Mercurial 4.3 release

2017-08-02

Mercurial 4.3 has been released and it's a pretty big release, you can find the release note here or below where we will highlight some changes:

Notable changes

  • Support for Python 2.6 has been dropped, so if you are still using it, stay on version 4.2 or update to Python 2.7.
  • Bundles now store phase information, you can now safely share draft and public changesets with your colleagues using bundles.
  • Two security issues have been fixed related to ssh repository URL and symlinks, for all details, see below.
  • The share extension now properly shares relevant caches between shares.
  • Server can now allow more concurrent pushes as long as they do not affect the same heads.

Multiple interesting experiments are going on:

  • Status is gaining a 'terse' mode. In this mode, the status of files is aggregated by directory.
  • A new option to improve manifest storage on repository with many branches has been added.
  • The sparse extensions have been integrated into the core distribution as experimental.

There have been a couple of update on the changeset evolution side:

  • Strip extension now removes relevant obsmarkers. So if you want to get rid of an amend, strip the successor and relevant obs-markers will be stripped too. The backup bundle will contain the stripped obsmarkers so they can be restored if necessary.
  • pushs and pulls now informs the user when changesets get obsoleted.

A handful of significant internal reworks happened in this release:

  • The support for Python 3 has been improved, every release brings us closer to be fully Python 3 compatible.
  • Transactions are now storing precise data about the changes it contains. This allows for accurate reports and more advanced extensions logic.
  • The rebase extensions internals were reworked to allow more advance rebase scenario with multiple destinations at the same time.
  • Config options can now be formally registered. This will unlock nice feature in the future like config validation and automatic documentation generation.

Security issue

The 4.3 release contains a fix for two CVE: CVE-2017-1000115 and CVE-2017-1000116. Upgrade as soon as possible. See the mailing-list for more information about the vulnerabilities.

Please be aware that the 4.3 release drop support for Python 2.7. Due to the vulnerabilities, a 4.2.3 version has been released with the security fixes. Upgrade to 4.2.3 if you are still using Python 2.6. You can download the release here.

We had to backport the security patches for Mercurial 4.1.3 for some of our customers. Due to the severity of the vulnerabilities, we made the patches available for everyone at the following address: https://bitbucket.org/octobus/mercurial-backport/branch/backport-4.1.

User-visible changes

Packaging

We did the necessary work to generate Linux wheels for the 4.3 release. For people who don't know what a wheel is, it's basically a compiled packaged of Mercurial installable by pip. We built wheels for Linux:

  • No need for a compiler
  • Declaratives metadata
  • Faster installation

On my laptop, the installation of Mercurial through pip went from 13 seconds with the default package to 1 second only.

If you want to test them, here are the instructions but be aware that we provide them as an experiment, you are using them at your own risk:

Run pip install -U --index-url=https://packagecloud.io/octobus/Mercurial/pypi/simple mercurial to update your mercurial installation with the 4.3 release of mercurial. Be warned that they are only Linux wheels, Windows wheels are available on Pypi and should be selected automatically. Mac OS X wheels are not available yet.

Behavior changes

There is a couple of backward incompatible changes, be sure to take a look at them! We compile a non-exhaustive list of the most notable ones:

  • The backup bundle name has changed for most operations.
  • The clonebundle hook arguments have changed.
  • hg update now show the commit it updated to in case of multiple heads.

Sharing caches

The share extension helps working with big repositories. It can share the 'store' (ie history) portion of a repository between multiple working copy. One thing was still painful, the various caches were not shared so each repository has to compute the phase cache, tag cache, bookmark cache, etc... which takes a significant amount of time per repository.

But that was before and with Mercurial 4.3 caches are now correctly shared, improving the performance when working with big repositories.

Concurrent pushes

When, pushing, a client first exchanges data with the server to discover the minimal information that actually needs to be pushed. Then it prepares a bundle containing these information and push it. It is possible for another client to update the repository between the discovery phase and the actual push phase. Voiding the validation that the client might have made before sending the data. Mercurial has a mechanism to detect such situation and abort the raced push.

This race detection can get in the way for repository with a lot of incoming traffic. So we introduced a new server.concurrent-push-mode option to control this behavior. The server config can be setup to accept pushed prepared concurrently if they affect unrelated heads.

    [server]
    concurrent-push-mode = related

Change in the Experimental realms

Manifest size

We have been investigating a sudden increase in the manifest size for some of our clients. The issue comes from a heuristic responsible of triggering new full snapshots. That heuristic fails badly when the number of concurrent branches becomes too large.

During the 4.3 cycle, we have upstreamed a new experimental option named 'maxdeltachainspan'. With this option set to 0, we have seen a reduction of manifest size in the order of 70x in our clients. This impacts push, pull and day-to-day commands durations. The manifest size exposition triggers when the number of parallel branch growth. So the more branches and merges you have, the bigger the win should be.

If you want to test it against your repositories:

  1. Make a new local clone
  2. Update the config to contains

        [format]
        aggressivemergedeltas = yes
        [experimental]
        maxdeltachainspan = 0 # no limit
    

  3. Run hg debugupgraderepo --optimize redeltaall --run --config format.aggressivemergedeltas=yes --config experimental.maxdeltachainspan=0 (this will likely take a long time).

Terse status

Pulkit Goyal started to integrate the terse status feature in the core hg status commands. When this flag is passed, directory summary will be used whenever possible. For example:

    $ hg status
    ? foo/x
    ? foo/y

Becomes:

    $ hg status
    ? foo/

[Experimental] - Sparse extension

The sparse extension is now shipped as an experimental extension. It is helpful with big mono-repositories when you only want a subset of your repository files to be checked out.

All the information can be found on the sparse extension documentation accessible with: hg help sparse.

Contribution statistics

To conclude, we compiled contribution in number of changesets from various contribution sources. Changesets are not an absolute metric, but it helps to get an idea of where contributions come from. Contributors from the same company have been grouped for clarity.

361 @octobus.net
291 @google.com
220 @fb.com
197 yuya@tcha.org
127 @mozilla.com
104 7895pulkit@gmail.com
84 matt_harbison@yahoo.com
61 foozy@lares.dti.ne.jp
18 sean@farley.io
12 kbullock@ringworld.org
6 wbruna@softwareexpress.com.br
6 @logilab.com
5 @nokia.com
4 demelier.david@gmail.com
4 danek.duvall@oracle.com
4 av6@dwimlabs.net
3 rishabhmadan96@gmail.com
3 marutosijp2@gmail.com
2 @unity.com
2 steve@borho.org
2 bamccaig@gmail.com
1 mtietze@gmx.com
1 cryo@cyanite.org
1 blacktrash@gmx.net
1 andrew.zwicky@gmail.com