Interesting to see them try to fix Git when Facebook considered it (due to similar constraints) and decided to go with Hg instead: https://code.facebook.com/posts/218678814984400/scaling-merc...
Microsoft took the same look and went the opposite direction, and decided that Git was easier for us to hack on. This was a no brainer for us. We build multiple tools that host peoples Git repositories (TFS and VSTS) and have Git repositories as deployment mechanisms for Azure. We have several contributors to tools in the Git ecosystem on staff, including people contributing to core git, and the maintainer for Git for Windows, libgit2 and LibGit2Sharp. But we have comparatively little institutional knowledge of Hg.
That post you linked is awesome. Facebook has done a lot of really impressive work scaling Hg and a lot of the lessons learned at Facebook and implementations in Hg are very similar to what we've done with Git.
This video from Durham Goode on Facebook's version control team (from Git Merge, amusingly) is also awesome: https://m.youtube.com/watch?v=gOVD-DrUpwQ
The ability to extend, swap and replace deep into the stack was always a core component of Mercurial, hence having to enable extensions for things many users would consider core features e.g. terminal colors or graph log
Facebook didn't hack around like monkeys, they built extensions. And when they could not do it as extensions, they upstreamed improvements to the core.
The alternative would have been to fork the codebase entirely.
Monkey patching the internals would have been significantly less maintainable: having used it as a library, I can tell you that none of the internal stuff is considered stable and pretty major components will change between point releases (I was using the diff-parsing and patch-application routines for something else, the API changed basically every minor release, forking would at least give you a heads-up conflict when upstream changed, monkey-patching would either blow up at runtime or not go through the patch anymore)
> They're now talking about implementing parts in rust, which, ironically, would have prevented them from doing what they originally chose mercurial for.
No.