## 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.)

## Discuss your design

The project welcomes submissions but please let everyone know what you're working on if you want it to become part of the main repository.

Before undertaking to write something new for the Go project, send mail to the mailing list to discuss what you plan to do. This 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 guarantees that the design is sound before code is written; the code review tool is not the place for high-level discussions.

In short, send mail before you code. And don't start the discussion by mailing a change list!

## 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 `make all` 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extension helps here: its `hg submit` command automatically checks for and warns about the local repository being out of date compared to the remote one.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.

### 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. Unless explicitly told otherwise, such as in the discussion leading up to sending in the change list, leave the reviewer field blank. This means that the golang-dev@googlegroups.com mailing list will be used as the reviewer.

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, prefixed by the primary affected package, 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:
    	math: improved Sin, Cos and Tan 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 comments 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 mail 99999

again to upload the latest copy and send mail asking the reviewers to please take another look (`PTAL`). 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.

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`.

### Copyright

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:

- If you are the copyright holder, you will need to agree to the individual contributor license agreement, which can be completed online.
- If your organization is the copyright holder, the organization will need to agree to the corporate contributor license agreement. (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.)

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

Code that you contribute should use the standard copyright header:

    // Copyright 2012 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.