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When a project grows to a certain size, it will probably need a way to share code between multiple software packages they release. In the context of Gnome, one example is the sharing of the libbackground code between Nautilus and gnome-control-center. The simplest way to do this is to just copy over the files in question and manually synchronise them. This is a pain to do, and can lead to problems if changes are made to both copies, so you’d want to avoid it if possible. So most version control systems provide some way to share code in this way. As with the previous articles, I’ll focus on Bazaar, CVS and Subversion

Unlike the common operations each system implements this feature in a different way, so I’ll go over each one in turn and then compare them.

CVS

When you run the “cvs checkout module” command, CVS will look in the CVSROOT/modules file for the repository. For example, the file might contain the following:

module foobar

This would tell CVS to check out the foobar directory from the repository into a directory named module when the user asks for module. If no entry is found for a particular name, the directory by that name is checked out from the repository.

To compose multiple modules into a single working copy, the ampersand syntax can be used:

module foo &bar &baz
bar othermodule/bar

With this modules file, “cvs checkout module” would give the following working copy:

Working Copy	Repository
module	foo
module/bar	othermodule/bar
module/baz	baz

Operations like tag, commit, update, etc will descend into included modules, so for the most part a user can treat the resulting working copy as a single tree. If a particular branch tag exists on all the included modules, you can even check out a branch of the combined working copy. There are some problems with the support though:

• While “cvs update” will update the working copy, it won’t take into account any changes in CVSROOT/modules.
• If you’ve only got write access to part of the repository, and can’t write to CVSROOT/modules, then you can’t change configurations.
• While CVS lets you check out old versions of code, you still use the latest version of CVSROOT/modules. This can make it difficult to check out historical versions of the tree.
• Since “cvs tag” descends into included modules, you can end up with many branch tags on some modules. For instance, the gnome-common/macros directory in Gnome CVS has 282 branch tags, which makes it almost impossible to feed fixes to all those branches.

Subversion

Rather than a single repository-wide file describing the module configuration for checkouts, Subversion makes use of the svn:externals property on directories.

Any directory can have such a property attached. Each line in the property is of the form:

subdir [-rrevnum] absolute-uri-of-tree-to-include

This will check out each the given tree at the given sub dir when ever “svn checkout” or “svn update” are used. However unlike CVS, “svn commit” will not descend into the included modules.

Some of the benefits of this approach include:

• Inclusions can be placed close to the location they are included.
• It reduces the permissions problems: if you can commit to the directory where the inclusion will occur, you can add the inclusion.
• Can include modules from other repositories. In this case, it is actually useful that “svn commit” doesn’t descend into the included module because it is likely that the user won’t have write access to the external modules.
• When checking out a historic version of the module, the historic version of the svn:externals properties get used.

Some of the down sides to the approach include:

• Module inclusion directives can be scattered throughout the tree. There isn’t a single place to look for such directives.
• When including something from the same repository, you still need to use an absolute URI to identify the module. It is not uncommon for committers to use a different URI to access the repository to those who only have read access (e.g. svn+ssh://hostname for committers, svn://hostname or http://hostname for read-only users). So which URI do you use in the svn:externals property? You’ll need to choose between a tree that read-only users can’t check out or a tree that committers can’t commit to …
• If you want to branch a set of related modules in the repository, you’ll need to alter the svn:externals properties to point at the branched versions of the modules. When performing merges back to the mainline, you need to make sure you don’t merge the svn:externals property changes.
• When checking out historic versions, although historic svn:externals definitions get used, you will get the up-to-date versions of the included modules unless a particular revision of the included module was specified in the property.
• If the hosting arrangements for an included module change, the historical values of svn:externals properties will be invalid.

Bazaar

The module inclusion system in Bazaar is handled through “configurations”. These are simple files stored in a branch with lines of the form:

subdirectory archivename/branchname[--patch-NNNN]

After checking out a branch, you can check out the various included modules by running the following command from the base of the working copy:

baz build-config file-name

To update a working copy and all the included modules, you need two commands:

baz update
baz build-config -u file-name

(the -u flag is only available in the 1.5 prereleases. Previously you needed a command like “baz cat-config file-name | xargs -n2 baz update -d“).

The name of the configuration file is not special, and it is possible to have multiple configurations stored in a single branch. In fact it is common to have a branch that stores nothing but configurations, and assemble the source tree in a subdirectory.

One common use of multiple configs is similar to the use of non-branch tags in CVS: recording a particular configuration used for a particular release. This can be done by taking a snapshot of the configuration, which adds fixed revision numbers to the branches checked out:

baz cat-config --snap development.config > release-0.42.config

If anyone builds this configuration, they will see the tree as it was when that snapshot was taken. Some benefits of this system include:

• It is easy to maintain multiple configurations for a set of branches.
• Since configurations are stored in the same way as other files on the branch, anyone can modify them (either by committing to the branch, or by creating a new branch and making the change there).
• Use of the arch namespace to identify branches, so is somewhat immune to branch location changes (it is still vulnerable to referenced branches disappearing altogether).

Some of the down sides of the approach include:

• Requires the user to run a second command after checking out the branch containing the configuration.
• No standard name for configurations, so the user needs to know the config file name in addition to the branch name when checking things out.

Summary

Here is a summary of how the three systems stand up against each other in this respect:

	CVS	Subversion	Bazaar
Who can change configs?	Committers to CVSROOT	Committers	Anyone
Build historic configs?	No	Yes	Sort of (snapshot configs)
Supports multiple parallel configurations of same code?	Yes	Yes	Yes
commit command crosses module inclusion boundaries?	Yes	No	No
Configs built by checkout command?	Yes	Yes	No
Configs built by update command?	No	Yes	No
Resistant to project hosting changes?	Yes	No	Yes
Same config usable for committers and read-only users?	Yes	Yes for DAV access No for svn+ssh:// access	Yes

Each system is slightly different with its benefits and problems. It isn’t particularly surprising then that configs are not handled well by the various version control migration scripts. For example, the cvs2svn script doesn’t handle them at all (e.g. the KDE Subversion repository doesn’t contain any svn:externals properties in historic versions migrated from CVS).