Mercurial Plugin for Trac (#1847)

There is an experimental plugin for Trac 0.10 which enables ​Mercurial to be used instead of ​Subversion as the VersioningSystemBackend for Trac.

The plugin being experimental, expect some rough edges and a somewhat unstable set of features/documentation…

Change Logs:

• log:sandbox/mercurial-plugin
• log:sandbox/vc-refactoring

Download and Installation

Trac

The plugin needs a slightly modified version of Trac 0.10. This version can be found in source:sandbox/vc-refactoring.

Simply do one of (see revision table below):

1. Use Trac 0.10 (see TracDownload) and install it (see TracInstall)

2. or use the version in trunk (0.11dev). The advantage being that you don't need ClearSilver anymore.

   svn co http://svn.edgewall.com/repos/trac/trunk trac
   svn co http://svn.edgewall.org/repos/genshi/trunk genshi
   

and install from there:

$ cd trac
$ python setup.py install
$ cd ..
$ cd genshi
$ python setup.py install

(for Genshi, you might prefer to use a packaged release, see ​Genshi:GenshiDownload, or to easy_install it)

The plugin itself is available from source:sandbox/mercurial-plugin

Check it out, too:

svn co http://svn.edgewall.com/repos/trac/sandbox/mercurial-plugin 

and create an "egg" from there

$ cd mercurial-plugin
$ python setup.py bdist_egg

Note that you'll need ​setuptools

0.6 for that (I used setuptools-0.6a9).

Releases

Version	mercurial-plugin	Trac vc-refactoring	Compatible with hg
0.1	r2514	r2511	0.7, tip without 1d7d0c07
	r2620	r2620	0.7, tip with 1d7d0c07
0.2	r2905	r2905	0.7, 0.8, tip
0.2	r3014	trunk@2900​	0.7, 0.8, tip
0.2	latest	0.10 or trunk​	0.7, 0.8, 0.9, tip

Mercurial

The plugin has been tested with recent development versions of Mercurial (upto Changeset 3324:34f08b8883cf from ​http://selenic.com/hg) and also with Mercurial 0.7 and 0.8. It won't work with earlier versions, in particular not with 0.6x.

You can download Mercurial itself from ​Hg:Download.

Configuration

The configuration has to be done on the Trac side, there's nothing to do on the Mercurial repository side, except for the fact that the repository should be made accessible as a local repository. Thanks to the distributed nature of Mercurial, that's always possible (if the repository is not already local, simply hg clone it).

Setting up the mercurial plugin

The TracMercurial-0.2 plugin egg should be added to the plugins folder of the environment, or it can be globally installed (python setup.py install or a python setup.py develop).

For general instructions about plugins, see also TracPlugins.

If you installed the egg globally and you're modifying an existing Trac environment to use the Mercurial backend, then you have to explicitely enable the plugin in TracIni:

[components]
tracvc.hg.* = enabled

Setting up a Trac environment

You can either reuse an existing Trac environment, or create a brand new one.

For general instructions, see TracInstall.

Since milestone:0.10, the TracAdmin initenv command has now a repository type argument besides repository directory.

For the repository type, specify hg instead of the default svn. For the repository directory, specify the location of the Mercurial repository (without the ending .hg).

Your <trac_environment>/conf/trac.ini configuration file should have a [trac] section similar to the following:

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

There's also a few Mercurial specific settings in TracIni:

[hg]
# -- Show revision number in addition to the changeset hash
show_rev = yes

# -- Changeset hash format
node_format = short
# hex:   Show the full SHA1 hash 
# short: Show a shortened hash for the changesets 

Features

The Mercurial support is pretty basic, but works well. I've tested that on the Mercurial repository itself and the performance is acceptable, even if there's currently no caching in the database (this is what I'm going to work on next).

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):

has to be re-added

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)
• Revision log ranges [xxx:yyy]
• View arbitrary diffs

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).

Note that the last comment on the above ticket suggests a good workaround for this problem.

To cache or not to cache?

When you try TracMercurial on the kernel repo, you quickly realize that it's way too slow without a db! Also, the way the diffs are produced currently (i.e. by Trac, from the full content of the files) is also too slow to be usable on such big repositories (e.g. the kernel changeset fc66195f585a took 7 minutes to be displayed on my machine). See #2591.

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.

There's an existing web implementation by Alexander Schremmer:

​http://moin.pocoo.org:8080/ (select a repo and then 'branchview' button on the top to see it in action)

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 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