Upgrading Mercurial

This document describes how to upgrade the Mercurial version deployed to hg.mozilla.org.

Managing Mercurial Packages

We generally don’t use Mercurial packages provided by upstream or from distros because they aren’t suitable or aren’t new enough. So, we often need to build them ourselves.

Building RPMs

Mercurial RPMs are created by invoking make targets in Mercurial’s build system. From a Mercurial source checkout:

$ make -j2 docker-centos6 docker-centos7

This will build Mercurial RPMs in isolated Docker containers and store the results in the packages/ directory. .rpm files can be found under packages/<distro>/RPMS/*.rpm. e.g. packages/centos7/RPMS/x86_64/mercurial-3.9-1.x86_64.rpm.


CentOS 6 RPMs are configured to use Python 2.6, as that is the Python used by CentOS 6 by default. Various Mercurial extensions at Mozilla require Python 2.7. So the system Mercurial RPMs produced via this method aren’t guaranteed to work with everything. Furthermore, Python 2.7 is faster than Python 2.6. For these reasons, it is recommended to avoid running Mercurial from the system Python on CentOS 6. Instead, run CentOS 7 or install Python 2.7 and run Mercurial from a virtualenv.

Building .deb Packages

The process for producing Debian .deb packages is similar: run Mercurial’s make targets for building packages inside Docker:

$ make docker-ubuntu-xenial

.deb files will be available in the packages/ directory.

Uploading Files to S3

Built packages are uploaded to the moz-packages S3 bucket.

CentOS 6 packages go in the CentOS6 folder. CentOS 7 packages in the CentOS7 folder.

When uploading files, they should be marked as world readable, since we have random systems downloading from this bucket.

Upgrading And Modifying Templates

The repository contains a vendored copy of Mercurial’s templates plus modifications in the hgtemplates/ directory. These templates are used by the hgweb server.

We have made several modifications to the templates. The most significant modification is the addition of the gitweb_mozilla theme, which is a fork of the gitweb theme. We have also made a number of changes to the json theme to facilitate rendering additional data not exposed by Mercurial itself.

Modifying Templates

When modifying a template, it isn’t enough to simply change a template file in hgtemplates/: you must also track that change by recording it somewhere in hgtemplates/.patches/.

Most modifications are tracked by patch files. Essentially, there exists a standalone patch file describing the change. To modify a template via patch file, do the following:

  1. Create a new Mercurial changeset like you normally would. i.e. make file changes and hg commit the result.
  2. Export the just-created changeset to a standalone patch file. e.g. hg export . > hgtemplates/.patches/my-change.patch.
  3. Track the new patch file via hg add hgtemplates/.patches/<name>.patch.
  4. Modify the hgtemplates/.patches/series file and add the new patch file to the list.
  5. Run run-tests /hgserver/tests/test-template-sync.t and verify the test passes.

test-template-sync.t verifies that the current state of the checkout matches what would be obtained if all modifications were performed on a fresh copy of the templates. In other words, it verifies we can reproduce the current state of the templates.

If the test passes, hg commit --amend or hg histedit your changesets so the modifications to hgtemplates/.patches are part of the changeset that modifies files in hgtemplates/. Then submit that for review and land when acceptable.

For other modifications (such as adding or removing a file), see the file lists at the top of hgtemplates/.patches/mozify-templates.py to influence behavior.

Upgrading Templates

When Mercurial is upgraded, we need to synchronize our vendored templates with the new templates from upstream.

To do that, we run the hgtemplates/.patches/mozify-templates.py script. This script will:

  1. Wipe away hgtemplates/.
  2. Copy the canonical templates from upstream into hgtemplates/.
  3. Perform special modifications to templates (notably adding and removing certain files and performing hard-coded template transforms).
  4. Attempt to apply and commit each patch listed in hgtemplates/.patches/series.


Ensure your working directory is clean and hgtemplates/ is free of untracked files before continuing. Run hg revert -C hgtemplates/ and hg purge hgtemplates/ to do this.

To perform an upgrade:

$ hgtemplates/.patches/mozify-templates.py /path/to/mercurial/templates \
    hgtemplates hgtemplates


$ hgtemplates/.patches/mozify-templates.py venv/mercurials/4.6.2/lib/python2.7/site-packages/mercurial/templates \
    hgtemplates hgtemplates

This tells the script to copy templates from the 1st argument, to grab patches and files from the 2nd argument, and to write the result into the 3rd argument.

If everything is successful, several commits would have been made. You can use e.g. hg show stack (assuming the show extension is enabled) to see them. These changesets can be removed via hg prune or hg strip without causing harm.

If the script fails, chances are it failed to run hg import to apply a patch. Your working directory may or may not be in a good state. Check that with hg status and resolve via hg revert etc as appropriate.

To recover from a non-working patch file, you’ll need to update the failed patch file until it applies cleanly. To do that, look at the process output for the name of the patch file that failed to apply. Next, you’ll attempt to apply it manually. e.g. if the foo.patch file fails:

$ hg import --partial hgtemplates/.patch/foo.patch

You will then need to resolve any conflicts, fix the files until they are in the state you want, etc. Then hg commit --amend the result. This will produce a new changeset with a working version of the patch.

Next, you will update the failing .patch file with the new version and commit the result. e.g.

$ hg export > hgtemplates/.patch/foo.patch $ hg commit -m ‘hgtemplates: update foo.patch for Mercurial 4.7 upgrade’

Then you need to start the template upgrade process over from the beginning with the modified .patch file in place. e.g.:

$ hg up @
$ hg rebase -s tip -d .
$ hgtemplates/.patches/mozify-templates.py venv/mercurials/4.6.2/lib/python2.7/site-packages/mercurial/templates \
    hgtemplates hgtemplates

You can also safely hg prune or hg strip the changesets produced by mozify-templates.py.

Once you’ve repeated this process and mozify-templates.py completes without error, hgtemplates/ now contains the upstream templates plus our modifications.

Then, modify hgserver/tests/test-template-sync.t so it picks up the Mercurial templates from the appropriate Mercurial version in its mozify-templates.py invocation. And run this test and verify all is happy. Then commit that change.

At this point, the repository has several commits. There could be modifications to hgtemplates/.patches/. There will be changesets tracking the upstream changes to hgtemplates/ and changes made by each patch. And there should be a changeset for the change to test-template-sync.t.

At this point, it is recommended to run hg histedit and roll all the changesets together. This will produce a unified changeset containing every change. It should effectively be a diff of the upstream changes plus whatever changes to patches were needed to accommodate upstream changes. This changeset should be suitable for review.