The Go project welcomes all contributors. The process of contributing to the Go project may be different than many projects you are used to. This document is intended as a guide to help you through the contribution process. This guide assumes you have a basic understanding of Git and Go.

In addition to the information here, the Go community maintains a CodeReview wiki page. Feel free to contribute to the wiki as you learn the review process.

Note that the gccgo front end lives elsewhere; see Contributing to gccgo.

Becoming a contributor

Overview

The first step is registering as a Go contributor and configuring your environment. Here is a very quick checklist of the required steps, that you will need to follow:

If you prefer, we have an automated tool that walks through these steps. Just run: 

$ go get -u golang.org/x/tools/cmd/go-contrib-init
$ cd /code/to/edit
$ go-contrib-init

The rest of this chapter elaborates on these steps. If you have completed the steps above (either manually or through the tool), jump to Making a change.

Step 0: Select a Google Account

A contribution to Go is made through a Google account, with a specific e-mail address. Make sure to pick one and use it throughout the process and for all your contributions. You may need to decide whether to use a personal address or a corporate address. The choice will depend on who will own the copyright for the code that you will be writing and submitting. Consider discussing this with your employer.

Google Accounts can either be Gmail email accounts, G-Suite organization accounts, or accounts associated with an external e-mail address. For instance, if you need to use an existing corporate e-mail that is not managed through G-Suite, you can create an account associated with your existing email address.

You also need to make sure that git is configured to author commits using the same e-mail address. You can either configure it globally (as a default for all projects), or locally (for a single specific project). You can check the current configuration with this command: 

$ git config --global user.email  # check current global config
$ git config user.email           # check current local config

To change the configured address:

$ git config --global user.email name@example.com   # change global config
$ git config user.email name@example.com            # change local config

Step 1: Contributor License Agreement

Before sending your first change to the Go project you must have completed one of the following two CLAs. Which CLA you should sign depends on who owns the copyright to your work.

You can check your currently signed agreements and sign new ones, through the Google Developers Contributor License Agreements website. If the copyright holder for your contribution has already completed the agreement in connection with another Google open source project, it does not need to be completed again.

If the copyright holder for the code you are submitting changes — for example, if you start contributing code on behalf of a new company — please send email to golang-dev and let us know, so that we can make sure an appropriate agreement is completed and update the AUTHORS file.

Step 2: Configure git authentication

Go development happens on go.googlesource.com, a git server hosted by Google. Authentication on the web server is made through your Google account, but you also need to configure git on your computer to access it. Follow this steps:

  1. Visit go.googlesource.com and click on "Generate Password" in the page's top right menu bar. You will be redirected to accounts.google.com to sign in.
  2. After signing in, you are taken to a page with the title "Configure Git". This page contains a personalized script that when run locally will configure git to have your unique authentication key. This key is paired with one generated server side, analogous to how SSH keys work.
  3. Copy and run this script locally in your command line terminal, to store your secret authentication token in a .gitcookies file. (On a Windows computer using cmd you should instead follow the instructions in the yellow box to run the command. If you are using git-bash use the same script as *nix.).

Step 3: Create a Gerrit account

Gerrit is an open-source tool used by Go maintainers to discuss and review code submissions.

To register your account, visit go-review.googlesource.com/login/ and sign in once using the same Google Account you used above.

Step 4: Install the git-codereview command

Changes to Go must be reviewed before they are accepted, no matter who makes the change. A custom git command called git-codereview, discussed below, helps to send changes to Gerrit.

Install the git-codereview command by running, 

$ go get -u golang.org/x/review/git-codereview

Make sure git-codereview is installed in your shell path, so that the git command can find it. Check that 

$ git codereview help

prints help text, not an error.

On Windows, when using git-bash you must make sure that git-codereview.exe is in your git exec-path. Run git --exec-path to discover the right location then create a symbolic link or simply copy the executable from $GOPATH/bin to this directory.

Before contributing code

The project welcomes submissions but please let everyone know what you're working on if you want to change or add to the Go repositories.

Before undertaking to write something new for the Go project, please file an issue (or claim an existing issue).

Check the issue tracker

Whether you already know what contribution to make, or you are searching for an idea, the issue tracker is always the first place to go. Issues are triaged to categorize them and manage the workflow.

Most issues will be marked with one of the following workflow labels:

Open an issue for any new problem

Excluding very trivial changes, all contributions should be connected to an existing issue. Feel free to open one and discuss what your plans are. This process gives everyone a chance to validate the design, helps prevent duplication of effort, and ensures that the idea fits inside the goals for the language and tools. It also checks that the design is sound before code is written; the code review tool is not the place for high-level discussions.

When planning work, please note that the Go project follows a six-month development cycle. The latter half of each cycle is a three-month feature freeze during which only bug fixes and doc updates are accepted. New contributions can be sent during a feature freeze but will not be accepted until the freeze thaws.

Significant changes must go through the change proposal process before they can be accepted.

Sensitive security-related issues should be reported to security@golang.org.

Sending a change via GitHub

First-time contributors that are already familiar with the GitHub flow are encouraged to use the same process for Go contributions. Even though Go maintainers use Gerrit for code review, a bot has been created to sync GitHub pull requests to Gerrit.

Open a pull request as you would normally do. Gopherbot will automatically sync the code and post a link to Gerrit. When somebody comments on the change, it will be posted in the pull request, so you will also get a notification.

Some things to keep in mind:

Sending a change via Gerrit

It is not possible to fully sync Gerrit and GitHub, at least at the moment, so we recommend learning Gerrit. It's different but powerful and familiarity with help you understand the flow.

Overview

This is an overview of the overall process:

The rest of this chapter describes these steps in more detail.

Step 1: Clone the Go source code

In addition to a recent Go installation, you need to have a local copy of the source checked out from the correct repository. You should check out the Go source repo anywhere you want as long as it's outside of your GOPATH. Either clone from go.googlesource.com or GitHub: 

$ git clone https://github.com/golang/go   # or https://go.googlesource.com/go
$ cd go

Step 2: Prepare changes in a new branch

Each Go change must be made in a separate branch, created from the master branch. You can use the normal git commands to create a branch and add changes to the staging area: 

$ git checkout -b mybranch
$ [edit files...]
$ git add [files...]

To commit changes, instead of git commit, use git codereview change. 

$ git codereview change
(open $EDITOR)

You can edit the commit description in your favorite editor as usual. git codereview change will automatically add a Change-Id line near the bottom. That line is used by Gerrit to match successive uploads of the same change. Do not edit or delete it. This is an example: 

commit fef82cf89a34935a41bd0e3c1e0c2d9d6de29ee2 (HEAD -> test)
Author: Giovanni Bajo 
Date:   Tue Feb 13 01:07:15 2018 +0100

    cmd/compile: test

    Change-Id: I2fbdbffb3aab626c4b6f56348861b7909e3e8990

git codereview change also checks that you've run go fmt over the source code, and that the commit message follows the suggested format.

If you need to edit the files again, you can stage the new changes and re-run git codereview change: each subsequent run will amend the existing commit.

Make sure that you always keep a single commit in each branch. If you add more commits by mistake, you can use git rebase to squash them together into a single one.

Step 3: Test changes

You've written and tested your code, but before sending code out for review, run all the tests for the whole tree to make sure the changes don't break other packages or programs: 

$ cd go/src
$ ./all.bash

(To build under Windows use all.bat; this also requires setting the environment variable GOROOT_BOOTSTRAP to the bootstrap compiler)

After running for a while, the command should print: 

"ALL TESTS PASSED".

Notice that you can use make.bash instead of all.bash to just build the compiler without running the testsuite. Once the compiler is built, you can run it directly from <GOCLONEDIR>/bin/go; see also the section on quickly test your changes.

Step 4: Send changes for review

Once the change is ready, send it for review. This is done via the mail sub-command which despite its name, doesn't directly mail anything, it just sends the change to Gerrit: 

$ git codereview mail

Gerrit assigns your change a number and URL, which git codereview mail will print, something like: 

remote: New Changes:
remote:   https://go-review.googlesource.com/99999 math: improved Sin, Cos and Tan precision for very large arguments

If you get an error instead, check the Troubleshooting mail errors section.

If your change relates to an open GitHub issue and you have followed the suggested commit message format, the issue will be updated in a few minutes by a bot, linking your Gerrit change in it.

Step 5: Revise changes after a review

Go maintainers will review your code on Gerrit, and you will get notifications via email. You can see the review on Gerrit, and comment on them. You can also reply via email if you prefer.

When you're ready to revise your submitted code, edit the files in correct branch, add them to the git staging area, and then amend the commit with git codereview change: 

$ git codereview change     # amend current commit
(open $EDITOR)
$ git codereview mail       # send new changes to Gerrit

If you don't need to change the commit description, just save and exit from the editor. Remember not to touch the special Change-Id line.

Make sure that you always keep a single commit in each branch. If you add more commits by mistake, you can use git rebase to squash them together into a single one.

Writing good commit messages

Commit messages in Go follow a specific convention. Read this chapter to learn more about it. This is an example of a good one: 

math: improve Sin, Cos and Tan precision for very large arguments

The existing implementation has poor numerical properties for
large arguments, so use the McGillicutty algorithm to improve
accuracy above 1e10.

The algorithm is described at http://wikipedia.org/wiki/McGillicutty_Algorithm

Fixes #159

First line

The first line of the change description is conventionally a one-line summary of the change, prefixed by the primary affected package. Keep the first line as short as possible (ideally, under 76 characters).

It should be written so to complete the sentence "This change modifies Go to _____."

Main content

The rest of the description elaborates and should provide context for the change and explain what it does. Write in complete sentences with correct punctuation, just like for your comments in Go. No Markdown should be used.

Referencing issues

The special notation "Fixes #159" associates the change with issue 159 in the Go issue tracker. When this change is eventually applied, the issue tracker will automatically mark the issue as fixed.

If the change is a partial step towards the resolution of the issue, uses the notation "Updates #159". This will leave a comment in the issue linking back to the change in Gerrit, but it will not close the issue when the change is applied.

If you are sending a change against a subrepository, you must use the fully-qualified syntax supported by GitHub, to make sure the change is linked to the issue in the main repository. The correct form is "Fixes golang/go#159".

The review process

This section explains the review process in details, and how to approach reviews after a change was submitted.

Common beginner mistakes

When a change is submitted to Gerrit, it is usually triaged in the next few days. A maintainer will give a look and submit some initial review, that for first-time contributors usually focus on basic cosmetics and common mistakes. For instance:

Trybots

After an initial reading of your patch, maintainers will trigger trybots, a cluster of servers that will run the full testsuite on several different architectures. Most trybots run complete in a few minutes, and a link will be posted in Gerrit where you can see the results.

If the trybot run fails, follow the link and check the full logs of the platforms on which the tests failed. Try to understand what broke, and update your patch. Maintainers will trigger a new trybot run to see if the problem was fixed.

Sometimes, the tree can be broken on some platforms for a few hours; if the failure in trybot logs doesn't seem related to your patch, go to the Build Dashboard and check if the same failures appears in the recent commits, on the same platform. In this case, feel free to write a comment in Gerrit to mention that the failure is unrelated to your change, to help maintainers understanding the situation.

Reviews

The Go team values very thorough reviews. Consider each line comment like a ticket: you are expected to somehow "close" it by acting on it, either by implementing the suggestion or convincing the reviewer otherwise.

After you update the change, go through line comments and make sure to reply on every one. You can click the "Done" button to reply indicating that you've implemented the reviewer's suggestion; otherwise, click on "Reply" and explain why you have not.

It is absolutely normal for changes to go through several round of reviews, in which the reviewer make new comments every time and then wait for an updated change to be uploaded. This also happens for experienced contributors, so don't feel discouraged by it.

Voting conventions

At some point, reviewers will express a vote on your change. This is the voting convention:

Submitting an approved change

After the code has been +2'ed, an approver will apply it to the master branch using the Gerrit UI. This is called "submission".

The two steps are separate because in some cases maintainers may want to approve it but not to submit it right away (e.g. the tree could be temporarily frozen).

Submission checks the change into the repository. The change description will include a link to the code review, and the code review will be updated with a link to the change in the repository. Since the method used to integrate the changes is "Cherry Pick", the commit hashes in the repository will be changed by the "Submit" operation.

If your change has been approved for a few days without being submitted, feel free to write a comment in Gerrit requesting submission.

More information

In addition to the information here, the Go community maintains a CodeReview wiki page. Feel free to contribute to this page as you learn the review process.

Advanced topics

This section contains more in-depth topics on how to contribute to Go. Read it to get a better understanding of the contribution process.

Files in the Go repository don't list author names, both to avoid clutter and to avoid having to keep the lists up to date. Instead, your name will appear in the change log and in the CONTRIBUTORS file and perhaps the AUTHORS file. These files are automatically generated from the commit logs periodically. The AUTHORS file defines who “The Go Authors”—the copyright holders—are.

New files that you contribute should use the standard copyright header:

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

Files in the repository are copyright the year they are added. Do not update the copyright year on files that you change.

Troubleshooting mail errors

The most common way that the git codereview mail command fails is because the email address in the commit does not match the one that you used during the registration process.
If you see something like... 

remote: Processing changes: refs: 1, done
remote:
remote: ERROR:  In commit ab13517fa29487dcf8b0d48916c51639426c5ee9
remote: ERROR:  author email address XXXXXXXXXXXXXXXXXXX
remote: ERROR:  does not match your user account.

You need to set this repo to use the email address that you registered with. First, let's change the email address for this repo so this doesn't happen again. You can change your email address for this repo with the following command: 

$ git config user.email email@address.com

Then change the commit to use this alternative email address. You can do that with: 

$ git commit --amend --author="Author Name <email@address.com>"

Finally try to resend with: 

$ git codereview mail

Quickly testing your changes

Running all.bash for every single change to the code tree is burdensome. Even though it is strongly suggested to run it before sending a change, during the normal development cycle you may want to quickly compile and locally test your change.

Contributing to subrepositories (golang.org/x/...)

If you are contributing a change to a subrepository, obtain the Go package using go get. For example, to contribute to golang.org/x/oauth2, check out the code by running: 

$ go get -d golang.org/x/oauth2/...

Then, change your directory to the package's source directory ($GOPATH/src/golang.org/x/oauth2), and follow the normal contribution flow.

Specifying a reviewer / CCing others

Unless explicitly told otherwise, such as in the discussion leading up to sending in the change, it's better not to specify a reviewer. All changes are automatically CC'ed to the golang-codereviews@googlegroups.com mailing list. If this is your first ever change, there may be a moderation delay before it appears on the mailing list, to prevent spam.

You can specify a reviewer or CC interested parties using the -r or -cc options. Both accept a comma-separated list of email addresses: 

$ git codereview mail -r joe@golang.org -cc mabel@example.com,math-nuts@swtch.com

Synchronize your client

While you were working, others might have submitted changes to the repository. To update your local branch, run 

$ git sync

(In git terms, git sync runs git pull -r.)

Reviewing code by others

As part of the review process reviewers can propose changes directly (in the GitHub workflow this would be someone else attaching commits to a pull request). You can import these changes proposed by someone else into your local Git repository. On the Gerrit review page, click the "Download ▼" link in the upper right corner, copy the "Checkout" command and run it from your local Git repo. It should look something like this: 

$ git fetch https://go.googlesource.com/review refs/changes/21/1221/1 && git checkout FETCH_HEAD

To revert, change back to the branch you were working in.

Set up git aliases

The git-codereview command can be run directly from the shell by typing, for instance, 

$ git codereview sync

but it is more convenient to set up aliases for git-codereview's own subcommands, so that the above becomes, 

$ git sync

The git-codereview subcommands have been chosen to be distinct from Git's own, so it's safe to do so. To install them, copy this text into your Git configuration file (usually .gitconfig in your home directory): 

[alias]
	change = codereview change
	gofmt = codereview gofmt
	mail = codereview mail
	pending = codereview pending
	submit = codereview submit
	sync = codereview sync

Sending multiple dependent changes

Gerrit allows for changes to be dependent on each other, forming a dependency chain. This is an indication for maintainers to better review your code, even though each change will technically need to be approved and submitted separately.

To submit a group of dependent changes, keep each change as a different commit under the same branch, and then run: 

$ git codereview mail HEAD
Make sure to explicitly specify HEAD, which is usually not required when sending single changes.