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| =Migrating from Hg to Git/Github=
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| In discussing a migration from Mercurial (Hg) to Git, there are really two issues to address:
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| # migration from Hg to Git
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| # migration from Mozilla hosted to third party hosted (Github) repositories
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| This page summarizes the benefits, issues, and requirements for both #1 and #2. The material is flagged by role to allow people to easily view the information relevant to them.
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| There are three primary roles that make use of Mozilla's source code management (SCM) systems: developers {{devs}}, release engineers {{releng}}, and IT system admins {{IT}}.
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| ==Migration from Hg to Git==
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| ===Benefits===
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| * Simplified onboarding for new contributors as Git is better known today than Hg {{devs}}
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| * Git supports new workflows not supported by Hg {{devs}}
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| * <strike>Git lets developers have a single repository (and multiple checkouts if so desired) for m-c, aurora, beta, etc, work {{devs}}</strike>
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| ** This is doable with the [http://mercurial.selenic.com/wiki/ShareExtension share extension]. See https://bitbucket.org/indygreg/gecko/ for a unified Mercurial repository.
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| * Better merging algorithm, fewer conflicts {{devs}}
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| * <strike>Easier source code archeology, we'll have history since the dawn of time (i.e. all of Hg and CVS) {{devs}}</strike>
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| ** This is doable with Mercurial. We'd need a flag day. But we could do it if we wanted.
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| * Git performs better than Hg (modulo in some operations on Windows, which we're working on) {{devs}}
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| * <strike>Hg occasionally corrupts repositories (known bug, wontfix), no such known issues with Git {{devs}} {{releng}}</strike>
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| ** Modern versions of Mercurial don't corrupt repositories with nearly the same frequency as older versions. If you are referring to {{bug|737865}}, this is because Mozilla is hosting Mercurial over NFS, which is not recommended.
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| * Mozilla currently makes use of Hg and Git, consolidation is easier for all devs {{devs}}
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| * <strike>Better review tools (Gerrit) available for devs {{devs}}</strike>
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| ** Most review tools are built on the concept of *take a patch against a parent revision* and this concept works equally well with Mercurial and Git. ReviewBoard, Rietveld, and Phabricator all support Mercurial and Git.
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| * Simplified debugging (no longer have to debug Hg) {{releng}} {{IT}}
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| ** You are just swapping Mercurial for Git.
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| * <strike>Simplified management as only one SCM system {{releng}} {{IT}}</strike>
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| ** IT will still manage hg.mozilla.org as not every project will switch to Git.
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| ===Issues===
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| * Overhead in having a large number of people learn new tools {{devs}}
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| * [http://msysgit.github.com/ Git for Windows] performance (per above) compared to Hg {{devs}}
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| * IT team has limited experience with Git at this point {{IT}}
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| * git.m.o has had limited usage, need to review and stress test to ensure deployment can support scale {{IT}}
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| * Cost to releng to update build and related tools {{releng}}
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| ===Requirements===
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| * Update build tools (Windows tools specifically) to support Git {{devs}}
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| ** jlebar's tools https://github.com/jlebar/moz-git-tools
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| * Must be able to create user repos as can do on Hg {{devs}}
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| * [https://hg.mozilla.org/hgcustom/pushlog/ Pushlog] equivalent needs to be created for Git {{devs}} {{releng}}
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| * [https://hg.mozilla.org/hgcustom/hghooks/file/93dc9c506add/mozhghooks Repository hooks] {{releng}}
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| **tree closure
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| **approval required hook
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| **try syntax
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| **l10n change protector for aurora + beta (not yet implemented - see {{bug|859358}})
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| * Update for Nightly about:buildconfig to link to Git revision {{releng}}
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| * Bugzilla must be updated to links to Git change sets, existing Hg links must not break {{devs}}
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| * [https://treeherder.mozilla.org treeherder.mozilla.org] (links to changeset id) must be updated to support Git or an equivalent system put in place {{devs}} {{releng}}
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| * [https://crash-stats.mozilla.com crash-stats] must be updated to support Git {{devs}}
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| * l10n tooling must be updated to support Git
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| * Will need to scale Git infrastructure (currently 2 machines) but should be able to reuse Hg machines {{IT}}
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| * Replacements for third party tools currently in use by dev community
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| ** Dave Townsend's [http://hgchanges.fractalbrew.com/ HG Change Feed] ([https://github.com/Mossop/hgchangefeed source])
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| ==Moving from Mozilla hosted to third party (Github) hosted==
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| ===Benefits===
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| * Easier onboarding/discoverability for community {{devs}}
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| * Nice change review system {{devs}}
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| ===Issues===
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| * Security of repo on hosted provider (can we trust our code to live there?) {{releng}} {{IT}}
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| * Up-time of hosted repos (Mozilla up-time is very good, have had sync problems with GitHub repos) {{releng}}
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| * Latency and failed pulls, some builds must pull from ~30 repos {{releng}}
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| * Some teams (NSS, security) require internal hosting {{devs}} {{IT}}
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| * Stability of repo / long term viability {{devs}} {{releng}} {{IT}}
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| * Github issues cause confusion as work is tracked in Bugzilla (can issues be disabled?) {{devs}}
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| * No pre-commit hooks, only post-commit hooks {{releng}}
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| ** no ability to block code commits, only react to bad commits
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| * Limited ability to add new features
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| ===Requirements===
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| * Must be able to back out changes without leaving history {{releng}}
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| * Must have ability to duplicate current repository hooks (see requirements) functionality {{releng}}
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| ==References==
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| * [https://etherpad.mozilla.org/value-of-github Value of GitHub etherpad]
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| * gps blog post: [http://gregoryszorc.com/blog/2013/05/12/thoughts-on-mercurial-(and-git) Thoughts on Mercurial (and Git)]
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| * gps blog post: [http://gregoryszorc.com/blog/2013/05/13/the-state-of-mercurial-at-mozilla The State of Mercurial at Mozilla]
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