<h2 id="Introduction">Introduction</h2>

This document explains how to contribute changes to the Go project.

It assumes you have followed the

<a href="/doc/install/source">installation instructions</a> and

have <a href="code.html">written and tested your code</a>.

(Note that the <code>gccgo</code> frontend lives elsewhere;

see <a href="gccgo_contribute.html">Contributing to gccgo</a>.)

<h2 id="Design">Discuss your design</h2>

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 <a href="https://golang.org/issue/new">file an issue</a>

(or claim an <a href="https://golang.org/issues">existing issue</a>).

Significant changes must go through the

<a href="https://golang.org/s/proposal-process">change proposal process</a>

before they can be accepted.

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

<a href="https://golang.org/wiki/Go-Release-Cycle">six-month

development cycle</a>. The latter half of each cycle is a three-month

feature freeze during which only bug fixes and doc updates are accepted.

New work cannot be submitted during a feature freeze.

<h2 id="Testing">Testing redux</h2>

You've <a href="code.html">written and tested your code</a>, 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 <code>all.bat</code>.)

After running for a while, the command should print

"<code>ALL</code> <code>TESTS</code> <code>PASSED</code>".

<h2 id="Code_review">Code review</h2>

Changes to Go must be reviewed before they are accepted,

no matter who makes the change.

A custom git command called <code>git-codereview</code>,

discussed below, helps manage the code review process through a Google-hosted

<a href="https://go-review.googlesource.com/">instance</a> of the code review

system called <a href="https://code.google.com/p/gerrit/">Gerrit</a>.

<h3>Set up authentication for code review</h3>

Gerrit uses Google Accounts for authentication. If you don't have

a Google Account, you can create an account which

<a href="https://www.google.com/accounts/NewAccount">includes

a new Gmail email account</a> or create an account associated

<a href="https://accounts.google.com/SignUpWithoutGmail">with your existing

email address</a>.

The email address associated with the Google Account you use will be recorded in

the <a href="https://go.googlesource.com/go/+log/">change log</a>

and in the <a href="/CONTRIBUTORS">contributors file</a>.

To set up your account in Gerrit, visit

<a href="https://go.googlesource.com">go.googlesource.com</a>

and click on "Generate Password" in the page's top right menu bar.

You will be redirected to accounts.google.com to sign in.

Once signed in, you are returned back to go.googlesource.com to "Configure Git".

Follow the instructions on the page.

(If you are on a Windows computer, you should instead follow the instructions

in the yellow box to run the command.)

Your secret authentication token is now in a <code>.gitcookie</code> file

and Git is configured to use this file.

<h3>Register with Gerrit</h3>

Now that you have your authentication token,

you need to register your account with Gerrit.

To do this, visit

<a href="https://go-review.googlesource.com/login/">

go-review.googlesource.com/login/</a>. You will immediately be redirected

to Google Accounts. Sign in using the same Google Account you used above.

That is all that is required.

<h3>Contributor License Agreement</h3>

<p>Gerrit serves as the gatekeeper and uses your e-mail address as the key.

To send your first change to the Go project from a given address,

you must have completed one of the contributor license agreements:

If you are the copyright holder, you will need to agree to the

<a href="https://developers.google.com/open-source/cla/individual">individual

contributor license agreement</a>, which can be completed online.

If your organization is the copyright holder, the organization

will need to agree to the

<a href="https://developers.google.com/open-source/cla/corporate">corporate

contributor license agreement</a>.

(If the copyright holder for your code has already completed the

agreement in connection with another Google open source project,

it does not need to be completed again.)

You can use the links above to create and sign the contributor license agreement

or you can show your current agreements and create new ones through the Gerrit

interface. <a href="https://go-review.googlesource.com/login/">Log into Gerrit</a>,

click your name in the upper-right, choose "Settings", then select "Agreements"

from the topics on the left. If you do not have a signed agreement listed here,

you can create one by clicking "New Contributor Agreement" and following the steps.

This rigmarole only needs to be done for your first submission for each email address.

<h3>Install the git-codereview command</h3>

Now install the <code>git-codereview</code> command by running,

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

Make sure <code>git-codereview</code> is installed in your shell path, so that the

<code>git</code> command can find it. Check that

$ git codereview help

prints help text, not an error.

Note to Git aficionados: The <code>git-codereview</code> command is not required to

upload and manage Gerrit code reviews. For those who prefer plain Git, the text

below gives the Git equivalent of each git-codereview command. If you do use plain

Git, note that you still need the commit hooks that the git-codereview command

configures; those hooks add a Gerrit <code>Change-Id</code> line to the commit

message and check that all Go source files have been formatted with gofmt. Even

if you intend to use plain Git for daily work, install the hooks in a new Git

checkout by running <code>git-codereview</code> <code>hooks</code>.

<h3>Set up git aliases</h3>

The <code>git-codereview</code> 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 <code>git-codereview</code>'s own

subcommands, so that the above becomes,

$ git sync

The <code>git-codereview</code> subcommands have been chosen to be distinct from

Git's own, so it's safe to do so.

The aliases are optional, but in the rest of this document we will assume

they are installed.

To install them, copy this text into your Git configuration file

(usually <code>.gitconfig</code> in your home directory):

[alias]

change = codereview change

gofmt = codereview gofmt

mail = codereview mail

pending = codereview pending

submit = codereview submit

sync = codereview sync

<h3>Understanding the git-codereview command</h3>

<p>After installing the <code>git-codereview</code> command, you can run</p>

$ git codereview help

to learn more about its commands.

You can also read the <a href="https://godoc.org/golang.org/x/review/git-codereview">command documentation</a>.

<h3>Switch to the master branch</h3>

Most Go installations use a release branch, but new changes should

only be made based on the master branch.

(They may be applied later to a release branch as part of the release process,

but most contributors won't do this themselves.)

Before making a change, make sure you start on the master branch:

$ git checkout master

$ git sync

(In Git terms, <code>git</code> <code>sync</code> runs

<code>git</code> <code>pull</code> <code>-r</code>.)

<h3>Make a change</h3>

The entire checked-out tree is writable.

Once you have edited files, you must tell Git that they have been modified.

You must also tell Git about any files that are added, removed, or renamed files.

These operations are done with the usual Git commands,

<code>git</code> <code>add</code>,

<code>git</code> <code>rm</code>,

and

<code>git</code> <code>mv</code>.

If you wish to checkpoint your work, or are ready to send the code out for review, run</p>

$ git change <i>&lt;branch&gt;</i>

from any directory in your Go repository to commit the changes so far.

The name <i>&lt;branch&gt;</i> is an arbitrary one you choose to identify the

local branch containing your changes.

(In Git terms, <code>git</code> <code>change</code> <code>&lt;branch&gt;</code>

runs <code>git</code> <code>checkout</code> <code>-b</code> <code>branch</code>,

then <code>git</code> <code>branch</code> <code>--set-upstream-to</code> <code>origin/master</code>,

then <code>git</code> <code>commit</code>.)

Git will open a change description file in your editor.

(It uses the editor named by the <code>$EDITOR</code> environment variable,

<code>vi</code> by default.)

The file will look like:

# Please enter the commit message for your changes. Lines starting

# with '#' will be ignored, and an empty message aborts the commit.

# On branch foo

# Changes not staged for commit:

# modified: editedfile.go

#

At the beginning of this file is a blank line; replace it

with a thorough description of your change.

The first line of the change description is conventionally a one-line

summary of the change, prefixed by the primary affected package,

and is used as the subject for code review mail.

The rest of the

description elaborates and should provide context for the

change and explain what it does.

If there is a helpful reference, mention it here.

After editing, the template might now read:

math: improved 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

# Please enter the commit message for your changes. Lines starting

# with '#' will be ignored, and an empty message aborts the commit.

# On branch foo

# Changes not staged for commit:

# modified: editedfile.go

#

The commented section of the file lists all the modified files in your client.

It is best to keep unrelated changes in different change lists,

so if you see a file listed that should not be included, abort

the command and move that file to a different branch.

The special notation "Fixes #159" associates the change with issue 159 in the

<a href="https://golang.org/issue/159">Go issue tracker</a>.

When this change is eventually submitted, the issue

tracker will automatically mark the issue as fixed.

(There are several such conventions, described in detail in the

<a href="https://help.github.com/articles/closing-issues-via-commit-messages/">GitHub Issue Tracker documentation</a>.)

Once you have finished writing the commit message,

save the file and exit the editor.

If you wish to do more editing, re-stage your changes using

<code>git</code> <code>add</code>, and then run

$ git change

to update the change description and incorporate the staged changes. The

change description contains a <code>Change-Id</code> line near the bottom,

added by a Git commit hook during the initial

<code>git</code> <code>change</code>.

That line is used by Gerrit to match successive uploads of the same change.

Do not edit or delete it.

(In Git terms, <code>git</code> <code>change</code> with no branch name

runs <code>git</code> <code>commit</code> <code>--amend</code>.)

<h3>Mail the change for review</h3>

Once the change is ready, mail it out for review:

$ git mail

You can specify a reviewer or CC interested parties

using the <code>-r</code> or <code>-cc</code> options.

Both accept a comma-separated list of email addresses:

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

Unless explicitly told otherwise, such as in the discussion leading

up to sending in the change list, it's better not to specify a reviewer.

All changes are automatically CC'ed to the

<a href="https://groups.google.com/group/golang-codereviews">golang-codereviews@googlegroups.com</a>

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.

(In Git terms, <code>git</code> <code>mail</code> pushes the local committed

changes to Gerrit using <code>git</code> <code>push</code> <code>origin</code>

<code>HEAD:refs/for/master</code>.)

If your change relates to an open issue, please add a comment to the issue

announcing your proposed fix, including a link to your CL.

The code review server assigns your change an issue number and URL,

which <code>git</code> <code>mail</code> 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

<h3>Reviewing code</h3>

Running <code>git</code> <code>mail</code> will send an email to you and the

reviewers asking them to visit the issue's URL and make comments on the change.

When done, the reviewer adds comments through the Gerrit user interface

and clicks "Reply" to send comments back.

You will receive a mail notification when this happens.

You must reply through the web interface.

(Unlike with the old Rietveld review system, replying by mail has no effect.)

<h3>Revise and upload</h3>

You must respond to review comments through the web interface.

(Unlike with the old Rietveld review system, responding by mail has no effect.)

When you have revised the code and are ready for another round of review,

stage those changes and use <code>git</code> <code>change</code> to update the

commit.

To send the update change list for another round of review,

run <code>git</code> <code>mail</code> again.

The reviewer can comment on the new copy, and the process repeats.

The reviewer approves the change by giving it a positive score

(+1 or +2) and replying <code>LGTM</code>: looks good to me.

You can see a list of your pending changes by running <code>git</code>

<code>pending</code>, and switch between change branches with <code>git</code>

<code>change</code> <code><i>&lt;branch&gt;</i></code>.

<h3>Synchronize your client</h3>

While you were working, others might have submitted changes to the repository.

To update your local branch, run

$ git sync

(In git terms, <code>git</code> <code>sync</code> runs

<code>git</code> <code>pull</code> <code>-r</code>.)

If files you were editing have changed, Git does its best to merge the

remote changes into your local changes.

It may leave some files to merge by hand.

For example, suppose you have edited <code>sin.go</code> but

someone else has committed an independent change.

When you run <code>git</code> <code>sync</code>,

you will get the (scary-looking) output:

$ git sync

Failed to merge in the changes.

Patch failed at 0023 math: improved Sin, Cos and Tan precision for very large arguments

The copy of the patch that failed is found in:

/home/you/repo/.git/rebase-apply/patch

When you have resolved this problem, run "git rebase --continue".

If you prefer to skip this patch, run "git rebase --skip" instead.

To check out the original branch and stop rebasing, run "git rebase --abort".

If this happens, run

$ git status

to see which files failed to merge.

The output will look something like this:

rebase in progress; onto a24c3eb

You are currently rebasing branch 'mcgillicutty' on 'a24c3eb'.

(fix conflicts and then run "git rebase --continue")

(use "git rebase --skip" to skip this patch)

(use "git rebase --abort" to check out the original branch)

Unmerged paths:

(use "git reset HEAD &lt;file&gt;..." to unstage)

(use "git add &lt;file&gt;..." to mark resolution)

<i>both modified: sin.go</i>

The only important part in that transcript is the italicized "both modified"

line: Git failed to merge your changes with the conflicting change.

When this happens, Git leaves both sets of edits in the file,

with conflicts marked by <code>&lt;&lt;&lt;&lt;&lt;&lt;&lt;</code> and

<code>&gt;&gt;&gt;&gt;&gt;&gt;&gt;</code>.

It is now your job to edit the file to combine them.

Continuing the example, searching for those strings in <code>sin.go</code>

might turn up:

arg = scale(arg)

&lt;&lt;&lt;&lt;&lt;&lt;&lt; HEAD

if arg &lt; 1e9 {

=======

if arg &lh; 1e10 {

&gt;&gt;&gt;&gt;&gt;&gt;&gt; mcgillicutty

largeReduce(arg)

Git doesn't show it, but suppose the original text that both edits

started with was 1e8; you changed it to 1e10 and the other change to 1e9,

so the correct answer might now be 1e10. First, edit the section

to remove the markers and leave the correct code:

arg = scale(arg)

if arg &lt; 1e10 {

largeReduce(arg)

Then tell Git that the conflict is resolved by running

$ git add sin.go

If you had been editing the file, say for debugging, but do not

care to preserve your changes, you can run

<code>git</code> <code>reset</code> <code>HEAD</code> <code>sin.go</code>

to abandon your changes.

Then run <code>git</code> <code>rebase</code> <code>--continue</code> to

restore the change commit.

<h3>Reviewing code by others</h3>

You can import a change 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 &amp;&amp; git checkout FETCH_HEAD

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

<h3>Submit the change after the review</h3>

After the code has been <code>LGTM</code>'ed, an approver may

submit it to the master branch using the Gerrit UI.

There is a "Submit" button on the web page for the change

that appears once the change is approved (marked +2).

This 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.

<h3>More information</h3>

In addition to the information here, the Go community maintains a <a href="https://golang.org/wiki/CodeReview">CodeReview</a> wiki page.

Feel free to contribute to this page as you learn the review process.

<h2 id="copyright">Copyright</h2>

<p>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

<a href="https://golang.org/change">change log</a>

and in the <a href="/CONTRIBUTORS"><code>CONTRIBUTORS</code></a> file

and perhaps the <a href="/AUTHORS"><code>AUTHORS</code></a> file.

<p>The <a href="/CONTRIBUTORS"><code>CONTRIBUTORS</code></a> file

defines who the Go contributors&mdash;the people&mdash;are;

the <a href="/AUTHORS"><code>AUTHORS</code></a> file defines

who &ldquo;The Go Authors&rdquo;&mdash;the copyright holders&mdash;are.

These files will be periodically updated based on the commit logs.

<p>Code that you contribute should use the standard copyright header:</p>

// Copyright 2015 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. It is not

necessary to update the copyright year on files that you change.