Git/Conversion

This page discusses efforts to convert away from our current Subversion repository to Git. MediaWiki core and extensions used on Wikimedia sites have now switched to Git, but moving additional affected projects, and improving our new development infrastructure with tools integrating Gerrit and Git into our workflow, will continue to take engineering time until we completely switch our Subversion repository to read-only in the summer of 2013.

Rationale
Our current Subversion-based version control system has served us well, but we're in need of a more suitable version control system for our development effort. Our community is very distributed, with many parallel efforts and needs to integrate many different feature efforts. After long consideration, we've decided to move from Subversion to Git.

Some advantages of git:


 * "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

Affected development projects
MediaWiki core (/trunk/phase3/) and MediaWiki extensions that WMF deploys moved to Git in March 2012. Afterwards, any other extensions, tools, or projects that wish to move can do so. These might include operations, fundraising, pywikipediabot, etc.

We will leave some codebases in Subversion and not bother migrating them, because those extensions or tools have been abandoned. Some developers will choose to move their projects to Github or some other git site. We will also leave svn.wikimedia.org up for at least multiple years; for the subdirectories holding projects that have moved to git, the repository will be read-only.

The Git conversion team will publicize any changeover date with at least 2 weeks' notice. As of right now (1 Feb 2011) there are no specific cutover dates set.

Chad would like to gradually migrate all projects currently on Wikimedia's Subversion repository so that he can make all of svn.wikimedia.org read-only by the middle of 2013 -- they can use Git/New repositories. They could move to WMF's git repo, or to another host; Chad can help them decide and migrate.


 * MediaWiki extensions (not used by WMF)
 * Starting in March or April 2012, Chad will move alphabetically through all extensions (that are not deployed on Wikimedia Foundation sites) and offer each of them choices as to when and whether to shift.
 * Pywikipediabot
 * The pywikipediabot community has not yet decided on whether to move, but is strongly leaning towards staying with SVN for now. Sumana Harihareswara, Wikimedia Foundation Volunteer Development Coordinator 00:23, 8 February 2012 (UTC)
 * Wikimedia Foundation fundraising
 * Fundraising extensions (including DonationInterface, FundraiserLandingPage, ContributionReporting, et al) can move along with core
 * Fundraising stuff in the Wikimedia repository can move on a timeline TBD
 * Wikimedia Foundation operations
 * Ops is pretty much aware of this since they've already started the git move. Happening piecemeal by them as they're ready.
 * Ariel Glenn's dumps infrastructure
 * Just two paths, /trunk/backups and /branches/ariel/. Should be pretty trivial, history's not complicated.
 * Can convert: as soon as we're ready, just give Ariel a day's notice or so.
 * Moved to operations/dumps.git on 15-Feb-2012. svn made r/o.
 * Wikimedia Foundation data mining and analytics, including Community Department
 * Toolserver internationalisation
 * In active use but maintainers don't have time right now to deal with migration. Another time.
 * Daniel Kinzler's WikiWord project
 * Per IRC: No rush, will move casually after main migration. Not under active development right now.
 * mwdumper
 * Not being actively developed right now. Can move this whenever.
 * Has been moved to mediawiki/tools/mwdumper.git on 15-Feb-2012. svn made r/o.
 * WM planet configuration
 * Was moved into operations/puppet.git (in files/planet) by Dzhan. This whole system needs redoing anyway (see bug 27208, GSoC 2012 project idea), but it'll do for now.
 * Should probably make svn r/o for this.
 * Wikimedia Mobile
 * Currently being done on github -- moving will be easy, just have to talk to mobile team about adjusting their workflow.
 * Continuous integration, for example TestSwarm
 * Not yet migrated to Git
 * Continuous integration, for example TestSwarm
 * Not yet migrated to Git

December 2011

 * Preliminary test conversions early in month ✅, December 5ish
 * Git workflow architecture review ✅, December 19
 * Agree on implementation strategies regarding remaining development process questions, e.g. how to handle multi-repo commits ✅ as of 1 Feb

January 2012

 * CI tests and linting get run when a developer chooses to push to the stage between their branch and the mainline branch (see Ideal Workflow Document and Git-review) (still in progess as of 9 March -- Antoine is working on this. right now, only happens on the master)

February 2012

 * Finish code review on trunk (progress at Code Review stats and MediaWiki 1.19/Revision report) Code review backlog went back up at the end of the month.
 * Cut 1.19 release branch - ✅, Branched at r110996 on 9th February, 2012
 * Ask people to stop creating new extensions in Subversion now - ✅, 11 Feb
 * Communicate to larger community about migration
 * techblog post - ✅, 15 Febuary 2012
 * Make Gerrit behave like we want it to -- TODO [in progress, see March]
 * work on permissions ✅ as of March 9th
 * work on hooks - ✅ as of March 30th
 * Training documentation and interactive training
 * Update Git/Workflow ✅
 * Video training. Set up a test repo, tell 3 people to submit patches simultaneously, walk them through that, then walk them through reviewing it ✅

March 2012
(Chad Horohoe unavailable March 10th-19th; Antoine Musso is his backup.)
 * Finish up specific Git management scripts / changes
 * to support WMF workflow
 * stage git-based tree on fenari (Aaron S to work on fenari git stuff; this blocks 1.20 deployment/release)
 * update documentation
 * i18n updates Bug 34137 ✅ This is nearly complete by March 30th. l10n updates can be pushed from translatewiki, finishing the auto-approval process.
 * Automate the translators' process as much as possible--it currently takes about 20mins/day for Raymond to do and that's a huge timesink. Chad to follow up.  This has to be ready before ANY migration of core to git starts.
 * MediaWiki 1.19RC1 release from Subversion. [Week of 5 March] ✅
 * Code review stats goal by end-of-week: 20 new on phase3, 100 new on phase3+extensions WMF deploys. ✅ (late)
 * 2 weeks before migration of MediaWiki core, start communicating about cutover date -- give date & links to all the documentation with the 3 most frequently asked questions [Week of 5 March]
 * wikitech-l, mediawiki-l [ongoing]
 * add !gitconversion to mw-bot ✅
 * Keep Extension:ExtensionDistributor working. See 27812. [In progress - Sam]
 * MediaWiki 1.19.0 release from Subversion Git. [Week of 12 March]
 * MediaWiki 1.19.0beta2 release from git ✅ March 22nd.
 * Code review stats goal by end-of-week: as close to zero as possible in phase3+extensions WMF deploys. ✅
 * Mass-create Gerrit accounts from the SVN users whose USERINFO has an email address in it, and tell them via wikitech-l that they should just go to the password reset page on labsconsole to start logging in. [Ryan, March 12th] ✅
 * Make Gerrit behave like we want it to -- [[labsconsole:Gerrit bugs that matter|TODO] (better TODO to come)
 * Making permissions right ✅
 * Allow anyone to create a gerrit account without needing human intervention from a gatekeeper [in discussion]
 * Making hooks correct ✅
 * Documentation and training
 * Chad to update the code review guides, with help from Guillaume, before the migration of core. This will include:
 * docs on how current SVN committers can link their LDAP accounts, get passwords put on them, and thus get gerrit accounts (currently on labs, need improvement)
 * Create a page where people can request Gerrit accounts [Sumana, March 13th]: ✅ labsconsole:Help talk:Access
 * procedure for adding and removing people from gerrit project owner groups, including for WMF deployment branch and WMF master ✅ When/how we'll add, remove people from Gerrit project owner groups, Git/Gerrit project ownership
 * change links on mediawiki.org
 * Developer Hub
 * How to become a MediaWiki hacker
 * Commit access
 * Code review guide
 * Download from SVN
 * templates for extensions
 * Writing an extension for deployment
 * Subversion
 * Links to svn.wikimedia.org -- Not super high priority, none of this is disappearing anytime soon.
 * and [//www.mediawiki.org/w/index.php?title=Special:LinkSearch/svn.wikimedia.org&limit=5000&offset=0&target=http%3A%2F%2Fsvn.wikimedia.org more]!
 * and moar!!
 * and MOAR, interwiki links
 * Interwiki map doesn't seem to have a usable way to link stuff on git/gerrit
 * There's now gerrit: as an interwiki link, also git:. Might also want more to gitweb.
 * Git migration -- core & extensions [scheduled for Wednesday, March 21] ✅
 * Make trunk/phase3 & trunk/extensions read-only ✅
 * do deauth of SVN as a pre-commit hook to output an informative error message in case someone tries to commit to MW core -- "Subversion is dead, we have moved to git, read Git/Conversion"
 * pre-commit hooks are in puppet, when someone does this (probably Chad)
 * Right now it's a hard de-auth in authz
 * Get native packages on Windows and Mac for git-review since it's so much easier and better than the manual commit messages hook process. See 35145. ❌

April 2012

 * 2 weeks before migration of MediaWiki extensions not deployed on WMF sites, start communicating about cutover date to extensions developers -- give date & links to all the documentation with the 3 most frequently asked questions (wikitech-l, mediawiki-l)
 * What will switch to gated trunk?
 * Git migration -- ANYTHING ELSE
 * Make paths read-only on case-by-case basis.
 * Ongoing, slow process.
 * Possibly 1.20wmf1 (first mini deployment untethered to release schedule, first of many...1.20wmf2, 1.20wmf3, etc) from git.
 * Move towards git-based development and release process
 * First deployment from git mainline development branch
 * Move towards continuous integration via git, goalpost: weekly deployment
 * Jenkins (Testswarm/PHPUnit tests) on git branches
 * 34141
 * MediaWiki 1.20, first deployment and release from git mainline development branch
 * [//bugzilla.wikimedia.org/showdependencytree.cgi?id=22596&hide_resolved=1 Unsorted "blockers"]

Split up and convert repositories
A naïve  conversion of the entire repository (with branches) weighs in at around 7.8GB (November 2011). It makes no sense to make one Git MediaWiki repository, it should be split up.

In Subversion everything gets squashed into one giant repository. In Git repositories are split at the boundaries over which code does not cross.

Splitting
We have a test repository up, but in February 2012 will redo the split to create a permanent git repo.


 * MediaWiki will go in mediawiki/core.git
 * Extensions will go in mediawiki/extensions/foo.git
 * There will be an extension "meta repository" at mediawiki/extensions.git which will contain all extensions as submodules.
 * Other things across SVN need to find new homes in Git

/Splitting tests

Converting

 * ✅ Every commit needs to be rewritten to give name/email pairs to SVN users. We are using username@users.mediawiki.org for a unified e-mail address scheme for all old commits.
 * Only for those without a known mailaddress or all?
 * What about username@svn.wikimedia.org instead?

Unscheduled items
Other cool things to do (not blockers):
 * ❌ Convert the Bugzilla code to recognize the new SHA-1 commits. Come up with a shorthand to autolink from BZ to gerrit changeset
 * Would Mark H. mind looking into this, since Bugzilla is his baby?
 * Filed as 35144. Workaround: For now, when referring to a Git diff, please paste changeset "Change ID"s, or the changeset number in the Gerrit changeset URL, into the BZ comment. Both are globally unique.
 * ❌ Create database of SVN revision ids -> Git SHA-1's for useful lookups.
 * Info is included in Git commits, just need to make a DB mapping of them. Good weekend project after the conversion is complete if someone is feeling bored.
 * Coren may work on this.
 * ❌ Rusty and Roan's effort to turn every Bugzilla patch into a git pull request.
 * ❌ change MediaWiki CR to readonly, no new comments or statuschanges. This is in the very long run: how to write redirects for viewvc -> gitweb?  But that can wait till like 2013.
 * ❌ Enforce commit message format http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html

Done:
 * ✅ Change the configs of automatic bots -- when I commit, where will it spit out?
 * Isn't this done? gerrit-wm now posts to #mediawiki by default.

Wontfix:
 * ❌ Bump gerrit ids to some big number (eg. 200000) greater than the largest svn revision (so that they could be treated as 'logical' revisions, without conflicting the svn ones).
 * Not easy -- we've already started using gerrit in a production setting and we've got over 2k changesets.
 * We can live with those 2k changesets having the same number of MW ones, as they are different repos. For the actual change, it seems as easy as running ALTER TABLE tbl_name AUTO_INCREMENT = N.
 * WONTFIX

Open issues after migration

 * Niklas is unable to do code review as he used to
 * (bug 35455) Gerrit breaks Unicode in commit messages
 * (bug 35534) How to do tagging of revisions (especially for collaborative projects; "topic" can't be edited after commit)
 * Email notifications are useless/broken
 * (bug 35531) Mailing list with all changes
 * (bug 35533) Link to the unified diff (for all files) and other necessary stuff (very hidden on gitweb)
 * (bug 35532) Unified diff directly in the notification
 * No codereview list either, probably harder and less useful? all actions are sent to IRC
 * Review tool should load fast, display all diffs inline (?)
 * "Gitweb pages load slowly, and for some reason they seem to disable the bfcache in Firefox"
 * (bug 35612) Loading of a change for review in Gerrit is often very slow
 * Path filtering also for unmerged commits
 * Unable to push for review if there are local uncommitted changes (like debugging hacks)
 * This is by design so you don't lose changes. If you've got local changes you don't want to commit but want to hang on to, use git stash.
 * If i understand correctly, the "local uncommitted changes" are changes that were committed to the developer's local repo and submitted using `git review' and were not yet reviewed and merged. See Talk:Git/Workflow, Committing followups: please no --amend and consecutive commits in Gerrit. --Amir E. Aharoni (talk) 08:36, 27 March 2012 (UTC)
 * No. These are changes that are not going to be committed upstream.

Ideal state
This is what we'd love to see:

History

 * See history of MediaWiki version control

Working on the conversion

 * User:^demon
 * Antoine Musso
 * Roan Kattouw

Would like to see it happen

 * 
 * Aryeh Gregor
 * Ashar Voultoiz (already use git locally)
 * Daniel Friesen
 * Gregory Varnum

Documents

 * User requirements:
 * Specifications: Gerrit bugs that matter
 * Software design document:
 * Test plan:
 * Documentation plan:
 * User interface design docs: Gerrit bugs that matter
 * Schedule: see Timeline above
 * Task management: Task list from Bugzilla (bug 22596)
 * Release management plan:
 * Communications plan:
 * Status updates

Communications

 * draft plan
 * announcement of test repository
 * "git boot camp" from October 2011 NOLA hackathon GitBootcamp
 * https://blog.wikimedia.org/2012/02/15/wikimedia-engineering-moving-from-subversion-to-git/
 * Git, Gerrit, and You! or, Gerrit training available starting Monday 27 February
 * Postponing Git migration until March 21
 * When/how we'll add, remove people from Gerrit project owner groups
 * Git migration: documentation and short-term considerations
 * MediaWiki core deployments starting in April, and how that might work