Gerrit Introduction

Note, if you are a first time Gerrit user, please take a look at the Setting up Gerrit page.

Overview

Main Features

Gerrit is a web-based tool that is used for code review. Its main features are the side-by-side difference viewing and inline commenting, which makes code reviews a quick and simple task. It is used together with the Git version control system. Gerrit allows authorized contributors to merge Changes to the Git repository, after reviews are done. Contributors can get their code reviewed with little effort, and get their Changes quickly through the system.

Basic Workflow

Gerrit usage has two stages: First, the contributor uploads Changes to Gerrit with Git, and second, peers use the web browser to make reviews. The review process includes the following steps:

• Review Changes
• Publish comments
• Approve or abandon Changes

Repository Structure

Gerrit can manage multiple repositories (projects). Branches are fully supported by Gerrit, each repository can have any number of branches.

Gerrit stores Changes, which are normal commits, as references in the refs/changes/ namespace. When contributors push Changes, they prepend refs/for/ to the target branch. For example, when contributor uploads a commit to the dev branch, the target ref will be refs/for/dev.

Terminology

Common terms used in Gerrit:

Term	Description
Change	The unit of review. Results in a single commit when merged to the Git repository. Change numbers are unique and never change.
Patch Set	A revision of a Change. Each time a Change is modified, it will receive a new Patch Set. Patch Set numbering starts from 1. Technically, a Patch Set is a unique Git commit.
Approval Category	Name for a scope that is checked during review process. Qt is using the categories Code Review and Sanity Review.
Score	A value in an Approval Category. Indicates if a Change is approved and can be submitted to the Git repository.
Submit	An action that allows Gerrit to merge a Change to the Git repository.
Abandon	Action that archives a Change. An abandoned Change can be restored later.
Project	A Git repository.

Creating and Uploading Contributions

Preparation

Before any contribution can be created, a Git clone of the target repository must be obtained and properly configured. The necessary steps are explained in Setting up Gerrit.

Creating a New Contribution

All contributions are uploaded with a regular Git push. Gerrit handles reviews at the commit level. A single contribution can easily result in several reviewable Changes in Gerrit. The contributor prepares a contribution by following these steps:

1. Creating or updating the local repository
2. Optionally creating a topic branch
3. Creating commits
4. Uploading the commit(s) to Gerrit
5. Soliciting feedback

Creating a topic branch

Keep code organized in topic branches. This is what Git excels in. "next-big-thing" is used as an example topic branch below:

$ git checkout -b next-big-thing
Switched to a new branch 'next-big-thing'
$

Creating Commits

See the Git Introduction to get started.

Here are a few notes specific to Qt/Gerrit:

• If you correctly set up the clone, the commit message editor will present you with a template for the message.
• The guidelines for creating good commit messages - and creating good commits in general, for that matter - are outlined in the Commit Policy.
• On initial creation of a commit, if the commit-msg hook is set up properly, an additional line in the form of "Change-Id: …" should have been added.

Uploading Commits

Changes are pushed to Gerrit with git push. Note that a special target ref is used. Gerrit reports how many Changes were created and provides links to these Changes.

$ git push gerrit HEAD:refs/for/dev
Counting objects: 6, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (4/4), 407 bytes, done.
Total 4 (delta 2), reused 0 (delta 0)
remote: Resolving deltas: 100% (2/2)
remote: Processing changes: new: 1, refs: 1, done    
remote: 
remote: New Changes:
remote:   https://codereview.qt-project.org/12345
remote: 
To ssh://qtcontributor@codereview.qt-project.org:29418/qt/qtbase
 * [new branch] HEAD -> refs/for/dev
$

When pushing to Gerrit, a typical refspec uses HEAD as the source ref and a special Gerrit ref for the target branch. The target ref has the format refs/for/<branch name>. Pushes to this target ref causes Gerrit to create new Changes for all commits pushed this way. To group your Changes, push to a topic by using the format refs/for/<branch name>/<topic name>. Note that it is possible to use any other ref as source ref instead of HEAD when necessary. See Branch Guidelines to decide about the target branch.

It is recommended that you use the git-gpush command from our qtrepotools repository (make sure to add the bin directory to your PATH for maximal convenience):

$ git gpush
[same output as above]

Once a Change has been created, it can be viewed in the web browser:

Soliciting Feedback

The contributor requests feedback by adding reviewers to the Change. This is typically done via the web browser. Access the Change with the web browser and use the "Add Reviewer" button to add any other registered user(s) to the review.

Alternatively, reviewers can be added already when uploading a contribution. Use the git-gpush tool for that purpose:

$ git gpush +approver@example.com
[same output as above]

Review Workflow

It is easy to get an overview of pending contributions in the "My Changes" page, also known as the "Dashboard":

It is possible to find Changes by issuing search queries. Links for the Change owner, project name and branch name in the search result listings can be used to quickly search for related Changes.

The navigation bar at the top of the Gerrit web view provides quick access to common search queries under the "My" and "All" items.

Reviewing Contributions

The review process starts with choosing a Change to review. After choosing a Change, changed files can be viewed side-by-side and comments can be posted in-line to each file. Contributions are typically reviewed by Approvers, but anyone can make a review.

The following steps are needed to complete a code review:

1. Opening the Change page (entered from the Dashboard or following a link in a notification mail)
2. Reviewing changed files
3. Publishing comments and reviewing results

Viewing the Change Overview

Changes can have several Patch Sets. When Changes get updated after review, the Patch Set number increases.

Reviewing Changed Files

Changes are compared side-by-side or in the unified diff view. The reviewer can also compare differences between specific Patch Sets (also called "inter-diffs").

Comments are posted by double clicking on a line. Comments are saved as drafts until they are published.

Note that comments are formatted as flowed text, discarding individual line breaks. A simple Wiki-like syntax is supported.

Gerrit keeps track which files have been reviewed and which files have comments ready to be published. If an ongoing review is interrupted, the reviewer can return later and resume where they left off. Draft comments can be edited and deleted before they are published.

Downloading Changes

It is also possible to download Changes for local review. The easiest way to do that is to copy the Download link from Gerrit to the command line:

$ git fetch ssh://qtcontributor@codereview.qt-project.org:29418/qt/qtbase refs/changes/19/419/1 && git checkout FETCH_HEAD
From ssh://codereview.qt-project.org:29418/qt/qtbase
 * branch refs/changes/19/419/1 -> FETCH_HEAD
Note: checking out 'FETCH_HEAD'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.  

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

git checkout -b new_branch_name

HEAD is now at 9a006a3... My Feature
$ git checkout -b changes/419
Switched to a new branch 'changes/419'
$

This makes it possible to use the usual git commands to view changes, to build the code, and to run tests.

Publishing Comments

When all files are reviewed, the reviewer continues to the Publish Comments screen with the Review button.

The review is completed by writing an optional cover message and giving a score. The cover message uses the same Wiki-like syntax as inline comments.

The Code Review category has 5 levels. A Change can only be submitted after it receives a +2 score, and cannot be be submitted if it receives a -2 score. Giving these scores is limited to Approvers; regular contributors can give only advisory scores (-1 to +1).

A Sanity Review score should not be given unless the Sanity Bot made a mistake and needs to be overridden.

Publishing Review Results From The Command Line

An alternative to browser-based review is to use the Gerrit ssh command line interface. Please do not do this, as scripting a step which is meant to be visual and interactive removes the additional safety net.

Processing Review Feedback

The contributor can view feedback on their contributions by accessing the Change page and reading comments file-by-file. It is possible to reply to comments and have a discussion about the code review in-line.

The contributor processes the feedback by following these steps:

1. Opening the Change page (entered from the Dashboard or following a link in a notification mail)
2. Locating comments in the Change page
3. Reading and writing in-line comments
4. Publishing a reply and/or uploading a new Patch Set

Locating Comments in The Change

Comments are located below the list of Patch Sets. In the picture below the comments for Patch Set 2 are examined:

Reading in-line Comments

Replies can be posted by clicking the comment like in the picture below:

Note that the comments are not actually posted until you publish them.

Note: do not reply to the notification mails you get from Gerrit. The mails you send this way will not be publicly visible and archived.

Updating a Contribution With New Code

Most Changes require multiple iterations of the review process. The contributor needs to update the Change if it received some comments that require action, or it did not merge with the branch tip. Each time a Change is updated, it gains a new Patch Set.

Updating a Change is done by following these steps:

1. Accessing the Changes
2. Modifying the Change(s)
3. Pushing updated Changes

Accessing The Changes

If the original commit(s) are still available, it is possible to amend them right away. Otherwise, download the Change(s) the same way as for local review.

If Gerrit reported a merge conflict, now would be the time to rebase the Changes:

$ git rebase --onto origin/dev

If you are not using a separate branch, you can just use git pull --rebase.

Note: Please do not rebase unnecessarily, as this makes inter-diffs much less useful.

Modifying Commits

Modifications in the working tree are committed with the --amend option. If the contribution consists of multiple commits, you will need to use git rebase --interactive to revise earlier commits.

When you amend commit messages, make sure to preserve the Change-Id footer, as this is how Gerrit identifies new Patch Sets for existing Changes.

$ vim src/foo.cc
$ git add src/foo.cc
$ git commit --amend
[changes/419 2ea7773] My Feature
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 src/foo.cc
$

Pay even more attention to copying the Change-Id when you redo commits from scratch.

Pushing Updated Changes

The commit(s) are uploaded back to Gerrit with the same target ref as before. After the push, new Patch Sets will have been created for the target Change(s):

$ git push gerrit HEAD:refs/for/dev
Counting objects: 6, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (4/4), 419 bytes, done.
Total 4 (delta 2), reused 0 (delta 0)
remote: Resolving deltas: 100% (2/2)
remote: Processing changes: updated: 1, refs: 1, done
To ssh://qtcontributor@codereview.qt-project.org:29418/qt/qtbase.git
 * [new branch] HEAD -> refs/for/dev
$

If anything goes wrong at this stage, please see the troubleshooting chart. Otherwise, when you reload the gerrit page in your browser, you should see the change, like this:

Integrating Changes

There are two ways to submit a Change. If the project is using a continuous integration system, Changes will be merged to staging. Otherwise, they will be submitted directly to their destination branch.

• To submit a Change to the CI system press "Merge Patch Set <n> to Staging".
• To submit a Change directly to the target branch press "Submit Patch Set <n>"

It is usually best when the contributor submits their own Changes.

Even if there are multiple reviewers, only one +2 approval is required. However, if other reviewers have shown a specific interest in a contribution, such as by repeated comments, it is polite to give them some time to add a +1 or +2 before submitting.

Continuous Integration

Gerrit has been slightly customized for the Qt project. The continuous integration system that is running regular builds and tests has been incorporated to the workflow. Instead of directly submitting Changes, Changes can be merged to a staging branch. There is a staging branch for each normal branch. Staging branches are maintained by Gerrit and are not visible to the contributors.

As part of the customization, Changes gained new states. Changes in the New state are waiting for a review. After review, Changes are merged to a staging branch and marked as Staged. The continuous integration system will pick staged Changes at regular intervals and move them to the Integrating state. From the Integrating state Changes are either submitted or moved back to the New state. If builds succeed and tests pass, the Changes are submitted or merged into their destination branch. Otherwise they are moved back to the New state for further analysis. The contributor of the affected Change(s) is expected to analyze the failure and upload fixes as necessary.

Abandoning and Deferring Contributions

Changes which are inherently flawed or became inapplicable should be abandoned. An abandoned Change will disappear from open Changes lists and is considered closed. Abandoned Changes can be restored later if they become valid again or if they were abandoned by accident.

Changes which have potential but will not be worked on in the near future should be deferred. Deferred Changes are basically abandoned, but are more easily accessible via the "My Deferred Changes" menu item.

Abandoning is a normal operation that is used to maintain Gerrit and hide Changes that will not make their way to the Git repository, for any reason.

To abandon a Change, its owner or an Administrator can click the "Abandon Change" button:

Feature Branches

The Qt Project's handling of branches is documented by the Branch Guidelines.

Topics vs. Branches

Gerrit topics can be used as "feature branches". A topic is created when a target ref like refs/for/<branch name>/<topic name> is used. It is often good enough to push a set of commits for review to a topic. However, if the contributor intends to work with others on a long-living branch, the Gerrit Administrators should be contacted for a new branch in Gerrit. Also, the QA team has to be contacted so they can include the new branch in the CI system builds. This way the code can be reviewed already when working on it in the branch.

Merging Feature Branches

Merges between feature branches and the mainline are like all other commits and are pushed, reviewed and staged the usual way. However, only 'Merge Masters' can push merge commits. User may recruit the merge master from outside project team if it they do not have person knowledgeable on Git available. Note: Do not create a merge from commits which have not been integrated yet.

Merging Branches Alien to Gerrit

This should not happen often. Nevertheless, only a merge commit should be pushed in this case. 'Push Master' can be asked to import the branch into Gerrit.

Moving Changes to Different Branches

It sometimes happens that you push a change for the wrong branch. To correct this mistake, do not create a new change, but instead ask the server to move the existing change to a new branch by posting a cover comment with:

bot: move to new/branch

For example:

bot: move to 5.11

Take care to make subsequent pushes of the change to the new branch, as otherwise a duplicate will be created.

Closing Remarks

See Gerrit Caveats and Hints to avoid common traps.

There is a process for requesting new repositories.

Report bugs in our Gerrit to the bugtracker. Ideally, provide a link to an upstream issue (note that the issue may be already closed, as we are typically lagging by several versions).