Gerrit/Tutorial

Right now Git/Workflow is a better tutorial - this page is to develop a good hourlong step-by-step tutorial for the Berlin Hackathon 2012.

What is Git?
Git is a distributed version control system (dvcs) written in C originally developed by Linus Torvalds and others to manage the Linux kernel. In the past couple of years, it has taken off as a very robust and well-supported code repository. “Distributed” means that there is no central copy of the repository. With Subversion, Wikimedia’s servers host the repository and users commit their changes to it. In contrast, with Git, once you’ve cloned the repository, you have a fully functioning copy of the source code, with all the branches and tagged releases at your disposal.

What is Gerrit?
Gerrit is a free, web-based collaborative code review tool that integrates with Git. It has been developed at Google by Shawn Pearce (co-author of Git, founder of JGit) for the development of the Android project.

Starting from a set of patches for Rietveld, it became a fork and evolved into a full blown project when ACL patches wouldn't be merged into Rietveld by its author, Guido van Rossum.

Originally written in Python like Rietveld, it is now written in Java (Java EE Java Servlet) with SQL since version 2.

Why did Wikimedia engineering move from Subversion to Git
Three major reasons: To encourage participation: Since Git is distributed, it allows people to contribute with a much lower barrier to entry. Anyone will be able to clone the repository and make their own changes to keep track of them. And if you’ve got an account in our code review tool (Gerrit), you’ll be able to push changes for the wider community to review. To fix our technical process: Subversion has technical flaws that make life difficult for developers. Notably, the implementation of branching is not very easy to use, and makes it hard to use “feature branches”. Our community is very distributed, with many parallel efforts and needs to integrate many different feature efforts, so we’d like to use feature branches more. Git branches are very easy to work with and merge between, which should make things easier for our development community. (Several other large projects, such as Drupal and PostgreSQL, have made the same switch for similar reasons, and we’ve done our best to learn from their experiences.)

Some quotes from our community:

“I love git just because it allows me to commit locally (and offline).” – Guillaume Paumier

“[Y]ou can create commits locally and push them to the server later (great for working without wifi), you can tell it ‘save my work so I can go do something else now’ in one command, and it’ll allow us to review changes before they go into “trunk” (master)…. without human intervention in merging things into trunk. Gerrit automates this process.” – Roan Kattouw

And finally, to get improvements to users faster: with better branching and a more granular code review workflow that suits our needs better, plus our ongoing improvements to our automated testing infrastructure, we won’t have to wait months before deploying already-written features and bugfixes to Wikimedia sites.

Setting up Git
On Ubuntu you can install the Git command line tool via the following command: sudo apt-get install git-core For other Linux distributions please check your vendor documentation.

A windows version of Git can be found on the msysgit Project site.

The URL to this webpage is http://code.google.com/p/msysgit/.

Configure Git
Now that you have Git on your system, you’ll want to do a few things to customize your Git environment. You should have to do these things only once; they’ll stick around between upgrades. You can also change them at any time by running through the commands again.

Git comes with a tool called git config that lets you get and set configuration variables that control all aspects of how Git looks and operates. git config -l Check that you have a name and email address configured; if you don't, tell Git your username and e-mail address so gerrit can attach it to your accounts: git config --global user.email "preilly@wikimedia.org" git config --global user.name "preilly"

Download MediaWiki core using Git
You can currently download MediaWiki core using Git, as well as any extension currently installed on the Wikimedia Foundation server cluster. By July 2013, all extensions will either be available using Git or move to alternate version control hosts.

The first step is to clone the MediaWiki repository.

In order to collaborate with us, or if you want to get a copy of the MediaWiki repository so you can look at or use the code, you will clone it. You simply run the following on your command line: git clone https://gerrit.wikimedia.org/r/p/mediawiki/core.git



This will copy the entire history of the MediaWiki repository so you have it locally and it will give you a working directory of the main branch so you can look at the code or start editing it. If you change into the new directory, you can see the .git subdirectory - that is where all the project data is.

By default, Git will create a directory that is the same name as the project in the URL you give it - basically whatever is after the last slash of the URL. If you want something different, you can just put it at the end of the command, after the URL. So, in this example you will have a core directory.



Prepare to work with gerrit
In order to properly work with gerrit, you need to have a pre-commit hook that adds a "change id" to your commit summary. That unique ID will let Gerrit track your commit. Since Git commit identification is based upon a sha1 of the commit id, whenever you amend a commit, the sha1 will change and gerrit would have lost track of it.

Installing git-review
First, install the release version of git-review using python package installer : $ sudo apt-get install python-pip $ sudo pip install git-review





Note: if you don't have  but have   installed, you can use this: $ sudo easy_install pip $ sudo pip install git-review pip

Setting up git-review
After cloning a repository, you need to set up for using git-review. This will automatically happen the first time you try to submit a commit, but it's generally better to do it right after cloning. $ git review -s This may ask you for your git username, if it's different from the shell username you're using.

Internally, this does the following:
 * checks whether accessing the remote repository works
 * if it doesn't, asks for a username and tries again
 * creates a remote called 'gerrit' that points to gerrit
 * installs the commit-msg hook

How to submit a patch
The main avenue for submitting changes to MediaWiki is to first join the MediaWiki development community so you can submit changes to Gerrit, our code review tool. Getting developer access is relatively easy.

Update master
Make sure that your master branch (the branch created when you initially cloned the repository) is up to date: git pull --rebase origin master

Create a branch
First, create a local branch for your new change. Give the branch a short but reasonably descriptive name. git checkout -b BRANCHNAME master This will create a new branch (BRANCHNAME) from 'master' and check it out for you. This is equivalent to doing git branch BRANCHNAME master git checkout BRANCHNAME Use a descriptive branch name. If you are working on a bug, include its id, for example "2012/bug12345".

Usual way
Modify your local code in some fashion.



Then check the changes you've made, within the file(s) and within the directory: git diff Without any extra arguments, a simple git diff will display in unified diff format (a patch) what code or content you've changed in your project since the last commit that are not yet staged for the next commit snapshot.

Then check the changes you've made, within the file(s) and within the directory: git status You run git status to see if anything has been modified and/or staged since your last commit so you can decide if you want to commit a new snapshot and what will be recorded in it.

This will show all modified files. To prepare submitting file, you should add your change to the index (the staging area between your working copy and your local repositroy), that is done by using the git add command. git add includes/api/ApiFormatXml.php

You run git add on a file when you want to include whatever changes you've made to it in your next commit snapshot. Anything you've changed that is not added will not be included - this means you can craft your snapshots with a bit more precision.

While doing so you can always review the change already added to the staging area by invoking git status and look at the diff with git diff --cached. The git diff --cached command will show you what contents have been staged. That is, this will show you the changes that will currently go into the next commit snapshot.

Once you are happy with the change list, you can add them to your local repository by using git commit. Now that you have staged the content you want to snapshot with the git add command, you run git commit</tt> to actually record the snapshot.

You will then be prompted with your favorite editor to add descriptive message for this commit. This is what other people will see when you will later push that commit to another repository.



You can repeat this step over and over until you have a set of changes that you want to have pushed to the master branch. One of the cool things about git is that when you git commit</tt>, you are committing to your local copy. This means you can commit as often as you like without potentially screwing things up for another developer on the project, unlike in SVN where you would want to be very careful that the changes you commit would not cause things to break.

Hence the workflow is something like:

$ git add $ git status $ git diff --cached $ git commit
 * 1) Add change:
 * 1) Verify list of files added to the staging area
 * 1) Review diff of changes staged:
 * 1) repeat until you are happy with your changes

Prepare to push your change set to Gerrit
Before your changes can be merged into master, they must undergo review in Gerrit.

But first, it's a good idea to synchronize your change set with any changes that may have occurred in master while you've been working. From within the branch you've been working on, excute the following command: git pull origin master git rebase master git pull</tt> will update the code in your local copy of the master branch. Then, git rebase</tt> will temporarily set aside the changes you've made in your branch, apply all of the changes that have happend in master to your working branch, then merge all of the changes you've made back into the branch. Doing this will help avoid future merge conflicts. Plus, it gives you an opportunity to test your changes against the latest code in master.

Once you are satisfied with your change set and you've rebased against master, you are ready to push your code to Gerrit for review.

Push your change set to Gerrit
If you installed, the command to push changes to Gerrit is very simple: git review -R Upon success, you'll get a confirmation and a link to the changeset in Gerrit.  New patchset: preilly; "Add some more detailed info about the xslt param of format=xml" [test/mediawiki/core] (master) - https://gerrit.wikimedia.org/r/8939 </tt>

You can view this change in the Gerrit Web UI:

If your commit addresses a bug in Bugzilla, please comment on that bug to note that the commit is in the merge queue, and link to its changeset in Gerrit.

Amending a change
Sometimes, you might need to amend a submitted change. You can amend your own changes as well as changes submitted by someone else, as long as the change hasn't been merged yet.

If you have git-review, checkout the change like this: git review -d

For example: git review -d 8939

Make changes. vim includes/api/ApiFormatXml.php the files as needed, then commit the change (ensuring you are amending the commit): git add includes/api/ApiFormatXml.php git commit --amend

NOTE: DO NOT use the  flag to specify a commit summary: that will override the previous summary and regenerate the Change-Id. Instead, use the editor to change the commit summary if needed, and keep the Change-Id line intact.

Push the change git review -R
 * The -R is important here. It tells git-review to not rebase your change against master, which clutters diffs between patch set 1 and 2.



New patchset: preilly; "Add some more detailed info about the xslt param of format=xml" [test/mediawiki/core] (master) - https://gerrit.wikimedia.org/r/8939</tt>



git review complains about multiple commits
If git review</tt> asks you if you really want to submit multiple commits, and lists a bunch of unrelated commits from different branches, try this: git fetch --all git remote update

How we review code
The things we look for in code won't change, but the workflow is different than it was before the Git switch.

Review before merge
It's important to us to have a review-before-merge workflow, for MediaWiki core and also for any extension we deploy. We will also offer that option to any extensions author who wants it for their extension. The one exception is i18n commits, which will be able to be pushed without review.

Who can review? Gerrit project owners
Who has the ability to do code review?

We use gerrit to manage code review. Anyone can ask for a Gerrit account (Get an account!). Within Gerrit, anyone can comment on commits and signal their criticisms and approvals. Anyone can give a nonbinding "+1" to any commit. However, for any given repository ("Gerrit project"), only a small group of people will have the ability to approve code within Gerrit and merge it into the repository. (Within gerrit, this superapproval is a "+2" even though that's a misleading name, because two +1 approvals DO NOT add up to a +2.) These people are "Gerrit project owners". To learn about becoming a Gerrit project owner, see Git/Gerrit project ownership.

Even within a Gerrit project, we can also specify particular branches that only specific people can pull into.

How to comment on, review, and merge code in Gerrit
Anyone can comment on code in Gerrit.

Viewing and commenting on code

 * Make sure you have a https://gerrit.wikimedia.org login (Get an account!). If you don't know, try logging in at https://labsconsole.wikimedia.org; the username and password should be the same.  If you can't, ask in  for someone to help.
 * Log in to Gerrit. If you know the changeset you want to look at (URL will look like https://gerrit.wikimedia.org/r/#change,8939 ), go to that. Otherwise, use the search box and try searching.  There is no fulltext search in Gerrit, but you can search by author ("Owner"), Gerrit project, branch, changesets you've starred, etc.
 * The changeset has a few important fields, links and buttons:
 * Reviewers. 'jenkins-bot' is the autoreviewer that auto-verifies anything that passes the Jenkins tests. If it passes, you see a green checkmark. If it fails, a red X.  A changeset needs a green checkmark before anyone can merge it.
 * Add reviewer (manually ping someone to request their review. It'll show up in their Gerrit stream)
 * Side-by-side diff button:
 * Opens the diff in a new tab. You can double-click on a line and comment on that line, then save a draft comment! Then, click "Up to change" to go back to the changeset.
 * Abandon Change button (you'll see this if you wrote this diff. This action removes the diff from the merge queue, but leaves it in Gerrit for archival purposes)
 * Review button:
 * On this page you can leave an overall comment, view inline comments from the diff that are still in draft form and awaiting publication, and signal your thoughts on the commit. You can signal approval with a "+1" and disapproval with a "-1".  These numbers are nonbinding, won't cause merges or rejections, and have no formal effect on the code review.  To publish your draft inline comments plus your comments on the diff as a whole, click Publish.

Formally reviewing and merging or rejecting code
If you are one of the Gerrit project owners, you'll also see:
 * Abandon Change button
 * on the Review page, additional Code Review options to +2 (approve) or -2 (veto) a diff, and a Publish And Submit button (publish your comment and merge diff into the branch, in 1 step)
 * Submit Patch Set 1 button (merge -- only useful if you or someone else has already given a +2 approval to the diff, but not merged it)

And once you've merged something into the example Gerrit project you'll see it in https://gerrit.wikimedia.org/r/gitweb?p=test/mediawiki/extensions/examples.git;a=summary.

If you merge a commit that references a Bugzilla bug, please go to that bug and mark it RESOLVED: FIXED and reference the merge ID.