adding 'reviewer delegation' to MozReview

4th Jul 2016


One of the first projects I worked on when I moved to the MozReview team was “review delegation”. The goal was to add the ability for a reviewer to redirect a review request to someone else. It turned out to be a change that was much more complicated than expected.

MozReview is a Mozilla developed extension to the open source Review Board product; with the primary focus on working with changes as a series of commits instead of as a single unified diff. This requires more than just a Review Board extension, it also encompasses a review repository (where reviews are pushed to), as well as Mercurial and Git modules that drive the review publishing process. Autoland is also part of our ecosystem, with work started on adding static analysis (eg. linters) and other automation to improve the code review workflow.

I inherited the bug with a patch that had undergone a single review. The patch worked by exposing the reviewers edit field to all users, and modifying the review request once the update was OK’d. This is mostly the correct approach, however it had two major issues:

  1. According to Review Board and Bugzilla, the change was always made by the review’s submitter, not the person who actually made the change
  2. Unlike other changes made in Review Board, the review request was updated immediately instead of using a draft which could then be published or discarded


Review Board has a simple permissions model - the review request’s submitter (aka patch author) can make any change, while the reviewers can pretty much only comment, raise issues, and mark a review as “ready to ship” / “fix it”. As you would expect, there are checks within the object layer to ensure that these permission boundaries are not overstepped. Review Board’s extension system allows for custom authorisation and permissions check, however the granularity of these are course: you can only control if a user can edit a review request as a whole, not on a per-field level.

In order to allow the reviewer list to be changed, we need to tell Review Board that the submitter was making the change.

Performing the actions as the submitter instead of the authenticated user is easy enough, however when the changes were pushed to Bugzilla they were attributed to the wrong user. After a few false starts, I settled on storing the authenticated user in the request’s meta data, performing the changes as the submitter, and updating the routines that touch Bugzilla to first check for a stored user and make changes as that user instead of the submitter. Essentially “su”.

This worked well except for on the page where the meta data changes are displayed - here the change was incorrectly attributed to the review submitter. Remember that under Review Board’s permissions model only the review submitter can make these changes, so the name and gravatar were hard-coded in the django template to use the submitter. Given the constraints a Review Board extension has to operate in, this was a problem, and developing a full audit trail for Review Board would be too time consuming and invasive. The solution I went with was simple: hide the name and gravatar using CSS.

Drafts and Publishing

How Review Board works is, when you make a change, a draft is (almost) always created which you can then publish or discard. Under the hood this involves duplicating the review request into a draft object, against which modifications are made. A review request can only have one draft, which isn’t a problem because only the submitter can change the review.

Of course for reviewer delegation to work we needed a draft for each user. Tricky.

The fix ended up needing to take a different approach depending on if the submitter was making the change or not.

When the review submitter updates the reviewers, the normal review board draft is used, with a few tweaks that show the draft banner when viewing any revision in the series instead of just the one that was changed. This allows us to correctly cope with situations where the submitter makes changes that are broader than just the reviewers, such as pushing a revised patch, or attaching a screenshot.

When anyone else updates the reviewers, a “draft” is created in local storage in their browser, and a fake draft banner is displayed. Publishing from this fake draft banner calls the API endpoint that performs the permissions shenanigans mentioned earlier.

Overall it has been an interesting journey into how Review Board works, and highlighted some of the complications MozReview hits when we need to work against Review Board’s design. We’ve been working with the Review Board team to ease some of these issues, as well as deviating where required to deliver a Mozilla-focused user experience.