Pull request workflow

The so-called “PR workflow” used by Godot is common to many projects using Git, and should be familiar to veteran free software contributors. The idea is that only a small number (if any) commit directly to the master branch. Instead, contributors fork the project (i.e. create a copy of it, which they can modify as they wish), and then use the GitHub interface to request a pull from one of their fork’s branch to one branch of the original (often named upstream) repository.

The resulting pull request (PR) can then be reviewed by other contributors, which might approve it, reject it, or most often request that modifications be done. Once approved, the PR can then be merged by one of the core developers, and its commit(s) will become part of the target branch (usually the master branch).

We will go together through an example to show the typical workflow and associated Git commands. But first, let’s have a quick look at the organisation of Godot’s Git repository.

Git source repository

The repository on GitHub is a Git code repository together with an embedded issue tracker and PR system.

The Git version control system is the tool used to keep track of successive edits to the source code - to contibute efficiently to Godot, learning the basics of the Git command line is highly recommended. There exist some graphical interfaces for Git, but they usually encourage users to take bad habits regarding the Git and PR workflow, and we therefore recommend not to use them (especially GitHub’s online editor).

See also

The first sections of Git’s “Book” are a good introduction to the tool’s philosophy and the various commands you need to master in your daily workflow. You can read them online on the Git SCM website.

The branches on the Git repository are organized as follows:

  • The master branch is where the development of the next major version (3.0, 3.1, 4.0, etc.) occurs. As a development branch, it can be unstable and is not meant for use in production. This is where PRs should be done in priority.
  • The stable branches are named after their version, e.g. 2.0 and 2.1. They are used to backport bugfixes and enhancements from the master branch to the currently maintained stable release (e.g. 2.0.1 or 2.1.3). As a rule of thumb, the last stable branch is maintained until the next major version (e.g. the 2.0 branch was maintained until the release of Godot 2.1). If you want to make PRs against a maintained stable branch, you will have to check if your changes are also relevant for the master branch.
  • There might be feature branches at time, usually meant to be merged into the master branch at some time.

Forking and cloning

The first step is to fork the godotengine/godot repository on GitHub. To do so, you will need to have a GitHub account and to be logged in. In the top right corner of the repository’s GitHub page, you should see the “Fork” button as shown below:

../../_images/github_fork_button.png

Click it, and after a while you should be redirected to your own fork of the Godot repo, with your GitHub username as namespace:

../../_images/github_fork_url.png

You can then clone your fork, i.e. create a local copy of the online repository (in Git speak, the origin remote):

$ git clone https://github.com/USERNAME/godot

Note

In our examples, the “$” character denotes the command line prompt on typical UNIX shells. It is not part of the command and should not be typed.

After a little while, you should have a godot directory in your current working directory. Move into it (cd godot), and we will set up a useful reference:

$ git remote add upstream https://github.com/godotengine/godot
$ git fetch upstream

This will create a reference named upstream pointing to the original godotengine/godot repository. This will be useful when you want to pull new commits from its master branch to update your fork. You have another remote reference named origin, which points to your fork.

You only need to do the above steps once, as long as you keep that local godot folder (which you can move around if you want, the relevant metadata is hidden in its .git subfolder).

Note

Branch it, pull it, code it, stage it, commit, push it, rebase it... technologic.

This bad take on Daft Punk’s Technologic shows the general conception Git beginners have of its workflow: lots of strange commands to learn by copy and paste, hoping they will work as expected. And that’s actually not a bad way to learn, as long as you’re curious and don’t hesitate to question your search engine when lost, so we will give you the basic commands to know when working in Git.

In the following, we will assume that you want to implement a feature in Godot’s project manager, which is coded in the editor/project_manager.cpp file.

Branching

By default, the git clone should have put you on the master branch of your fork (origin). To start your own feature development, we will create a feature branch:

// Create the branch based on the current branch (master)
$ git branch better-project-manager

// Change the current branch to the new one
$ git checkout better-project-manager

This command is equivalent:

// Change the current branch to a new named one, based on the current branch
$ git checkout -b better-project-manager

If you want to go back to the master branch, you’d use:

$ git checkout master

You can see which branch you are currently on with the git branch command:

$ git branch
  2.1
* better-project-manager
  master

Updating your branch

This would not be needed the first time, just after you forked the upstream repository. However, the next time you want to work on something, you will notice that your fork’s master is several commits behind the upstream master branch: pull requests from other contributors would have been merged in the meantime.

To ensure there won’t be conflicts between the feature you develop and the current upstream master branch, you will have to update your branch by pulling the upstream branch.

$ git pull upstream master

However, if you had local commits, this method will create a so-called “merge commit”, and you will soon hear from fellow contributors that those are not wanted in PRs. Then how to update the branch without creating a merge commit? You will have to use the --rebase option, so that your local commits are replayed on top of the updated upstream master branch. It will effectively modify the Git history of your branch, but that is for the greater good.

Then command that you should (almost) always use is there:

$ git pull --rebase upstream master

Making changes

You would then do your changes to our example’s editor/project_manager.cpp file with your usual development environment (text editor, IDE, etc.).

By default, those changes are unstaged. The staging area is a layer between your working directory (where you make your modifications) and the local git repository (the commits and all the metadata in the .git folder). To bring changes from the working directory to the git repository, you need to stage them with the git add command, and then to commit them with the git commit command.

There are various commands you should know to review your current work, before staging it, while it is staged, and after it has been committed.

  • git diff will show you the current unstaged changes, i.e. the differences between your working directory and the staging area.
  • git checkout -- <files> will undo the unstaged changes to the given files.
  • git add <files> will stage the changes on the listed files.
  • git diff --staged will show the current staged changes, i.e. the differences between the staging area and the last commit.
  • git reset HEAD <files> will unstage changes to the listed files.
  • git status will show you what are the currently staged and unstaged modifications.
  • git commit will commit the staged files. It will open a text editor (you can define the one you want to use with the GIT_EDITOR environment variable or the core.editor setting in your Git config) to let you write a commit log. You can use git commit -m "Cool commit log" to write the log directly.
  • git log will show you the last commits of your current branch. If you did local commits, they should be shown at the top.
  • git show will show you the changes of the last commit. You can also specify a commit hash to see the changes for that commit.

That’s a lot to memorise! Don’t worry, just check this cheat sheet when you need to make changes, and learn by doing.

Here’s how the shell history could look like on our example:

// It's nice to know where you're starting from
$ git log

// Do changes to the project manager
$ nano editor/project_manager.cpp

// Find an unrelated bug in Control and fix it
$ nano scene/gui/control.cpp

// Review changes
$ git status
$ git diff

// We'll do two commits for our unrelated changes,
// starting by the Control changes necessary for the PM enhancements
$ git add scene/gui/control.cpp
$ git commit -m "Fix handling of margins in Control"

// Check we did good
$ git log
$ git show
$ git status

// Make our second commit
$ git add editor/project_manager.cpp
$ git commit -m "Add a pretty banner to the project manager"
$ git log

With this, we should have two new commits in our better-project-manager branch which were not in the master branch. They are still only local though, the remote fork does not know about them, nor does the upstream repo.

Pushing changes to a remote

That’s where git push will come into play. In Git, a commit is always done in the local repository (unlike Subversion where a commit will modify the remote repository directly). You need to push the new commits to a remote branch to share them with the world. The syntax for this is:

$ git push <remote> <local branch>[:<remote branch>]

The part about the remote branch can be ommitted if you want it to have the same name as the local branch, which is our case in this example, so we will do:

$ git push origin better-project-manager

Git will ask you for your username and password, and the changes will be sent to your remote. If you check the fork’s page on GitHub, you should see a new branch with your added commits.

Issuing a pull request

When you load your fork’s branch on GitHub, you should see a line saying “This branch is 2 commits ahead of godotengine:master.” (and potentially some commits behind, if your master branch was out of sync with the upstream master branch.

../../_images/github_fork_make_pr.png

On that line, there is a “Pull request” link. Clicking it will open a form that will let you issue a pull request on the godotengine/godot upstream repository. It should show you your two commits, and state “Able to merge”. If not (e.g. it has way more commits, or says there are merge conflicts), don’t create the PR, something went wrong. Go to IRC and ask for support :)

Use an explicit title for the PR and put the necessary details in the comment area. You can drag and drop screenshots, gifs or zipped projects if relevant, to showcase what your work implements. Click “Create a pull request”, and tadaa!

Modifying a pull request

While it is reviewed by other contributors, you will often need to make changes to your yet-unmerged PR, either because contributors requested them, or because you found issues yourself while testing.

The good news is that you can modify a pull request simply by acting on the branch you made the pull request from. You can e.g. make a new commit on that branch, push it to your fork, and the PR will be updated automatically:

// Check out your branch again if you had changed in the meantime
$ git checkout better-project-manager

// Fix a mistake
$ nano editor/project_manager.cpp
$ git add editor/project_manager.cpp
$ git commit -m "Fix a typo in the banner's title"
$ git push origin better-project-manager

That should do the trick, but...

Mastering the PR workflow: the rebase

On the situation outlined above, your fellow contributors who are particularly pedantic regarding the Git history might ask your to rebase your branch to squash or meld the last two commits together (i.e. the two related to the project manager), as the second commit basically fixes an issue in the first one.

Once the PR is merged, it is not relevant for a changelog reader that the PR author made mistakes; instead, we want to keep only commits that bring from one working state to another working state.

To squash those two commits together, we will have to rewrite history. Right, we have that power. You may read that it’s a bad practice, and it’s true when it comes to branches of the upstream repo. But in your fork, you can do whatever you want, and everything is allowed to get neat PRs :)

We will use the interactive rebase git rebase -i to do this. This command takes a commit hash as argument, and will let you modify all commits between that commit hash and the last one of the branch, the so-called HEAD. In our example, we want to act on the last two commits, so we will do:

// The HEAD~X syntax means X commits before HEAD
$ git rebase -i HEAD~2

This will open a text editor with:

pick 1b4aad7 Add a pretty banner to the project manager
pick e07077e Fix a typo in the banner's title

The editor will also show instructions regarding how you can act on those commits. In particular, it should tell you that “pick” means to use that commit (do nothing), and that “squash” and “fixup” can be used to meld the commit in its parent commit. The difference between “squash” and “fixup” is that “fixup” will discard the commit log from the squashed commit. In our example, we are not interested in keeping the log of the “Fix a typo” commit, so we use:

pick 1b4aad7 Add a pretty banner to the project manager
fixup e07077e Fix a typo in the banner's title

Upon saving and quitting the editor, the rebase will occur. The second commit will be melded into the first one, and git log and git show should now confirm that you have only one commit with the changes from both previous commits.

Note

You could have avoided this rebase by using git commit --amend when fixing the typo. This command will write the staged changes directly into the last commit (HEAD), instead of creating a new commit like we did in this example. So it is equivalent to what we did with a new commit and then a rebase to mark it as “fixup”.

But! You rewrote the history, and now your local and remote branches have diverged. Indeed, commit 1b4aad7 in the above example will have changed, and therefore got a new commit hash. If you try to push to your remote branch, it will raise an error:

$ git push origin better-project-manager
To https://github.com/akien-mga/godot
 ! [rejected]        better-project-manager -> better-project-manager (non-fast-forward)
error: failed to push some refs to 'https://akien-mga@github.com/akien-mga/godot'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart.

This is a sane behaviour, Git will not let you push changes that would override remote content. But that’s actually what we want to do here, so we will have to force it:

$ git push --force origin better-project-manager

And tadaa! Git will happily replace your remote branch with what you had locally (so make sure that’s what you wanted, using git log). This will also update the PR accordingly.