Hooks changes on hg.mozilla.org

[Gregory Szorc]

Some changes to server-side push hooks were just made to hg.mozilla.org.

First, we're in the process of rewriting hooks to a new extension-based
format so hooks are faster, easier to maintain, and more consistently
installed. e.g. we can now target hooks to run on all Firefox repositories
instead of manually having to enable hooks on a per-repo basis.

As part of this conversion, it is possible some hooks may be running or not
running on repos you expect them to. If you see any weird behavior, make
noise in #vcs.

Next, we have banned symlinks on all non-user repos. Existing symlinks
(find them with `hg files 'set:symlink()'`) can continue to exist. We allow
symlinks to be deleted and converted to regular files. But the symlink
target cannot be changed and new symlinks cannot be added. This was tracked
in bug 985087.

Finally, we have banned subrepositories on all non-user repos. They were
only ever used on 4 repos. So practically nobody is impacted by this. This
was tracked in bug 1388559.

The reason for the symlink and subrepositories ban is to mitigate potential
exploits due to known vulnerabilities that were disclosed today. It's worth
noting that subrepositories account for a disproportionate amount of the
CVEs in Mercurial and Git over the years. I think banning them to reduce
attack surface area is easily justified.

The justification for outright banning symlinks isn't as great. I'm not
opposed to curtailing the scope of the ban if it becomes problematic.

If you run into problems, make noise in #vcs.