I want to take a moment to elaborate on what makes a well formed commit
message. I think the best practices for commit message formatting is
one of the little details that makes Git great. Understandably, some of
the first commits to rails.git have messages of the really-long-line
variety, and I want to expand on why this is a poor practice.
Here’s a model Git commit message:
Capitalized, short (50 chars or less) summary
More detailed explanatory text, if necessary. Wrap it to about 72
characters or so. In some contexts, the first line is treated as the
subject of an email and the rest of the text as the body. The blank
line separating the summary from the body is critical (unless you omit
the body entirely); tools like rebase can get confused if you run the
two together.
Write your commit message in the imperative: "Fix bug" and not "Fixed bug"
or "Fixes bug." This convention matches up with commit messages generated
by commands like git merge and git revert.
Further paragraphs come after blank lines.
- Bullet points are okay, too
- Typically a hyphen or asterisk is used for the bullet, followed by a
single space, with blank lines in between, but conventions vary here
- Use a hanging indent
Let’s start with a few of the reasons why wrapping your commit messages
to 72 columns is a good thing.
git log doesn’t do any special special wrapping of the commit
messages. With the default pager of less -S, this means your
paragraphs flow far off the edge of the screen, making them difficult
to read. On an 80 column terminal, if we subtract 4 columns for the
indent on the left and 4 more for symmetry on the right, we’re left
with 72 columns.
git format-patch --stdout converts a series of commits to a series
of emails, using the messages for the message body. Good email
netiquette dictates we wrap our plain text emails such that there’s
room for a few levels of nested reply indicators without overflow in
an 80 column terminal. (The current rails.git workflow doesn’t
include email, but who knows what the future will bring.)
Vim users can meet this requirement by installing my vim-git runtime
files, or by simply setting the
following option in your git commit message file:
:set textwidth=72
For Textmate, you can adjust the “Wrap Column” option under the view
menu, then use ^Q to rewrap paragraphs (be sure there’s a blank line
afterwards to avoid mixing in the comments). Here’s a shell command to
add 72 to the menu so you don’t have to drag to select each time:
More important than the mechanics of formatting the body is the practice
of having a subject line. As the example indicates, you should shoot
for about 50 characters (though this isn’t a hard maximum) and always,
always follow it with a blank line. This first line should be a concise
summary of the changes introduced by the commit; if there are any
technical details that cannot be expressed in these strict size
constraints, put them in the body instead. The subject line is used all
over Git, oftentimes in truncated form if too long of a message was
used. The following are just a handful of examples of where it ends up:
git log --pretty=oneline shows a terse history mapping containing
the commit id and the summary
git rebase --interactive provides the summary for each commit in the
editor it invokes
if the config option merge.summary is set, the summaries from all
merged commits will make their way into the merge commit message
git shortlog uses summary lines in the changelog-like output it
produces
git format-patch, git send-email, and related tools use it as the
subject for emails
reflogs, a local history accessible with git reflog intended to help
you recover from stupid mistakes, get a copy of the summary
gitk has a column for the summary
GitHub uses the summary in various places in their user interface
The subject/body distinction may seem unimportant but it’s one of many
subtle factors that makes Git history so much more pleasant to work with
than Subversion.
In our previous post in the series, we took a look at how a Rails
core committer could best pull in a branch submitted by a contributer.
Today, we’re going to be looking at things from the opposite side: How
a contributer can best maintain a branch intended for pulling.
Note that contrary to the initial assumptions of many, git pull is not
necessarily the be-all end-all solution for getting your work included
upstream. For simple bug fixes and features, it’s generally easier for
all involved if you submit git format-patch --stdout output. The real
gains of a merge based workflow come when you start to collaborate with
others on a topic branch. Having said that, let’s get on with the
example.
Let’s start with the basics. We’ll be using GitHub for the upstream
repository in this example. If you haven’t already done, so grab the
clone the latest upstream over at GitHub.
$ git clone git://github.com/rails/rails.git
$ cd rails
Next, set up a public repository. With GitHub, this is straightforward:
sign in and click the fork button on the rails/rails.git repository
page. Once that’s done, take note of
your push URL and add it to your repository. Here, we use the name
mine, to distinguish it from the origin remote which is the upstream
we cloned from.
With that set up, we can get to work. Start by checking out a branch
for your feature. Below, we use git fetch to download the latest
updates, then create a new branch my_wonderful_feature based off the
master branch of the remote origin, which is our upstream.
Obviously, unless it’s a bug fix or feature targeted for the stable
line, we should start by building off the tip of the upstream master
branch, right? Well, not necessarily; that’s really a holdover thought
process from Subversion. There are several advantages to picking a
recent tag like v2.0.2 and building off of that instead:
No churn of constantly updating to the latest.
Not having to worry about breakages in the latest edge. If a test
breaks, you know (in theory) that you’re the one that broke it.
People can easily test your changes in their application without
testing everything else introduced by the particular revision of
edge that you forked.
Of course, this is a trade off, as there are advantages to working on
edge as well:
Get to leverage the very latest infrastructure changes in Rails.
Lesser chance of something that works in your version breaking once
it’s merged into a later revision.
The right choice depends both on your feature and the current state of
the upstream. Given that the last release of Rails happened back in
Subversion and the next is just around the corner, it generally makes
sense right now to base things off of a recent commit. However, once
2.1 hits, I’d seriously advise people to consider branching off of
v2.1.0 instead. Even if you do choose the edge route, don’t be afraid
to back up a few revisions if the current one has broken tests or
otherwise doesn’t suit your needs.
Okay, so you’ve thought it through and decided on origin/master for
the my_wonderful_feature branch you’re implementing. You’ve created
your branch, done a few commits, and you’re ready to publicize it.
Using the mine remote we created earlier, this is a simple process:
$ git push mine my_wonderful_feature
If you want the public branch name to be named differently, that’s a
snap too:
With that done, your branch is ready to be pulled. Here’s the command a
core committer would use to merge your work, which can be mentioned in
the Lighthouse ticket, or on #rails-contrib, or however you want to
communicate it to the powers that be:
Great. So a few days later, you get some feedback, and decide to do
some further work on your branch. If you’re a Subversion veteran, your
first reaction is probably to update to the latest edge before
continuing:
$ git pull
$ git merge origin/master
But wait! Is there a reason you need to update? Frequently updating
complicates the history graph and adds noise to the logs:
Merge branch 'master' of git://github.com/rails/rails
There are certainly times when it’s appropriate to merge the latest
upstream, like if there are conflicts that need to be resolved before it
can be added upstream. When these times come, go ahead and merge, but
give a descriptive commit message:
$ git merge v2.0.3 -m 'Synchronize with 2.0.3 release'
$ git merge origin/master -m 'Leverage new Rails.public_path method'
$ git merge origin/master -m 'Resolve conflicts with upstream'
If you’d like to try merging (for example, to test that tests still
pass) but discard it when you’re done (for example, you’re planning on
doing more work on the branch before publishing), use the following
pattern. HEAD@{1} means “the previous commit referenced by HEAD”,
that is, the commit before the merge.
All right, you made the needed changes and your contribution was merged
upstream. Here’s the syntax to delete a branch from your remote
repository so it doesn’t clutter things up:
Welcome to my series of articles providing example workflows for working
with the Rails core Git repository. My first post is designed for Rails
core committers and explains one way to deal with contributions that
come in the form of a Git URL and a branch name.
Based on my interactions with the core, I’d deduced the following are
common requirements when merging a contributor’s work, and modeled my
example around them:
Preserve history: don’t squash to a single commit
Provide a paper trail of which core committer merged the contribution
Add a CHANGELOG entry on top
First, a recommended configuration option. This will add a short
summary of the changes a merge introduces to the commit message.
$ git config --global merge.summary true
On to the merge. The first thing we want to do is bring our master
branch up to date. The assumption here is that we don’t have any local
work done in master: it’s a clean branch tracking the upstream, so
that git pull will simply fast forward the latest changes.
We use --no-ff to disallow fast forwarding, even if the
remote branch is up to date. This ensures there will always be a merge
commit, providing for the paper trail requirement mentioned earlier.
The --no-commit option gives us a chance to alter the work
tree as part of the merge commit. Putting hefty changes here would be
confusing, but it is the perfect chance to make CHANGELOG
edits.
Once the CHANGELOG is to your liking, we are ready to finalize it.
The commit message is already in place but feel free to replace it if it
doesn’t give a clear picture of what changed.
The Ruby on Rails core is now hosted on Git This is great news for
Git fans like myself. For those of Rails core contributors who are
coming late to the party, here’s a quick list of tips I’ve put together
especially for you. This no substitute for a proper tutorial but
rather a Rails biased supplement to one.
The first thing you do should be configure a real name and email. By
default, Git chooses a default name based on the GECOS data (which is
probably right) and a default email based on your login and hostname
(which is almost certainly wrong). Best practices dictate you use
your real name and email here, not your login, IRC handle, or any
other aliases you may have. These fields will be immortalized in the
repository history so make sure you get them right.
While you’re configuring, you may want to enable coloring for some
commands:
$ git config --global color.diff auto
$ git config --global color.status auto
$ git config --global color.branch auto
$ git config --global color.interactive auto
While Git will accept just about any commit message you feed to it,
sticking to best practices makes the log a lot easier to work with. A
model commit message is shown below.
Short (50 chars or less) summary of changes
More detailed explanatory text, if necessary. Wrap it to about 72
characters or so. In some contexts, the first line is treated as the
subject of an email and the rest of the text as the body. The blank
line separating the summary from the body is critical (unless you omit
the body entirely); tools like rebase can get confused if you run the
two together.
Write your commit message in the present tense: "Fix bug" and not "Fixed
bug." This convention matches up with commit messages generated by
commands like git merge and git revert.
Further paragraphs come after blank lines.
- Bullet points are okay, too
- Typically a hyphen or asterisk is used for the bullet, preceded by a
single space, with blank lines in between, but conventions vary here
As far as submitting your work to the Rails core, the workflow here is
still being fleshed out. For now, either give a public URL and branch
where your contribution can be found, or use the following series of
commands to get a file that can be easily applied by anyone with the
git am command to reconstruct your history locally.
Here’s a tip for keeping up to date: In lieu of using git pull to
download the latest changes, use git pull --rebase. Instead of
cluttering the history with a merge commit, it reapplies your changes to
the latest upstream. The only caveat is that you shouldn’t use this
method if you’ve already published the changes to another repository.
Doing so would cause problems for anyone who has already downloaded the
original commits.
