Contributing to Chromium: an illustrated guide

I gave a talk about how to get started contributing to Chromium, but it wasn’t recorded, and my slides by themselves look like cold-medicine induced hallucinations (which, to be fair, they were). So instead, here is a giant blog post that will take you through every step from “checking out the code” to “landing the code in the Chromium repo”. It will also come in super handy for mild to moderate cases of insomnia.

If you just want a TL;DR or a refresher of the commands you might need, check out the slides. They’re basically bullet points without the running commentary.

Warning: this is a long post. The bug we’re fixing is silly, but will get us writing actual Chromium code. If you want a real good-first-bug to fix after this, here is a nice list. Usually unassigned bugs (with no owner) are free for the taking, but it can also happen that a bug will be assigned to a human who is not actually working on it. Check the activity on it – if there haven’t been any activities in a while, leave a message on the bug or ping the owner and tell them you’d like to work on it!

Get your computer ready

Chrome is giant. It needs a beefy machine (we recommend a 64-bit OS, with at least 8GB of RAM. A separate SSD to hold/build your code will make your life infinitely more pleasant), and a couple dozen goat sacrifices. Even then, building Chromium from scratch is slow. Snails run the half mile faster (fact). This is something you might as well get used to.

We have a pretty solid set of instructions on how to get everything set up. I promise you this page has been used and reviewed a billion times, it’s up to date, and every step in it is important. Don’t skip steps because you think you don’t need them. You do.

However, I’ll tell you about the custom things that I use that aren’t on that page, which I’m pretty proud of.

Edit (Aug 9, 2016): Chromium just switched to gn, which means some of the comments below don’t apply. Check these mac build instructions for how to make your build fast again, using gn. Edit (Feb 17, 2016): I wrote a detailed answer that contains a whole bunch of extra tricks to make builds faster.

Get your body ready

Chromium has a code style. Do not panic if your first review will have 20 comments that are code style nits. It’s absolutely normal, and nobody thinks less of you. On the contrary, we try to be extra picky with new people, so that they learn everything as quickly as possible.

Chromium is hard. I’ve been working on it for two years, and it’s still hard. There’s a loooot of code, and you’ll spend a fair bit of time looking for the right bit of code you care about. Don’t be afraid to ask questions if you’re stuck. It took me forever not to be scared of asking questions, but it turns out all the people that told me that everyone is nice and helpful were right: everyone IS nice and helpful, because at some point they were you, the code was as scary then as it is now, and the compiler has never stopped barfing errors since the day it was born.

OMG let’s write some code!

We’re going to add a button to Chrome. It’s going to be the best thing ever (second to a freshly opened can of Pringles). For realsies. It will be in this bubble here, and it will open [redacted] (2019 update: I let that URL expire and now it apparently points to nsfw things so uhhhh don’t click it.) in a new tab. This is the before and after:

The before image has 2 buttons in the Chrome profile menu, 'switch person' and 'go incognito'. The after image has a third button, 'Can I have a pony?', with a star icon, that has been inserted above the other 2 buttons.

(side note: if you don’t see that button in your dev build of Chromium, launch it with --enable-new-avatar-menu. The UI is enabled by default on all of the released Chromes through a server side flag, but that bit of magic doesn’t run on dev builds, so you need to turn it on yourself)

I chose this dialog because the easiest way to find your way through code is for there to be a searchable string in there like “Switch Person”. Also I wrote this bubble, so it’s Pretty Clutch™.

0. Make a branch

First things first: always create a new branch for every bug/feature/bit of code you’re working on. Working directly on the master branch is bad news bears: 1) it’s very unlikely you’re working on one thing at a time, 2) pulling new code from the remote master to your local repo becomes an adventure. .TL; DR: don’t work on master evar. So,

(´ ▽`).。o♡ src on master ❥ git checkout -b add-pony-button origin/master
Branch add-pony-button set up to track remote branch master from origin by rebasing.
Switched to a new branch 'add-pony-button'
(´ ▽`).。o♡ src on add-pony-button ❥

1. Find the code

Hooman, meet codesearch. It’s your best friend in Chromium. It knows where all the codes are and who they’re called by, and where interfaces get implemented. I spend so much time with it, I’ll probably send it a Valentine’s Day card this year. Anyway, search for “Switch Person” in there, and get these results

The search gives two results, described below

First, generated_resources.grd is where most of the strings in Chrome live. A giant file makes internationalization sooper easy – you hand out the file to translators, they give you back the same file in a different language, and at startup, Chrome decides which file to load based on its locale. Bingo bango, localized UI.

Some of the results have ACCESSIBLE_NAME in them, which means that they’re accessibility strings (hint: they’re read out loud by VoiceOver apps). IDS_PROFILES_SWITCH_USERS_BUTTON looks promising though, so let’s see where it’s used.

The files that are relevant and appear in the search are '' and ''

Aside from the generated_resources.grd results from before, we have two new files!

I’m doing this demo on the Mac, so let’s look at I’ve written both of these files, so I promise you they’re SUPER similar.

2. Adding a button

Ok, so now I’m looking at and here’s how my brain would start nomming this code: that string ID is used in a button that lives in a method called -createOptionsViewWithRect:. This method is called by -buildProfileChooserView:, which in turn is called by -initMenuContentsWithView:. You can go down this rabbit hole for days, but the basic idea is that this is clearly the place where we draw buttons in this bubble.

If we look at -createOptionsViewWithRect: in particular, it does the following:

Hey! We should do the same thing! Let’s add our ponyButton right below the switchUsersButton (which, again, means it’s being drawn above it ARE YOU HAVING FUN YET???). The highlighted bits are the new code.

Code showing how to instantiate a pony button

The code we just wrote says that when you click on the ponyButton, we call a method called -goPoniesGo:. We should probably write it, so that we can actually test our code. It will only log something to the console for now, because logging code is the best code.

Code showing how to create the click handler, and add a logging statement to it

If you build and run this, your bubble should look like the “after” image described before, and clicking the button should spew things on the console.

3. Making the button go

This bit is a leap of faith. We want to open a URL in a new tab, but we don’t really know how. If you search for things like open in new tab, you can hope to hit some comments, but tabs are kind of like the prom queen of the browser so you’re going to get a crap load of useless results. Unfortunately for us, I know that we’re looking for a method called chrome::ShowSingletonTab (in chrome/browser/ui/ Had I not known this, I think I would have found it, for example, by checking how the “Settings” item in the hot dog menu (or hamburger menu, call it whatever food you wish) opens the “chrome://settings” tab. It will take some digging.

If you don’t know how to use ShowSingletonTab(), I would codesearch again for different uses of the function. This time, just by looking at the method signature, we can figure out we should write:

Code showing how to add a line to the click handler to open a new tab

Because the stars aligned and Mercury wasn’t in retrograde, we had all of the .h header files already included for this to work. Compile it, run it, and get a pony!

Send your code for review

I don’t know about you, but I’m preeeeeeetty proud of this feature, so I feel we’re ready to send it for review! Run git commit -am "added pony button" to commit this file, and git cl upload to upload it to codereview.

In your git cl upload message, write a meaningful description, a crbug ID, and a blurb about how to test this. This is what I would write:

Sample CL description. It has a title which is less than 72 characters, a summary, a crbug id, and detailed testing instructions

And if that goes well, this is what your CL (stands for change-list. Comes from the dark days of Perforce) should look like on the site! (that CL doesn’t exist anymore, but here’s a random CL as an example)

Sample CL as it renders on codereview

If you want to test that your CL didn’t break anything, run git cl try. This will look at what your code touches and run a whole bunch of tests on a whole bunch of platforms.

In Chromium, code lands only after it’s been LGTM-ed, which means that someone has reviewed it and gave you the thumbs up. If you don’t know to whom to send it for review, pick someone from your file’s OWNERS. In this case, look at the OWNERS file in chrome/browser/ui/cocoa. Owners are people who are responsible for the code, so they tend to know it best. If they’re too busy for a review or aren’t entirely familiar with your particular part of the code, they can direct you to a better reviewer.

(Side note: please don’t actually send this pony code out for review. People will be very confused, and not necessarily amused.)

Ship it, squirrel! :shipit:

When your CL is reviewed and ready to go, all you have to do is check the “Commit” checkbox, and the commit-queue bots will take care of it. This means that they will run a whole bunch of unit tests again, and if they all pass (or “come up green”), merge your code into the current master branch.

Oh noes, TROUBLE

Sometimes, something goes wrong (or even better, horribly wrong). Your bots could come up red, and then you’ll get an email from the commit-bot telling you your CL couldn’t land, because you done bungled some tests. This is totally fine – you didn’t break anything yet, because your code wasn’t merged. Either your tests are actually broken, or some code got committed before you and you need to rebase, or maybe you’ve just encountered a flaky test and don’t need to do anything. You can go back to your CL, fix your tests, and re-check the “Commit” box.

Sometimes you’ll even get to break the tree. I recently tried to land a change where all my bots were green and as soon as the change landed, it broke 165 tests on each bot. Suuuup.

It happens. You can revert your CL if you realize this in time, or that day’s sheriff might do it for you (especially if you’re an external committer, aren’t on IRC, etc). In this case, be nice to the sheriff and apologize a bit. Maybe send them a gif. Remember: if you’re stuck, ask for help!

Good first bugs

We have a list of bugs deemed as “good starter bugs”. Sometimes they’re more complicated that we thought, so don’t panic if that’s the case. It’s not you, it’s Chrome :). Protip: in the statuses, “Assigned” with a name in the owner means someone is actually looking at that issue, so it’s probably not a great one to pick.

Your turn!

That’s it! That’s how you commit code to Chromium! Good luck, and if you do end up landing a CL, send me an email or a tweet. I’d love to see it!


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