Requests for comment/Migrate code review and management to Phabricator from Gerrit

This is an RFC to migrate code-review from Gerrit to Phabricator's Differential.

Background
We have used Gerrit for code review since the migration from Subversion to Git, circa early 2012. In 2015, we migrated from Bugzilla to Phabricator for our bug tracking. Phabricator also includes code management and review tools, and in fact these tools were a large part of the reason we chose Phabricator for our development platform.

Problem
Gerrit has a very high learning curve for new developers (cf: ), is difficult to maintain and does not integrate well with Phabricator. Continued maintenance of Gerrit has a very high cost and is yet another software tool to maintain by a small team.

Process
See also:
 * The WMF Release Engineering Team's Differential Migration goal project page for context and a rough (quarterly, iow: 3 month chunks) timeline.
 * The #Gerrit-Migration project in Phabricator which holds all tasks blocking the migration

Some milestones along the way
(not all in a strict dependency order)
 * 1) Deprecate Gitblit (ie: git.wikimedia.org) and host all repositories in Phabricator ("Diffusion")
 * 2) * This is in-progress already, tracked via #gitblit-deprecate
 * 3) Import all repos from Gerrit -  - ✅
 * 4) Get early adopters using Differential for "small" self-contained projects -
 * 5) * Doing this will give us real-world examples of code-review in Phabricator
 * 6) Prototype CI with Differential -  - ✅
 * 7) Integrate CI with Differential -
 * 8) Broadcast Differential activity to IRC -
 * 9) Document code review workflows -
 * 10) Update Code Review related wiki pages -
 * 11) Have Phabricator take over Github replication -
 * 12) Stop taking in new projects to Gerrit
 * 13) Migrate open changes from Gerrit to Differential -
 * 14) Provide static dump of Gerrit -
 * 15) SWITCH OVER (things after this are not absolute blockers/needed before switch over)
 * 16) Make MetricsGrimoire/korma support gathering Code Review statistics from Phabricator's Differential -

Pros and Cons of migrating to Phabricator/Differential
see also the original Phabricator RFC

Pros

 * Everything would be in one place
 * This benefit can not be understated
 * Single login for everything (it uses your Wikimedia SUL account)
 * Built in integration of subsystems like bug tracker, code-review, etc
 * Single interface across subsystems (reduced cognitive load/confusion)
 * Integrates design and designers (via Pholio)
 * Conversations allowed to stay in one place (instead of spread across multiple platforms)
 * Reduce technical-debt maintained by our community by no longer maintaining a family of home-grown integration bots (see: [| grrrit-wm] and potentially wikibugs2) and instead use/extend the Phabricator Bot.
 * Reducing the technical debt maintained by the WMF by:
 * Decommissioning Gitblit
 * Decommissioning Gerrit
 * Mobile-friendly
 * Login from any mobile device and see what you can read and do. Now, open Bugzilla or Gerrit...
 * Support for code auditing, with automatic notification based on herald rules and conditions
 * Audit cowboy commits triggered automatically and appropriate teams can be notified.
 * Raise concern about a commit that is already past code review.
 * Similar UI to differential but simplified and with different states for a commit:
 * "Needs audit" for commits that bypassed code review
 * "Concern raised" for manually flagged commits
 * Must be accept by someone with "ownership" of the code in question.
 * Faster and much more enjoyable code review experience (especially large diffs), and improved commit/review procedures
 * arc patch D123 vs git review -d 239028
 * differential comments are actually readable. vs gerrit is completely illegible, and if that weren't bad enough, no amount of css can fix it because the markup is pathological. Gerrit code review: Intentionally 99% css-resistant in the 21st century.
 * Multi-commit single-branch review support instead of the approach in Gerrit where each change is a single commit that is amended for each new patch set
 * But phabricator supports the single amended commit workflow for those that prefer it.
 * Better visibility of requested reviews - patches awaiting review are easier to notice thanks to dashboards, an outstanding review counter on the phabricator main menu, and phabricator notifications.
 * Possibility to let users create their own repositories
 * ease of creating feature branches?

Cons

 * Code review workflow differs of the Gerrit workflow when several persons want to improve a single commit: Gerrit allows to smoothly add a new revision, with author still being the initial proponent. Differential requires you commandeer the revision: you become author, and so you can't approve it as you can't self review (side effect: we so need to enable self review in the config, even if we don't socially use it). It "steals" the revision from the author, which becomes a review subscriber, and the revision is now officially yours. In pratice, this means people won't do that and will block a review for an extraneous space instead of fix it quickly, approve it, and send it to Zuul for merge.
 * Arcanist required
 * not for much longer
 * Arcanist is easy to install on every platform, ArchLinux excepted, and is also available as a Docker container
 * Transition learning curve (inherent to any migration)
 * Can be partly addressed via trainings and documentation (which mostly already exists, e.g. Phabricator/Differential)
 * Single Point of Failure
 * isn't gerrit a single point of failure now?
 * Phabricator supports repo mirroring and high availability clustering
 * Can be mitigated
 * Repository callsigns. This is being addressed with
 * Rewrite a bunch of bots relating to Gerrit
 * Gerritbot is no more needed (Phabricator has a built-in IRC notifier)
 * The notifier code has been broken since May 2015. Yet, something custom could be built around Doorkeeper.
 * wikibugs needs to be updated, maybe? (see pros)
 * Impact on CI, needs the middleware glue to Jenkins/Nodepool

Investigate

 * Replication to GitHub
 * Yes, it does support it. Working on small scale already.
 * access lists inheritance (eg: operations/*   analytics/*   mediawiki/* ..)
 * this would use the acl*projects in phabricator
 * review dashboards
 * https://phabricator.wikimedia.org/differential/query/active/ for the default
 * search queries (ex: all changes I am a reviewer on and which I haven't voted CR yet: is:open reviewer:self label:code-review=0,self ie: https://gerrit.wikimedia.org/r/#/q/is:open+reviewer:self+label:code-review%253D0%252Cself,n,z )
 * https://phabricator.wikimedia.org/differential/query/advanced/
 * Set "reviewers" to you, and status "needs review"
 * see also "blocking reviewers" vs "reviewers" in Differential
 * dependencies between changes across repos
 * Differential allows you to indicate any other Diff as a dependency of any given Diff
 * private repos / patches
 * Bad feature of Gerrit, will not be re-doing.