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Contribution Guide

The Go project welcomes all contributors.

This document is a guide to help you through the process of contributing to the Go project, which is a little different from that used by other open source projects. We assume 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 checklist of the required steps to follow:

If you prefer, there is 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 instructions. 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 use the same account throughout the process and for all your subsequent 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. You might want to discuss this topic with your employer before deciding which account to use.

Google accounts can either be Gmail e-mail 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 e-mail address.

You also need to make sure that your Git tool is configured to create commits using your chosen e-mail address. You can either configure Git 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 at 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 mail to the golang-dev mailing list. This will let us know the situation so 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 will be taken to a page with the title "Configure Git". This page contains a personalized script that when run locally will configure Git to hold your unique authentication key. This key is paired with one that is generated and stored on the server, 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. If you are using a Windows computer and running cmd, you should instead follow the instructions in the yellow box to run the command; otherwise run the regular script.

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 simplifies sending 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 just copy the executable from $GOPATH/bin to this directory.

Before contributing code

The project welcomes submissions but to make sure things are well coordinated we ask that everyone to discuss any significant changes to the Go repositories before starting work. Best practice is to connect your work to the issue tracker, either by filing a new issue or by claiming 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 your plans. 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 documentation updates are accepted. New contributions can be sent during a feature freeze but will not be accepted until the freeze is over.

Changes in general other than bug and documentation fixes must go through the change proposal process before they can be accepted.

Sensitive security-related issues (only!) 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 called Gopherbot has been created to sync GitHub pull requests to Gerrit.

Open a pull request as you normally would. 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 section 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 can check out the Go source repo onto your local file system anywhere you want as long as it's outside your GOPATH. Either clone from go.googlesource.com or from 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. The git codereview change command will automatically add a unique 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. A Change-Id looks like this:

Change-Id: I2fbdbffb3aab626c4b6f56348861b7909e3e8990

The tool 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 while preserving the Change-Id.

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 your 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 directory holding the Go tree for the bootstrap compiler.)

After running for a while and printing a lot of testing output, the command should finish by printing,

ALL TESTS PASSED

You can use make.bash instead of all.bash to just build the compiler and standard packages without running the test suite. Once the go tool is built, it will be installed as bin/go under the directory in which you cloned the Go repository, and you can run it directly from there. See also the section on how to test your changes quickly.

Step 4: Send changes for review

Once the change is ready and tested over the whole tree, send it for review. This is done with 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 to it in the comments.

Step 5: Revise changes after a review

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

If you need to revise your change after the review, 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.

Again, 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.

Good commit messages

Commit messages in Go follow a specific set of conventions, which we discuss in this section.

Here 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 short one-line summary of the change, prefixed by the primary affected package.

A rule of thumb is that it should be written so to complete the sentence "This change modifies Go to _____." That means it does not start with a capital letter, is not a complete sentence, and actually summarizes the result of the change.

Follow the first line by a blank line.

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. Don't use HTML, Markdown, or any other markup language.

Add any relevant information, such as benchmark data if the change afects performance. The benchcmp tool is conventionally used to format benchmark data for change descriptions.

Referencing issues

The special notation "Fixes #12345" associates the change with issue 12345 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 #12345". 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, not the subrepository. All issues are tracked in the main repository's issue tracker. The correct form is "Fixes golang/go#159".

The review process

This section explains the review process in detail and how to approach reviews after a change has been mailed.

Common beginner mistakes

When a change is sent to Gerrit, it is usually triaged within a few days. A maintainer will have a look and provide some initial review that for first-time contributors usually focuses on basic cosmetics and common mistakes. These include things like:

Trybots

After an initial reading of your change, maintainers will trigger trybots, a cluster of servers that will run the full test suite on several different architectures. Most trybots complete in a few minutes, at which point 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, update your patch to fix it, and upload again. 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 reported by the trybot doesn't seem related to your patch, go to the Build Dashboard and check if the same failure appears in other 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 understand the situation.

Reviews

The Go community values very thorough reviews. Think of each review 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 the review comments and make sure to reply to 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, or what you have done instead.

It is perfectly normal for changes to go through several round of reviews, with one or more reviewers making new comments every time and then waiting for an updated change before reviewing again. This cycle happens even for experienced contributors, so don't be discouraged by it.

Voting conventions

As they near a decision, reviewers will make a "vote" on your change. The Gerrit voting system involves an integer in the range -2 to +2:

Submitting an approved change

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

The two steps (approving and submitting) are separate because in some cases maintainers may want to approve it but not to submit it right away (for instance, the tree could be temporarily frozen).

Submitting a change checks it into the repository. The change description will include a link to the code review, which will be updated with a link to the change in the repository. Since the method used to integrate the changes is Git's "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 more about the review process.

Miscellaneous topics

This section collects a number of other comments that are outside the issue/edit/code review/submit process itself.

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.

(Use the current year if you're reading this in 2019 or beyond.) Files in the repository are copyrighted 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 e-mail 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 configure Git for this repository to use the e-mail address that you registered with. To change the e-mail address for this doesn't happen again, run:

$ git config user.email email@address.com

Then change the commit to use this alternative e-mail address with this command:

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

Then retry by running:

$ 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 compile and test only the package you are developing.

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 e-mail 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 codereview sync

(Under the covers this 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 will look something like this:

$ git fetch https://go.googlesource.com/review refs/changes/21/13245/1 && 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 define these aliases. 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

Advanced users may want to stack up related commits in a single branch. Gerrit allows for changes to be dependent on each other, forming such a dependency chain. Each change will need to be approved and submitted separately but the dependency will be visible to reviewers.

To send out 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.