How to rebase a pull request

Source : https://github.com/edx/edx-platform/wiki/How-to-Rebase-a-Pull-Request

When many different people are working on a project simulateously, pull requests can go stale quickly. A "stale" pull request is one that is no longer up to date with the main line of development, and it needs to be updated before it can be merged into the project. The most common reason why pull requests go stale is due to conflicts: if two pull requests both modify similar lines in the same file, and one pull request gets merged, the unmerged pull request will now have a conflict. Sometimes, a pull request can go stale without conflicts: perhaps changes in a different file in the codebase require corresponding changes in your pull request to conform to the new architecture, or perhaps the branch was created when someone had accidentally merged failing unit tests to the master branch. Regardless of the reason, if your pull request has gone stale, you will need to rebase your branch onto the latest version of the master branch before it can be merged.

What is a rebase?

To understand this, we need to understand a bit about how git works. A git repository is a tree structure, where the nodes of the tree are commits. Here's an example of a very simple repository: it has four commits on the master branch, and each commit has an ID (in this case, a, b, c, and d). You'll notice that d is currently the latest commit (or HEAD) of the master branch.

Here, we have two branches: master and my-branch. You can see that master and my-branch both contain commits a and b, but then they start to diverge: master contains c and d, while my-branch contains e and f. b is said to be the "merge base" of my-branch in comparison to master -- or more commonly, just the "base". It makes sense: you can see that my-branch was based on a previous version of master.

So let's say that my-branch has gone stale, and you want to bring it up to date with the latest version of master. To put it another way, my-branch needs to contain c and d. You could do a merge, but that causes the branch to contain weird merge commits that make reviewing the pull request much more difficult. Instead, you can do a rebase.

When you rebase, git finds the base of your branch (in this case, b), finds all the commits between that base and HEAD (in this case, e and f), and re-plays those commits on the HEAD of the branch you're rebasing onto (in this case, master). Git actually creates new commits that represent what your changes look like on top of master: in the diagram, these commits are called e′ and f′. Git doesn't erase your previous commits: e and f are left untouched, and if something goes wrong with the rebase, you can go right back to the way things used to be.

Another thing to notice, however, is that git treats branches as merely labels. The master branch is whatever commit the master label is pointing to, as well as all of that commit's parents. When you rebase a branch, git moves the branch label to point at the newly-created commits: my-branch is no longer pointing at f, it's now pointing at f′. Going back to the way things used to be just consists of changing that branch label so that it points back at f.

Changing History

You'll notice that there is not a direct path from f to f′: from the point of view of anyone else watching my-branch, history has suddenly changed. In effect, c and d have been injected into my-branch's history, as if they had been there the entire time. Unlike most version control systems, git allows you to change the history of your project -- but it is very cautious about letting you do so. We'll come back to this point later.

How do I rebase?

So, now that you understand what a rebase is, the next step is learning how to do it. Let's say you have a fork of edx-platform, and you've created a branch, like so:

$ git clone https://github.com/my-username/edx-platform.git
$ cd edx-platform
$ git checkout -b my-branch

You've made some commits in your branch, pushed them to Github, and created a pull request. You're going through the code review process, responding to comments, and someone asks you to rebase your pull request. Here's what you do:

Add the official repo as a remote (first time only)

A "remote", in git terminology, is another clone of the repository that you can share commits with. When you forked the official edx/edx-platform project, you created a new git repository called my-username/edx-platform, but these two repositories can share commits with each other.

To add upstream as a remote, run this command in your repository:

$ git remote add upstream https://github.com/edx/edx-platform.git

You can verify that you did this successfully by running $ git remote -v: you should see upstream in the list of remotes. Note: this step only needs to be performed once per clone.

Fetch the latest version of master

Your computer needs to download information from Github about the official repo, so that it knows what's on the latest version of the master branch. Once you've got your remote set up, this is simple. Run this command in your repository:

$ git fetch upstream

Squash your changes (optional)

This step is optional, but recommended. It involves taking all the commits you've made on your branch, and squashing them all into one, larger commit. Doing this makes it easier for you to resolve conflicts while performing the rebase, and easier for us to review your pull request.

To do this, we're going to do an interactive rebase. First, find the commit that is base of your branch. You can do this by running:

$ git merge-base my-branch master

That command will return a commit hash. Use that commit hash in constructing this next command:

$ git rebase --interactive ${HASH}

Note that you should replace ${HASH} with the actual commit hash from the previous command. For example, if your merge base is abc123, you would run $ git rebase --interactive abc123. Your text editor will open with a file that lists all the commits in your branch, and in front of each commit is the word "pick". It looks something like this:

pick 1fc6c95 do something
pick 6b2481b do something else
pick dd1475d changed some things
pick c619268 fixing typos

For every line except the first, you want to replace the word "pick" with the word "squash". It should end up looking like this:

pick 1fc6c95 do something
squash 6b2481b do something else
squash dd1475d changed some things
squash c619268 fixing typos

Save and close the file, and a moment later a new file should pop up in your editor, combining all the commit messages of all the commits. Reword this commit message as you want, and then save and close that file as well. This commit message will be the commit message for the one, big commit that you are squashing all of your larger commits into. Once you've saved and closed that file, your commits have been squashed together, and you're done with this step!

Perform the rebase

Run this command in your repository:

$ git rebase upstream/master

Git will start replaying your commits onto the latest version of master. You may get conflicts while doing so: if you do, git will pause and ask you to resolve the conflicts before continuing. This is exactly like resolving conflicts with a merge: you can use git status to see which files are in conflict, edit the files to resolve the conflicts, and then use git add to indicate that the conflicts have been resolved. However, instead of running git commit, you instead want to run git rebase --continue to indicate to git that it should continue replaying your commits. If you've squashed your commits before doing this step, you'll only have to pause to resolve conflicts once -- if you didn't squash, you may have to resolve your conflicts multiple times.

Force-push to update your pull request

As explained above, when you do a rebase, you are changing the history on your branch. As a result, if you try to do a normal git push after a rebase, git will reject it because there isn't a direct path from the commit on the server to the commit on your branch. Instead, you'll need to use the -f or --force flag to tell git that yes, you really know what you're doing. When doing force pushes, it is highly recommended that you set your push.default config setting to simple, which is the default in Git 2.0. To make sure that your config is correct, run:

$ git config --global push.default simple

Once it's correct, you can just run:

$ git push -f

And check your pull request. It should be updated!