Introduction

This document explains how to contribute changes to the Go project. It assumes you have installed Go using the installation instructions and have written and tested your code. (Note that the gccgo frontend lives elsewhere; see Contributing to gccgo.)

Testing redux

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 $GOROOT/src
./all.bash

The final line printed by all.bash should be of the form: 

N known bugs; 0 unexpected bugs

The value of N varies over time, but the line must say “0 unexpected bugs” and must not add “test output differs.”

Code review

Changes to Go must be reviewed before they are submitted, no matter who makes the change. (In exceptional cases, such as fixing a build, the review can follow shortly after submitting.) A Mercurial extension helps manage the code review process. The extension is included in the Go source tree but needs to be added to your Mercurial configuration.

Caveat for Mercurial aficionados

Using Mercurial with the code review extension is not the same as using standard Mercurial.

The Go repository is maintained as a single line of reviewed changes; we prefer to avoid the complexity of Mercurial's arbitrary change graph. The code review extension helps here: its hg submit command automatically checks for and warns about the local repository being out of date compared to the remote one. The hg submit command also verifies other properties about the Go repository. For example, it checks that Go code being checked in is formatted in the standard style, as defined by gofmt, and it checks that the author of the code is properly recorded for copyright purposes.

To help ensure changes are only created by hg submit, the code review extension disables the standard hg commit command.

Mercurial power users: To allow Go contributors to take advantage of Mercurial's functionality for local revision control, it might be interesting to explore how the code review extension can be made to work alongside the Mercurial Queues extension.

Configure the extension

Edit $GOROOT/.hg/hgrc to add:

[extensions]
codereview = YOUR_GO_ROOT/lib/codereview/codereview.py

[ui]
username = Your Name <you@server.dom>

Replace YOUR_GO_ROOT with the value of $GOROOT. The Mercurial configuration file format does not allow environment variable substitution. The username information will not be used unless you are a committer (see below), but Mercurial complains if it is missing.

Log in to the code review site.

The code review server uses a Google Account to authenticate. (If you can use the account to sign in at google.com, you can use it to sign in to the code review server. The email address you use on the Code Review site will be recorded in the Mercurial change log and in the CONTRIBUTORS file. You can create a Google Account associated with any address where you receive email. 

$ cd $GOROOT
$ hg code-login
Email (login for uploading to codereview.appspot.com): rsc@golang.org
Password for rsc@golang.org:

Saving authentication cookies to /Users/rsc/.codereview_upload_cookies_codereview.appspot.com

Configure your account settings.

Edit your code review settings. Grab a nickname. Many people prefer to set the Context option to “Whole file” to see more context when reviewing changes.

Once you have chosen a nickname in the settings page, others can use that nickname as a shorthand for naming reviewers and the CC list. For example, rsc is an alias for rsc@golang.org.

Make a change

The entire checked-out tree is writable. If you need to edit files, just edit them: Mercurial will figure out which ones changed. You do need to inform Mercurial of added, removed, copied, or renamed files, by running hg add, hg rm, hg cp, or hg mv.

When you are ready to send a change out for review, run

$ hg change

from any directory in your Go repository. Mercurial will open a change description file in your editor. (It uses the editor named by the $EDITOR environment variable, vi by default.) The file will look like: 

# Change list.
# Lines beginning with # are ignored.
# Multi-line values should be indented.

Reviewer:
CC:

Description:
	<enter description here>

Files:
	src/pkg/math/sin.go
	src/pkg/math/tan.go
	src/pkg/regexp/regexp.go

The Reviewer line lists the reviewers assigned to this change, and the CC line lists people to notify about the change. These can be code review nicknames or arbitrary email addresses. If you don't know who is best to review the change, set the reviewer field to the golang-dev@googlegroups.com mailing list.

Replace “<enter description here>” with a description of your change. The first line of the change description is conventionally a one-line summary of the change and is used as the subject for code review mail; the rest of the description elaborates.

The Files section lists all the modified files in your client. It is best to keep unrelated changes in different change lists. In this example, we can include just the changes to package math by deleting the line mentioning regexp.go.

After editing, the template might now read: 

# Change list.
# Lines beginning with # are ignored.
# Multi-line values should be indented.

Reviewer: golang-dev@googlegroups.com
CC: math-nuts@swtch.com

Description:
	Sin, Cos, Tan: improved precision for very large arguments

	See Bimmler and Shaney, ``Extreme sinusoids,'' J. Math 3(14).
	Fixes issue 159.

Files:
	src/pkg/math/sin.go
	src/pkg/math/tan.go

The special sentence “Fixes issue 159.” associates the change with issue 159 in the Go issue tracker. When this change is eventually submitted, the issue tracker will automatically mark the issue as fixed.

Save the file and exit the editor.

The code review server assigns your change an issue number and URL, which hg change will print, something like: 

CL created: http://codereview.appspot.com/99999

If you need to re-edit the change description, run hg change 99999.

You can see a list of your pending changes by running hg pending (hg p for short).

Synchronize your client

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

$ hg sync

(For Mercurial fans, hg sync runs hg pull -u but then also synchronizes the local change list state against the new data.)

If files you were editing have changed, Mercurial 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 flag_test.go but someone else has committed an independent change. When you run hg sync, you will get the (scary-looking) output (emphasis added): 

$ hg sync
adding changesets
adding manifests
adding file changes
added 1 changeset with 2 changes to 2 files
getting src/pkg/flag/flag.go
couldn't find merge tool hgmerge
merging src/pkg/flag/flag_test.go
warning: conflicts during merge.
merging src/pkg/flag/flag_test.go failed!
1 file updated, 0 files merged, 0 files removed, 1 file unresolved
use 'hg resolve' to retry unresolved file merges
$

The only important part in that transcript is the italicized line: Mercurial failed to merge your changes with the independent change. When this happens, Mercurial leaves both edits in the file, marked by <<<<<<< and >>>>>>>. it is now your job to edit the file to combine them. Continuing the example, searching for those strings in flag_test.go might turn up: 

	VisitAll(visitor);
<<<<<<< local
	if len(m) != 7 {
=======
	if len(m) != 8 {
>>>>>>> other
		t.Error("VisitAll misses some flags");

Mercurial doesn't show it, but suppose the original text that both edits started with was 6; you added 1 and the other change added 2, so the correct answer might now be 9. First, edit the section to remove the markers and leave the correct code: 

	VisitAll(visitor);
	if len(m) != 9 {
		t.Error("VisitAll misses some flags");

Then ask Mercurial to mark the conflict as resolved: 

$ hg resolve -m flag_test.go

If you had been editing the file, say for debugging, but do not care to preserve your changes, you can run hg revert flag_test.go to abandon your changes, but you may still need to run hg resolve -m to mark the conflict resolved.

Mail the change for review

To send out a change for review, run hg mail using the change list number assigned during hg change:

$ hg mail 99999

You can add to the Reviewer: and CC: lines using the -r or --cc options. In the above example, we could have left the Reviewer and CC lines blank and then run: 

$ hg mail -r golang-dev@googlegroups.com --cc math-nuts@swtch.com 99999

to achieve the same effect.

Note that -r and --cc cannot be spelled --r or -cc.

Reviewing code

Running hg mail will send an email to you and the reviewers asking them to visit the issue's URL and make coments on the change. When done, the reviewer clicks “Publish and Mail comments” to send comments back.

Revise and upload

You will probably revise your code in response to the reviewer comments. When you have revised the code and are ready for another round of review, run 

$ hg upload 99999

to upload the latest copy. You might also visit the code review web page and reply to the comments, letting the reviewer know that you've addressed them or explain why you haven't. When you're done replying, click “Publish and Mail comments” to send the line-by-line replies and any other comments. A common acronym in such mails is PTAL: please take another look.

The reviewer can comment on the new copy, and the process repeats. The reviewer approves the change by replying with a mail that says LGTM: looks good to me.

Submit the change after the review

After the code has been LGTM'ed, it is time to submit it to the Mercurial repository. If you are a committer, you can run: 

$ hg submit 99999

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.

If your local copy of the repository is out of date, hg submit will refuse the change: 

$ hg submit 99999
local repository out of date; must sync before submit

If you are not a committer, you cannot submit the change directly. Instead, a committer, usually the reviewer who said LGTM, will run: 

$ hg clpatch 99999
$ hg submit 99999

The clpatch command imports your change 99999 into the committer's local Mercurial client, at which point the committer can check or test the code more. (Anyone can run clpatch to try a change that has been uploaded to the code review server.) The submit command submits the code. You will be listed as the author, but the change message will also indicate who the committer was. Your local client will notice that the change has been submitted when you next run hg sync.

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 Mercurial change log and in the CONTRIBUTORS file and perhaps the AUTHORS file.

The CONTRIBUTORS file defines who the Go contributors—the people—are; the AUTHORS file, which defines who “The Go Authors”—the copyright holders—are. The Go developers at Google will update these files when submitting your first change. In order for them to do that, you need to have completed one of the contributor license agreements:

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

Code that you contribute should use the standard copyright header:

// Copyright 2009 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.