The hghooks directory contains a number of Mercurial hooks used by Mozilla projects.

The content of this directory originally derived from its own repository. Changesets e11fee681380 through 1f927bcba52c contain the import of this repository.

This directory has its origins in the operation of the Mercurial server at Mozilla. It is an eventual goal to restructure the hooks to be usable on both client and server.

Available Hooks


Older versions of Mercurial had a bug where the set of modified files stored in the commit object were incomplete. Operations that relied on this cached set of changed files (hooks, some revset queries, log) could have inaccurate output if a buggy commit was present.

This hook looks for the presence of buggy metadata and rejects it.


This hook attempts to enforce that commit messages are well-formed. It is targeted towards the Firefox commit message standard.


This hook prevents file renames that only change the case of a file. e.g. renaming foo to FOO would be disallowed.

This hooks exists to prevent issues with case-insensitive filesystems.


This hook is used to prevent changes to strings in string frozen release heads without the explicit approval from l10n-drivers.


This hook prevents changes to WebIDL files that shouldn’t be made.

All WebIDL changes must be reviewed by a DOM peer and this hook enforces that.


This hook prints relevant information about a push that just completed.

It will print the URL of the changesets on https://hg.mozilla.org/. It will also print TreeHerder URLs for Try pushes.


This hook enforces that all Mercurial branches contain at most one head.


This hook prevents pushes to Firefox repositories that are currently closed.


This hook enforces the requirement that pushes to the Try repository contain Try job selection syntax.

Hook Development Standards

Hooks are written and loaded into Mercurial as Python modules. This goes against recommendations by the Mercurial project. However, we do this for performance reasons, as spawning new processes for hooks wastes valuable wall time during push. (Mercurial recommends against in-process hooks because they don’t make promises about the stability of the internal API.)

Hooks should be unit tested via .t tests and should strive for 100% code coverage. We don’t want any surprises in production. We don’t want to have to manually test hooks when upgrading Mercurial. We should have confidence in our automated tests.

Pre-commit (notably pretxnchangegroup) hooks should filter the strip source and always return success for these. If an hg strip operation is running, the changesets already got into the repository, so a hook has no business checking them again.

Any hook change touching a Mercurial API should be reviewed by someone who knows Mercurial internals. You should default to getting review from gps.

Hooks connecting to external systems or performing process that could be deferred will be heavily scrutinized. We want hg push operations to be fast. Slow services, networks, or CPU or I/O intensive hooks all undermine that goal.