Effective Pull Requests

Recently I’ve been sending a lot of pull requests to various GitHub-hosted projects. It had been a lot of trial and error before I settled on the git workflow which doesn’t involve "Nah, I’ll just rm -rf this folder and do a fresh git clone" somewhere. This post documents the workflow. In a nutshell, it is

  • do not use the master branch for pull requests

  • use the master branch to track upstream repository

  • automate

Note that hub utility exist to handle these issues automatically. I personally haven’t used for no real reason, you definitely should check it out!

Avoiding the master branch

The natural thing to do, when sending a pull request, is to fork the upstream repository, git clone your fork locally, make a fix, git commit -am and git push it to the master branch of your fork and then send a PR.

It even seems to work at first, but breaks down in these two cases:

  • You want to send a second PR, and now you don’t have a clean branch to base your work off.

  • The upstream was updated, your PR does not merge cleanly anymore, you need to do a rebase, but you don’t have a clean branch to rebase onto.

The fix here is to create a fresh branch immediately after cloning:

Tip 1: always start with creating a feature branch for PR
$ git clone git@github.com:matklad/cargo.git && cd cargo
$ git checkout -b long-and-descriptive-name-of-the-pr-branch
$ $EDITOR hack-hack-hack

However it is easy to forget this step, so it is important to be able to move to a separate branch after you erroneously committed code to master. It is also crucial to reset master to clean state, otherwise you’ll face some bewildering merge conflicts, when you try to update your fork several days later.

Tip 2: don’t forget to reset master after a mix-up
$ git clone git@github.com:matklad/cargo.git && cd cargo
$ $EDITOR hack-hack-hack
$ git commit -am'A very important fix'
$ echo "urgh, should have done this on a separate branch"

$ git branch pr-branch
$ git reset --hard origin/master
$ git checkout pr-branch

Syncing with upstream

If you work regularly on a particular project, you’d want to keep your fork in sync with upstream repository. One way to do that would be to add upstream repository as a git remote, and set the local master branch to track the master from upstream:

Tip 3: tracking remote repository
$ git clone git@github.com:matklad/cargo.git && cd cargo
$ git remote add upstream git@github.com:rust-lang/cargo.git
$ git fetch remote
$ git branch --set-upstream-to=upstream/master

With this setup, you can easily update your pull request if they don’t merge cleanly because of upstream changes:

Tip 4: updating a PR
$ git checkout master && git pull --rebase
$ git checkout pr-branch
$ git rebase master

Automating

There are several steps to get the repo setup just right, and doing it manually every time would lead to errors and mysterious merge conflicts. It might be useful to define a shell function to do this for you! It could look like this

# calling `gcf rust-lang/cargo` would clone github.com/matklad/cargo,
# and setup upstream properly
function gcf() {
    local userrepo=$1
    local repo=`basename $userrepo`
    git clone git@github.com:matklad/$repo.git
    pushd $repo
    git remote add upstream git@github.com:$userrepo.git
    git fetch upstream
    git checkout master
    git branch --set-upstream-to=upstream/master
    git pull --rebase --force
    popd
}

Bonus points

Bonus 1: another useful function to have is for reviewing PRs:
# called like `gpr 9262`, this function would checkout
# GitHub pull request #9262 to `pr-9262` branch
function gpr() {
    local pr=$1
    git fetch upstream pull/$pr/head:pr-$pr
    git checkout pr-$pr
}
Bonus 2

There are a lot of learning materials about Git out there. However, a lot of these materials are either comprehensive references, or just present a handful of most useful git commands. I’ve once accidentally stumbled upon Git from the bottom up and I highly recommend reading it: it is a moderately long article, which explains the inner mechanics of Git.