Gerrit/Tutorial

This is a tutorial which explains how to use Git to create and modify changes in Gerrit.

  • If you want to save time and are tech-savvy, use the very short how-to guide instead: Gerrit/Tutorial/tl;dr
  • For power users, Gerrit/Advanced usage has additional documentation.
  • If you only want to play with Gerrit and do not want to write a patch for a "real" Wikimedia software project, use our Gerrit test instance instead.

In this tutorial, commands to enter start with a dollar sign in a box, like this: command. Do not enter the $ prefix.
If a command also includes a variable which you must change yourself, then the variable is shown in red: command variable.

What is Git?

Git is a free and open source distributed version control system. “Distributed” means that there is no central copy of the repository. 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.

Create a Wikimedia developer account

If you do not have a Wikimedia developer account yet, go to wikitech.wikimedia.org and create an account. The same username and password will be used to log into Gerrit below.

Set up Git

These instructions explain how to install Git as a command-line (terminal window) tool. If you prefer a graphical user interface (GUI) instead of the command line, then check the list of clients maintained by the Git project. For alternate installation instructions see the official documentation.

Installation

Follow Installing Git to learn how to install git on your operating system.

Configure Git

Now that you have Git installed, it’s time to configure your personal information. You should have to do this only once. You can also change your personal information at any time by running these commands again.

Git tracks who makes each commit by checking the user’s name and email. In addition, this info is used to associate your commits with your Gerrit account.

Enter the two commands below to set your username and email address. Replace gerrituser with your own Gerrit username and replace gerrituser@example.com with your own email address:

git config --global user.email "gerrituser@example.com"

git config --global user.name "gerrituser"

To see your current configuration variables which control how Git behaves, use git config -l.

Set Up SSH Keys in Gerrit

We use an SSH key to establish a secure connection between your computer and Gerrit. The Wikimedia Security Team recommends, as of August 2021, that users creating SSH Keys use the ed25519 type for optimum security and performance.

Get your SSH key

Follow SSH keys#Generating a new SSH key.

Add SSH Public key to your Gerrit account

  • Log into the web interface for Gerrit. The username and password for your Gerrit are the same as for your Wikimedia Developer account.
  • Click on your username in the top right corner, then choose "Settings".
  • Click "SSH Keys" in the menu on the left.
  • Paste your SSH Public Key into the corresponding field and click "ADD NEW SSH KEY".

Test Gerrit SSH connection

Connect to the Gerrit server via ssh to check if everything works as expected. Replace gerrituser by your username as shown in your Gerrit settings:

ssh -p 29418 gerrituser@gerrit.wikimedia.org
  • Be mindful and compare that the "ed25519 key fingerprint" is the same as the SSH fingerprint for gerrit.wikimedia.org:29418. If it is the same, answer "Yes" to "Are you sure you want to continue connecting?". Then enter the passphrase for your key.
  • You should get a message "Welcome to Gerrit Code Review". The last line should show "Connection to gerrit.wikimedia.org closed."
  • If you run into problems, use ssh -p 29418 -v gerrituser@gerrit.wikimedia.org (replace gerrituser by your username). The -v will provide verbose output to help find problems. Then read Gerrit Troubleshooting.

An example Gerrit SSH connection success message looks like this:

Example:

Download code using Git

Sandbox

If you would like to practice using Gerrit you can download (also called "cloning") the repository this tutorial uses called "sandbox".

Run the following on the Git Bash command line:

git clone ssh://gerrituser@gerrit.wikimedia.org:29418/sandbox (Replace gerrituser by your Gerrit username. And make sure the URL begins with ssh: and not https:).

This will copy the entire history and the code base of the "sandbox" extension repository into your machine. You will have a working directory of the extension's main branch (usually also called "git master"). Enter the new directory (via the command cd sandbox). Now you can look at the code and start editing it.

Existing repositories

Cloning the Sandbox repository will not give you a development environment setup or a running MediaWiki installation. (Running will require MediaWiki Core and placing the code you checked out in a location expected by your web server.) See Download from Git how to download MediaWiki Core, extensions, skins, or any other project repository hosted at gerrit.wikimedia.org from Git.

Vagrant

If you have downloaded MediaWiki or extensions using Vagrant, make sure you have configured Git to push code using SSH instead of HTTPS.

Prepare to work with Gerrit

Gerrit requires that your commit message must have a "change ID". They look like Change-Id: Ibd3be19ed1a23c8638144b4a1d32f544ca1b5f97 starting with an I (capital i). Each time you amend a commit to improve an existing patch in Gerrit, this change ID stays the same, so Gerrit understands it as a new "patch set" to address the same code change.

There's a git add-on called git-review that adds a Change-ID line to your commits. Using git-review is recommended. It makes it easier to configure your Git clone, to submit a change or to fetch an existing one.

Installing git-review

Note that Wikimedia Gerrit requires git-review version 1.27 or later.

For more details, please see Gerrit/git-review#Installation.

  Linux

  Windows

  macOS

  • For OS X 10.11 El Capitan and later, follow Method 1.
  • On versions prior to 10.11, use the pip Python package installer by following Method 2.

Configuring git-review

Git's default remote host name is "origin". This name is also used by Wikimedia projects. We need to tell git-review to use that host. Replace gerrituser with your Gerrit username:

git config --global gitreview.remote origin

git config --global gitreview.username gerrituser

Setting up git-review

After downloading ("cloning") a repository, you need to set it up for 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. Make sure that you are in the directory of the project that you cloned (otherwise you will get an error "fatal: Not a git repository"). Then run this command:

git review -s --verbose

Towards the end of the output, you should see something like this:

Example:

This may ask you for your git username, if it's different from the shell username you're using.

If you could not install git-review, then you could use the Gerrit patch uploader or Gerrit/Web tutorial to submit a patch.

Submit a patch

Make sure that you cloned the code repository that you are interested in (see here).

Make sure that you are in the directory of the code repository (the command pwd tells you where exactly you are).

Update the main development branch

Make sure that the main development branch (the branch created when you initially cloned the repository) is up to date:

git pull origin master

However, note that some repositories use a different name for their main development branch (for example main instead of master, or the operations/puppet repository has a production instead of a master branch).

Create a branch

First, create a local branch for your new change. Replace BRANCHNAME below by a short but reasonably descriptive name (e.g. T1234 if a corresponding Phabricator task exists for your changes, cleanup/some-thing, or badtitle-error). Other people will also use this name to identify your branch.

git checkout -b BRANCHNAME origin/master

Example:

This will create a new branch (called BRANCHNAME) from the latest 'master' and check it out for you. In the example above, we called that new branch mentionWikimedia.

Make your changes

Make changes to your local code. Use your preferred text editor and modify a file. In the example below, we edit the file README.md and add a word.

Then close your text editor and check the changes you have made since the last commit, within the file(s) and within the directory:

git diff

Example:

git diff displays your changes in unified diff format: Removed lines have a minus (-) prefix and added lines have a plus (+) prefix. These changes are not yet "staged" (via git add) for the next commit.

Stage your changes for a commit

Run git status to decide which of your changes should become part of your commit. It will display a list of all file(s) that you have changed within the directory. At this point, the output will display "no changes added to commit" as the last line.

Use git add to make your changed file(s) become part of your next commit. In the example above we modified the file README.md, so the command would be:

git add README.md Any files you've changed that you have not passed to git add will be ignored when running git commit in the next step.

At any time you can always review the changes already staged by running git status. After you ran git add, git status will not show the line "no changes added to commit" anymore.
You can also use git diff --cached to see which changes are staged and will go into the next commit. The output will look the same as for the git diff command above.

Commit your staged changes

Once you are happy with the list of changes added via git add, you can turn these changes into a commit in your local repository by using

git commit

sandbox/.git/COMMIT_EDITMSG:

You will then be asked in your text editor to add a descriptive summary for your commit. You must follow the Commit message guidelines. This is what other people will see when looking at the history of changes in the code repository.

Save the commit message and close your text editor. A summary (the commit ID, your subject line, the files and lines changed) will be displayed.

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.

When you git commit, 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.

Prepare to push your commit to Gerrit

Synchronize your changeset with any changes that may have occurred in the master branch while you've been working ("rebasing"). From within your branch, run:

git pull --rebase origin master

Example:
git pull --rebase origin master will fetch new commits from the remote and then rebase your local commits on top of them.

It will temporarily set aside the changes you've made in your branch, apply all of the changes that have happened in master to your working branch, then merge (recommit) 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.

Now you are ready to push your code to Gerrit for review. If you made several related commits, consider merging them into one single commit for review.

Push your commit to Gerrit

If you followed #Prepare to work with Gerrit above and installed git-review and ran git review -s, then the command to push changes to Gerrit is:

git review -R

The -R option tells git-review not to perform a rebase before submitting the change to Gerrit.

Example:

Upon success, you'll get a confirmation and a link to the changeset in Gerrit. In the example above, that link is: https://gerrit.wikimedia.org/r/#/c/sandbox/+/563720

Congratulations! Your patch is in Gerrit and hopefully will get reviewed soon!

If git review -R fails

If you are asked to enter your git credentials (a username and password) after running git review -R and it responds that they are invalid, it's likely that the repo was cloned using https and not ssh. You should not recieve any prompt to enter credentials if you are using ssh. To switch the repo to ssh, run git remote set-url origin repository URL and replace repository URL with the one found in gitiles that begins with ssh://. Make sure to add your Gerrit username before gerrit.wikimedia.org. For example, the repository URL for the Echo repository would be ssh://gerrituser@gerrit.wikimedia.org:29418/mediawiki/extensions/Echo.

If you get a Permission denied (publickey). fatal: Could not read from remote repository., re-follow the instructions at SSH keys to make sure your ssh agent is running and your identity is added. If you close your Git Bash shell, you will be signed out and need to re-follow these instructions each time.

View the Change / Next Steps

Open the link to your Gerrit changeset in a web browser.

Under "Files", after you clicked the down arrow at the very right of any file in the list, you can see a diff of your changes per file: The old lines are shown in red color and your new lines are shown in green color.

Gerrit's diff algorithm (jGit) is slightly different from git's default diff algorithm. The differences displayed by Gerrit might not look like the differences displayed by Git on your machine.

If your commit addresses a ticket in Phabricator, a comment will be automatically added in the Phabricator task if you followed the Commit message guidelines. If you did not, you could either fix your commit message (by creating an updated patchset), or manually add a comment on that Phabricator ticket which includes a link to your changeset in Gerrit.

Other common situations

Also see Gerrit Advanced usage if your situation is not covered here.

Squash several commits into one single commit via rebase

If you made several related commits to your local repository prior to wanting to submit for review, you should squash (merge) those commits into one single commit.

The --interactive or -i option allows you to change (rewrite) your commit history. For each commit, you can modify and change the commit message, add or remove files, or perform other modifications.

First you need to tell git how far back you want to pull. To get a list of all changes in your branch:

git rebase -i origin/master

You can also limit the displayed list of recent changes. HEAD~3 means pull the last three commits:

git rebase -i HEAD~3

After you type this command, your text editor will display your commits in reverse order and a list of available commands:

Example:

Since we only want to send one commit to review, we will squash the last two commits into the first. Hence change all but the first "pick" to "squash":

pick aa8cf1d Adding method customFilterFunctionGetRiskyCountryCodeScore() to GatewayAdapter.
squash 38828e2 Adding $wgDonationInterfaceCustomFiltersFunctionsRiskyCountries to donationinterface.php
squash be33007 Fix a typo

When you finished picking and squashing and saved the file, another file will open in your text editor to allow you get to edit and merge your commit messages. Be careful to only keep one of the Change-Id lines and have it be at bottom of the message after one empty line.

Your messages from your previous commits will automatically be placed in this message:

Example:

Remember to put your (updated) summary message in the commit. In this case the new summary message will be:

(mingle-fr-2012-69) Adding a custom filter for risky countries.
In regards to which Change-Id you want to use, squashing a commit into an existing commit (one that's already in Gerrit), you need to pick the Change-Id that belongs to the one you meant to submit a new patchset for (the surviving commit). If your commits are new and are not in Gerrit, it does not matter which Change-Id you choose.

If all goes well, you should see a successful rebase message:

Example:

Afterwards, submit your patch for review:

git review

You should see a message like this showing your git review went to Gerrit (in this example, to https://gerrit.wikimedia.org/r/7187):

Example:

Amending a change (your own or someone else's)

Sometimes, you might need to amend a submitted change. You can amend a change as long as the change hasn't been merged yet.

You can amend your own changes. To amend changes submitted by someone else, you need to be a member of Gerrit's Trusted-Contributors group. To become a member of Trusted-Contributors, find someone who is a member and ask them to add you. The group is viral in that members can add new members, use your powers responsibly.

Rebasing

Rebase to bring your local branch up to date with the remote. It's best to make rebase updates a separate patch, so that your code reviewers have an easy time seeing what changes you've made. Assuming you are using Gerrit, you can do this by clicking the "Rebase Change" button when viewing your patch in Gerrit's web interface.

Hard reset and checkout the change with this command: (BEWARE: git review -d performs a hard reset that destroys all local changes. Stash or commit changes first which you wish to preserve!)

git review -d changeNumber For example: git review -d 814356

You can look in Gerrit to figure out the changeNumber. It is the six digit number in the URL of your code review page.

Or, if you already have the change in a branch on your local repository, you can just check it out:

git checkout BRANCHNAME For example: git checkout review/gerrituser/2012/bug12345

Make changes and push

Next, make some changes with your favorite text editor.

git add the files as needed, then commit the change (ensuring you are amending the commit):

git add Example/Example.body.php

git commit --amend

DO NOT use the -m flag to specify a commit summary: that will override the previous summary and regenerate the Change-Id. Instead, use your text editor to change the commit summary (in the file .git/COMMIT_EDITMSG 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.

Push to a branch different than master

Above, the commit was pushed to the master branch. The branch name only appeared as the topic of the commit in the Gerrit UI. If you really want to push to a different branch than master, you have to push via git review <branch name>.

Troubleshooting

For problems and how to solve them, see Gerrit/Troubleshooting .

See also

Also useful are these pages:

Third party guides to Git

References