Migrating to Mercurial - Open Source Mercurial: The Definitive Guide

by Bryan O'Sullivan

A common way to test the waters with a new revision control tool is to experiment with switching over an existing project, rather than starting a new project from scratch.

In this appendix, we discuss how to import a project’s history into Mercurial, and what to look out for if you are used to a different revision control system.

Mercurial: The Definitive Guide  book cover

This excerpt is from Mercurial: The Definitive Guide . This definitive guide takes you step by step through ways to track, merge, and manage both open source and commercial software projects with Mercurial, using Windows, Mac OS X, Linux, Solaris, and other systems.

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Importing History from Another System

Mercurial ships with an extension named convert, which can import project history from most popular revision control systems. At the time this book was written, Mercurial could import history from the following systems:

  • Subversion

  • CVS

  • Git

  • Darcs

  • Bazaar

  • Monotone

  • GNU Arch

  • Mercurial

(To see why Mercurial itself is supported as a source, see the section called “Tidying Up the Tree”.)

You can enable the extension in the usual way, by editing your ~/.hgrc file.

[extensions]
convert =

This will make an easy-to-use hg convert command available. For instance, the following command will import the Subversion history for the Nose unit testing framework into Mercurial.

$ hg convert http://python-nose.googlecode.com/svn/trunk

The convert extension operates incrementally. In other words, after you have run hg convert once, running it again will import any new revisions committed after the first run began. Incremental conversion will only work if you run hg convert in the same Mercurial repository that you originally used, because the convert extension saves some private metadata in a non-revision controlled file named .hg/shamap inside the target repository.

When you want to start making changes using Mercurial, it’s best to clone the tree in which you are doing your conversions, and leave the original tree for future incremental conversions. This is the safest way to let you pull and merge future commits from the source revision control system into your newly active Mercurial project.

Converting Multiple Branches

The hg convert command given above converts only the history of the trunk branch of the Subversion repository. If we instead use the URL http://python-nose.googlecode.com/svn, Mercurial will automatically detect the trunk, tags, and branches layout that Subversion projects usually use, and it will import each as a separate Mercurial branch.

By default, each Subversion branch imported into Mercurial is given a branch name. After the conversion completes, you can get a list of the active branch names in the Mercurial repository using hg branches -a. If you would prefer to import the Subversion branches without names, pass the --config convert.hg.usebranchnames=false option to hg convert.

Once you have converted your tree, if you want to follow the usual Mercurial practice of working in a tree that contains a single branch, you can clone that single branch using hg clone -r mybranchname.

Mapping Usernames

Some revision control tools save only short usernames with commits, and these can be difficult to interpret. The norm with Mercurial is to save a committer’s name and email address, which is much more useful for talking to them after the fact.

If you are converting a tree from a revision control system that uses short names, you can map those names to longer equivalents by passing the --authors option to hg convert. This option accepts a filename that should contain entries of the following form.

arist = Aristotle <aristotle@phil.example.gr>
soc = Socrates <socrates@phil.example.gr>

Whenever convert encounters a commit with the username arist in the source repository, it will use the name Aristotle <aristotle@phil.example.gr> in the converted Mercurial revision. If no match is found for a name, it is used verbatim.

Tidying Up the Tree

Not all projects have pristine history. There may be a directory that should never have been checked in, a file that is too big, or a whole hierarchy that needs to be refactored.

The convert extension supports the idea of a “file map” that can reorganize the files and directories in a project as it imports the project’s history. This is useful not only when importing history from other revision control systems, but also to prune or refactor a Mercurial tree.

To specify a file map, use the --filemap option and supply a filename. A file map contains lines of the following forms.

# This is a comment.
# Empty lines are ignored.	

include path/to/file

exclude path/to/file

rename from/some/path to/some/other/place

The include directive causes a file, or all files under a directory, to be included in the destination repository. This also excludes all other files and directories not explicitly included. The exclude directive causes files or directories to be omitted, and others not explicitly mentioned to be included.

To move a file or directory from one location to another, use the rename directive. If you need to move a file or directory from a subdirectory into the root of the repository, use . as the second argument to the rename directive.

Improving Subversion Conversion Performance

You will often need several attempts before you hit the perfect combination of user map, file map, and other conversion parameters. Converting a Subversion repository over an access protocol like ssh or http can proceed thousands of times slower than Mercurial is capable of operating due to network delays. This can make tuning that perfect conversion recipe very painful.

The svnsync command can greatly speed up the conversion of a Subversion repository. It is a read-only mirroring program for Subversion repositories. The idea is that you create a local mirror of your Subversion tree, then convert the mirror into a Mercurial repository.

Suppose we want to convert the Subversion repository for the popular Memcached project into a Mercurial tree. First, we create a local Subversion repository.

$ svnadmin create memcached-mirror

Next, we set up a Subversion hook that svnsync needs.

$ echo '#!/bin/sh' > memcached-mirror/hooks/pre-revprop-change
$ chmod +x memcached-mirror/hooks/pre-revprop-change

We then initialize svnsync in this repository.

$ svnsync --init file://`pwd`/memcached-mirror \
  http://code.sixapart.com/svn/memcached

Our next step is to begin the svnsync mirroring process.

$ svnsync sync file://`pwd`/memcached-mirror

Finally, we import the history of our local Subversion mirror into Mercurial.

$ hg convert memcached-mirror

We can use this process incrementally if the Subversion repository is still in use. We run svnsync to pull new changes into our mirror, then hg convert to import them into our Mercurial tree.

There are two advantages to doing a two-stage import with svnsync. The first is that it uses more efficient Subversion network syncing code than hg convert, so it transfers less data over the network. The second is that the import from a local Subversion tree is so fast that you can tweak your conversion setup repeatedly without having to sit through a painfully slow network-based conversion process each time.

Migrating from Subversion

Subversion is currently the most popular open source revision control system. Although there are many differences between Mercurial and Subversion, making the transition from Subversion to Mercurial is not particularly difficult. The two have similar command sets and generally uniform interfaces.

Philosophical Differences

The fundamental difference between Subversion and Mercurial is of course that Subversion is centralized, while Mercurial is distributed. Since Mercurial stores all of a project’s history on your local drive, it only needs to perform a network access when you want to explicitly communicate with another repository. In contrast, Subversion stores very little information locally, and the client must thus contact its server for many common operations.

Subversion more or less gets away with not having a well-defined notion of a branch: which portion of a server’s namespace qualifies as a branch is a matter of convention, with the software providing no enforcement. Mercurial treats a repository as the unit of branch management.

Scope of commands

Since Subversion doesn’t know what parts of its namespace are really branches, it treats most commands as requests to operate at and below whatever directory you are currently visiting. For instance, if you run svn log, you’ll get the history of whatever part of the tree you’re looking at, not the tree as a whole.

Mercurial’s commands behave differently, by defaulting to operating over an entire repository. Run hg log and it will tell you the history of the entire tree, no matter what part of the working directory you’re visiting at the time. If you want the history of just a particular file or directory, simply supply it by name, e.g., hg log src.

From my own experience, this difference in default behaviors is probably the most likely to trip you up if you have to switch back and forth frequently between the two tools.

Multi-user operation and safety

With Subversion, it is normal (though slightly frowned upon) for multiple people to collaborate in a single branch. If Alice and Bob are working together, and Alice commits some changes to their shared branch, Bob must update his client’s view of the branch before he can commit. Since at this time he has no permanent record of the changes he has made, he can corrupt or lose his modifications during and after his update.

Mercurial encourages a commit-then-merge model instead. Bob commits his changes locally before pulling changes from, or pushing them to, the server that he shares with Alice. If Alice pushed her changes before Bob tries to push his, he will not be able to push his changes until he pulls hers, merges with them, and commits the result of the merge. If he makes a mistake during the merge, he still has the option of reverting to the commit that recorded his changes.

It is worth emphasizing that these are the common ways of working with these tools. Subversion supports a safer work-in-your-own-branch model, but it is cumbersome enough in practice to not be widely used. Mercurial can support the less-safe mode of allowing changes to be pulled in and merged on top of uncommitted edits, but this is considered highly unusual.

Published versus local changes

A Subversion svn commit command immediately publishes changes to a server, where they can be seen by everyone who has read access.

With Mercurial, commits are always local, and must be published via a hg push command afterwards.

Each approach has its advantages and disadvantages. The Subversion model means that changes are published, and hence reviewable and usable, immediately. On the other hand, this means that a user must have commit access to a repository in order to use the software in a normal way, and commit access is not lightly given out by most open source projects.

The Mercurial approach allows anyone who can clone a repository to commit changes without the need for someone else’s permission, and they can then publish their changes and continue to participate however they see fit. The distinction between committing and pushing does open up the possibility of someone committing changes to their laptop and walking away for a few days having forgotten to push them, which in rare cases might leave collaborators temporarily stuck.

Quick Reference

Table A.1. Subversion commands and Mercurial equivalents

SubversionMercurialNotes
svn addhg add
svn blamehg annotate
svn cathg cat
svn checkouthg clone
svn cleanupn/aNo cleanup needed
svn commithg commit; hg pushhg push publishes after commit
svn copyhg cloneTo create a new branch
svn copyhg copyTo copy files or directories
svn delete (svn remove)hg remove
svn diffhg diff
svn exporthg archive
svn helphg help
svn importhg addremove; hg commit
svn infohg parentsShows what revision is checked out
svn infohg showconfig paths.parentShows what URL is checked out
svn listhg manifest
svn loghg log
svn mergehg merge
svn mkdirn/aMercurial does not track directories
svn move (svn rename)hg rename
svn resolvedhg resolve -m
svn reverthg revert
svn statushg status
svn updatehg pull -u

Useful Tips for Newcomers

Under some revision control systems, printing a diff for a single committed revision can be painful. For instance, with Subversion, to see what changed in revision 104654, you must type svn diff -r104653:104654. Mercurial eliminates the need to type the revision ID twice in this common case. For a plain diff, type hg export 104654. For a log message followed by a diff, type hg log -r104654 -p.

When you run hg status without any arguments, it prints the status of the entire tree, with paths relative to the root of the repository. This makes it tricky to copy a filename from the output of hg status into the command line. If you supply a file or directory name to hg status, it will print paths relative to your current location instead. So to get tree-wide status from hg status, with paths that are relative to your current directory and not the root of the repository, feed the output of hg root into hg status. You can easily do this as follows on a Unix-like system:

$ 
      hg status `hg root`
If you enjoyed this excerpt, buy a copy of Mercurial: The Definitive Guide.