Mercurial Plugin for Trac (#1847)

There is an experimental plugin for Trac 0.9 which enables ​Mercurial to be used instead of ​Subversion for the VersioningSystemBackend.

Download and Installation

Trac

The plugin needs a slightly modified version of Trac 0.9, which offers support for pluggable SCM backends. This version can be found in

source:branches/cboos-dev/vc-refactoring

Simply do:

svn co http://svn.edgewall.com/repos/trac/branches/cboos-dev/vc-refactoring trac-for-hg

and install from there (cd trac-for-hg; python setup.py install).

The plugin itself is available from

source:branches/cboos-dev/hg-plugin

Check it out, too:

svn co http://svn.edgewall.com/repos/trac/branches/cboos-dev/hg-plugin .

and create an "egg" from there (cd hg-plugin; python setup.py bdist_egg). Note that you'll need ​setuptools

0.6 for that (I used setuptools-0.6a5).

The initial release of the plugin was made at r2472 and works well with the vc-refactoring branch at that revision.

Mercurial

The plugin has been tested with Mercurial 0.7 and a few later versions (upto Changeset 1481:acd1832ebf31 from ​http://selenic.com/hg). It won't work with earlier versions. You can download hg from ​Hg:Download.

Configuration

The plugin egg can be added to the plugins folder of an existing environment, or to a brand new environment. Note that if you create a new environment, it's currently necessary to not specify the repository directory during the initenv step (this will probably be fixed soon).

In either case, you'll need to specify the backend scheme to be used (here hg:) in front of the actual repository location, in TracIni:

[trac]
repository_dir = hg:/path/to/my/hg/repository

Also, you'll need to explicitely disable the SVN components:

[components]
trac.versioncontrol.svn_fs.* = disabled

Note: that constraint will most certainly be lifted in the future

Features

The Mercurial support is pretty basic, but works well. I've tested that on the Mercurial repository itself and the performance is quite good, even if there's currently no caching in the database (I'm still not decided if that's a feature or a bug).

For those used to Subversion in general and Subversion repository browsing in Trac in particular, there are a few differences worth noting.

Mercurial Changesets

Changeset Navigation

In Mercurial, the Previous Changeset/Next Changeset navigation is not purely sequential, as it is in Subversion. Instead of a flat history of successive changesets, we actually navigate a DAG of changesets. This means a changeset can have multiple parents (0, 1 or 2) and multiple children as well (0 to n).

Therefore, Previous Changeset is a link to the first parent, and Next Changeset is a link to the first children. In case there are additional parents or children, these are shown as additional changeset properties (Parents or Children), placed below the Author property and above the Message property.

Another additional changeset property is the list of Tags that might be associated with a changeset.

Wiki syntax

The Wiki syntax has been extended a bit, to cope with the hexadecimal notation of hg changesets. E.g [8ef2] would link to the changeset 8ef2ba892518c115170398ec754bd1c27cab271f … Also, it is possible to refer to changesets using the changeset: prefix (or cset: or chgset:, for hgweb compatibility). The tag: prefix can be used to refer to symbolic tags, although this is not a requirement (using. e.g. cset:tip would work too). Finally, the branch: prefix has a special meaning, as this will not select the specified revision, but the head which is reachable from that revision.

TracBrowser changes

The TracBrowser View revision form has been extended with pulldown menus for jumping to a given tag or branch (in Mercurial, a branch within a repository corresponds to a head, i.e. a changeset without children):

Bugs and Limitations

There are still a lot of things that can be improved.

Features that Trac+svn has but not currently implemented for Trac+hg

• History doesn't follow copy/move operations
• No path history mode (i.e. show all create/delete operations that affected a given path)

Multi-repository support

First and foremost, even if Mercurial allows intra-repository branching, it strongly supports the use of branching by cloning the full repository. Therefore, it is common to have a lot of hg repositories around, each devoted to the implementation of some particular feature.

Trac should support this by the way of multiple repository support within a single environment (see #2086).

Arbitrary diff support

The TracDiff features should be supported by TracMercurial too.

To cache or not to cache?

I happen to like to current database-free implementation of the Mercurial support. It makes it quite easy to change the repository_dir on the fly, and this flexibility is good. Also, hg being remarkably efficient, as everybody knows, the performance is quite good… Except in one case, getting the history of a directory. There, I suspect that the database cache would be quite useful. But I have yet to try on a big repository (e.g. the kernel) with either approach before concluding. In any case, thanks to the vc refactoring, the possibility will be offered to choose between a direct access or a cached access, as it's done for Subversion, svn: (cached) or direct-svn-fs: (direct access).

Cool Features

Wild ideas section…

Visualize branches and merges

There should be a way to show graphically the branch and merge points within the revision log view. Not something as fancy as hgk, but nonetheless something that will make the changeset relationships immediately obvious.

Search over the source

A search provider could do the equivalent of an hg grep.

Highlight Conflict Resolution

While visualizing changeset diffs for merge changesets, we already show the changes relative to both parents, which helps to understand how conflicts (if any) were solved. But this can be improved by specifically highlighting lines that differs from both parents.

Add your cool feature here…

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Implementation Notes

I'm interested in feedback concerning the code, in particular:

• concerning Trac, there are only a few changes to the standard modules, ChangesetModule (the additional properties) and BrowserModule (the tags and branches support). Those changes could possibly be promoted to the generic level, therefore removing the need for having to explicitely disable the SVN components…
• concerning Mercurial, I'm pretty sure I did things in a sub-optimal way, as I was discovering the guts of hg while writing the plugin. Therefore, I'll be pleased to get tips for improvements.

— ChristianBoos