gil/sample/CppLogging/modules/CppBenchmark - DUPLICATE!
~/gil/sample/CppLogging/modules/CppCommon/modules/CppBenchmark - DUPLICATE!

The first problem if you updated CppBenchmark and want to get the fix in all dependent repositories you have to update them manually with the set of git subtree pull commands:

# Commit and push changes in CppBenchmark repository
cd CppBenchmark
git commit -a -m "Bugfix in CppBenchmark"
git push
cd ..

# Update CppCommon repository
cd CppCommon
git subtree pull --prefix modules/CppBenchmark https://github.com/chronoxor/test-CppBenchmark.git main --squash
git push
cd ..

# Update CppLogging repository
cd CppLogging
git subtree pull --prefix modules/CppBenchmark https://github.com/chronoxor/test-CppBenchmark.git main --squash
git subtree pull --prefix modules/CppCommon https://github.com/chronoxor/test-CppCommon.git main --squash
git push
cd ..

The second problem if you found a bug in CppBenchmark while debugging CppLogging and want to fix it in place, push the changes and update all repositories properly. In this case you need to make the following steps:

# Commit and push changes in CppBenchmark but from CppLogging repository
cd CppLogging
git commit -a -m "Bugfix in CppBenchmark (found in CppLogging repository)"
git push
git subtree push --prefix modules/CppBenchmark https://github.com/chronoxor/test-CppBenchmark.git main --squash
cd ..

# Update CppBenchmark repository
cd CppBenchmark
git pull
cd ..

# Update CppCommon repository
cd CppCommon
git subtree pull --prefix modules/CppBenchmark https://github.com/chronoxor/test-CppBenchmark.git main --squash
git push
cd ..

# Update CppLogging repository
cd CppLogging
git subtree pull --prefix modules/CppBenchmark https://github.com/chronoxor/test-CppBenchmark.git main --squash
git subtree pull --prefix modules/CppCommon https://github.com/chronoxor/test-CppCommon.git main --squash
git push
cd ..

As the result too much manual work should be done to make all repositories up to date. Definitely all commands should be stored into scripts with care. And this job requires some efforts and testing because it is very easy to make a mistake. Git subtrees are good and easy to use for small repositories without complicated hierarchy. But working with huge complicated repositories using git subtrees is a big challenge as git subtrees do not suppot recursing pull/push/update commands.