Vendor Branches
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## Vendor Branches

As is especially the case when developing software, the data that you maintain under version control is often closely related to, or perhaps dependent upon, someone else's data. Generally, the needs of your project will dictate that you stay as up to date as possible with the data provided by that external entity without sacrificing the stability of your own project. This scenario plays itself out all the time—anywhere that the information generated by one group of people has a direct effect on that which is generated by another group.

For example, software developers might be working on an application that makes use of a third-party library. Subversion has just such a relationship with the Apache Portable Runtime (APR) library (see the section called “The Apache Portable Runtime Library”). The Subversion source code depends on the APR library for all its portability needs. In earlier stages of Subversion's development, the project closely tracked APR's changing API, always sticking to the “bleeding edge” of the library's code churn. Now that both APR and Subversion have matured, Subversion attempts to synchronize with APR's library API only at well-tested, stable release points.

Now, if your project depends on someone else's information, you could attempt to synchronize that information with your own in several ways. Most painfully, you could issue oral or written instructions to all the contributors of your project, telling them to make sure they have the specific versions of that third-party information that your project needs. If the third-party information is maintained in a Subversion repository, you could also use Subversion's externals definitions to effectively “pin down” specific versions of that information to some location in your own working copy directory (see the section called “Externals Definitions”).

But sometimes you want to maintain custom modifications to third-party code in your own version control system. Returning to the software development example, programmers might need to make modifications to that third-party library for their own purposes. These modifications might include new functionality or bug fixes, maintained internally only until they become part of an official release of the third-party library. Or the changes might never be relayed back to the library maintainers, existing solely as custom tweaks to make the library further suit the needs of the software developers.

Now you face an interesting situation. Your project could house its custom modifications to the third-party data in some disjointed fashion, such as using patch files or full-fledged alternative versions of files and directories. But these quickly become maintenance headaches, requiring some mechanism by which to apply your custom changes to the third-party code and necessitating regeneration of those changes with each successive version of the third-party code that you track.

The solution to this problem is to use vendor branches. A vendor branch is a directory tree in your own version control system that contains information provided by a third-party entity, or vendor. Each version of the vendor's data that you decide to absorb into your project is called a vendor drop.

Vendor branches provide two benefits. First, by storing the currently supported vendor drop in your own version control system, you ensure that the members of your project never need to question whether they have the right version of the vendor's data. They simply receive that correct version as part of their regular working copy updates. Second, because the data lives in your own Subversion repository, you can store your custom changes to it in-place—you have no more need of an automated (or worse, manual) method for swapping in your customizations.

### General Vendor Branch Management Procedure

Managing vendor branches generally works like this: first, you create a top-level directory (such as `/vendor`) to hold the vendor branches. Then you import the third-party code into a subdirectory of that top-level directory. You then copy that subdirectory into your main development branch (e.g., `/trunk`) at the appropriate location. You always make your local changes in the main development branch. With each new release of the code you are tracking, you bring it into the vendor branch and merge the changes into `/trunk`, resolving whatever conflicts occur between your local changes and the upstream changes.

An example will help to clarify this algorithm. We'll use a scenario where your development team is creating a calculator program that links against a third-party complex number arithmetic library, libcomplex. We'll begin with the initial creation of the vendor branch and the import of the first vendor drop. We'll call our vendor branch directory `libcomplex`, and our code drops will go into a subdirectory of our vendor branch called `current`. And since svn import creates all the intermediate parent directories it needs, we can actually accomplish both of these steps with a single command:

    $ svn import /path/to/libcomplex-1.0 \
                 http://svn.example.com/repos/vendor/libcomplex/current \
                 -m 'importing initial 1.0 vendor drop'
    …

We now have the current version of the libcomplex source code in `/vendor/libcomplex/current`. Now, we tag that version (see the section called “Tags”) and then copy it into the main development branch. Our copy will create a new directory called `libcomplex` in our existing `calc` project directory. It is in this copied version of the vendor data that we will make our customizations:

    $ svn copy http://svn.example.com/repos/vendor/libcomplex/current  \
               http://svn.example.com/repos/vendor/libcomplex/1.0      \
               -m 'tagging libcomplex-1.0'
    …
    $ svn copy http://svn.example.com/repos/vendor/libcomplex/1.0  \
               http://svn.example.com/repos/calc/libcomplex        \
               -m 'bringing libcomplex-1.0 into the main branch'
    …

We check out our project's main branch—which now includes a copy of the first vendor drop—and we get to work customizing the libcomplex code. Before we know it, our modified version of libcomplex is now completely integrated into our calculator program. [25]

A few weeks later, the developers of libcomplex release a new version of their library—version 1.1—which contains some features and functionality that we really want. We'd like to upgrade to this new version, but without losing the customizations we made to the existing version. What we essentially would like to do is to replace our current baseline version of libcomplex 1.0 with a copy of libcomplex 1.1, and then re-apply the custom modifications we previously made to that library to the new version. But we actually approach the problem from the other direction, applying the changes made to libcomplex between versions 1.0 and 1.1 to our modified copy of it.

To perform this upgrade, we check out a copy of our vendor branch and replace the code in the `current` directory with the new libcomplex 1.1 source code. We quite literally copy new files on top of existing files, perhaps exploding the libcomplex 1.1 release tarball atop our existing files and directories. The goal here is to make our `current` directory contain only the libcomplex 1.1 code and to ensure that all that code is under version control. Oh, and we want to do this with as little version control history disturbance as possible.

After replacing the 1.0 code with 1.1 code, svn status will show files with local modifications as well as, perhaps, some unversioned files. If we did what we were supposed to do, the unversioned files are only those new files introduced in the 1.1 release of libcomplex—we run svn add on those to get them under version control. If the 1.1 code no longer has certain files that were in the 1.0 tree, it may be hard to notice them; you'd have to compare the two trees with some external tool and then svn delete any files present in 1.0 but not in 1.1. (Although it might also be just fine to let these same files live on in unused obscurity!) Finally, once our `current` working copy contains only the libcomplex 1.1 code, we commit the changes we made to get it looking that way.

Our `current` branch now contains the new vendor drop. We tag the new version as 1.1 (in the same way we previously tagged the version 1.0 vendor drop), and then merge the differences between the tag of the previous version and the new current version into our main development branch:

    $ cd working-copies/calc
    $ svn merge http://svn.example.com/repos/vendor/libcomplex/1.0      \
                http://svn.example.com/repos/vendor/libcomplex/current  \
                libcomplex
    … # resolve all the conflicts between their changes and our changes
    $ svn commit -m 'merging libcomplex-1.1 into the main branch'
    …

In the trivial use case, the new version of our third-party tool would look, from a files-and-directories point of view, just like the previous version. None of the libcomplex source files would have been deleted, renamed, or moved to different locations—the new version would contain only textual modifications against the previous one. In a perfect world, our modifications would apply cleanly to the new version of the library, with absolutely no complications or conflicts.

But things aren't always that simple, and in fact it is quite common for source files to get moved around between releases of software. This complicates the process of ensuring that our modifications are still valid for the new version of code, and things can quickly degrade into a situation where we have to manually re-create our customizations in the new version. Once Subversion knows about the history of a given source file—including all its previous locations—the process of merging in the new version of the library is pretty simple. But we are responsible for telling Subversion how the source file layout changed from vendor drop to vendor drop.

### svn_load_dirs.pl

Vendor drops that contain more than a few deletes, additions, and moves complicate the process of upgrading to each successive version of the third-party data. So Subversion supplies the svn_load_dirs.pl script to assist with this process. This script automates the importing steps we mentioned in the general vendor branch management procedure to make sure mistakes are minimized. You will still be responsible for using the merge commands to merge the new versions of the third-party data into your main development branch, but svn_load_dirs.pl can help you more quickly and easily arrive at that stage.

In short, svn_load_dirs.pl is an enhancement to svn import that has several important characteristics:

- It can be run at any point in time to bring an existing directory in the repository to exactly match an external directory, performing all the necessary adds and deletes, and optionally performing moves, too.

- It takes care of complicated series of operations between which Subversion requires an intermediate commit—such as before renaming a file or directory twice.

- It will optionally tag the newly imported directory.

- It will optionally add arbitrary properties to files and directories that match a regular expression.

svn_load_dirs.pl takes three mandatory arguments. The first argument is the URL to the base Subversion directory to work in. This argument is followed by the URL—relative to the first argument—into which the current vendor drop will be imported. Finally, the third argument is the local directory to import. Using our previous example, a typical run of svn_load_dirs.pl might look like this:

    $ svn_load_dirs.pl http://svn.example.com/repos/vendor/libcomplex \
                       current                                        \
                       /path/to/libcomplex-1.1
    …

You can indicate that you'd like svn_load_dirs.pl to tag the new vendor drop by passing the `-t` command-line option and specifying a tag name. This tag is another URL relative to the first program argument.

    $ svn_load_dirs.pl -t libcomplex-1.1                              \
                       http://svn.example.com/repos/vendor/libcomplex \
                       current                                        \
                       /path/to/libcomplex-1.1
    …

When you run svn_load_dirs.pl, it examines the contents of your existing “current” vendor drop and compares them with the proposed new vendor drop. In the trivial case, no files will be in one version and not the other, and the script will perform the new import without incident. If, however, there are discrepancies in the file layouts between versions, svn_load_dirs.pl will ask you how to resolve those differences. For example, you will have the opportunity to tell the script that you know that the file `math.c` in version 1.0 of libcomplex was renamed to `arithmetic.c` in libcomplex 1.1. Any discrepancies not explained by moves are treated as regular additions and deletions.

The script also accepts a separate configuration file for setting properties on files and directories matching a regular expression that are added to the repository. This configuration file is specified to svn_load_dirs.pl using the `-p` command-line option. Each line of the configuration file is a whitespace-delimited set of two or four values: a Perl-style regular expression against which to match the added path, a control keyword (either `break` or `cont`), and then optionally a property name and value.

    \.png$              break   svn:mime-type   image/png
    \.jpe?g$            break   svn:mime-type   image/jpeg
    \.m3u$              cont    svn:mime-type   audio/x-mpegurl
    \.m3u$              break   svn:eol-style   LF
    .*                  break   svn:eol-style   native

For each added path, the configured property changes whose regular expression matches the path are applied in order, unless the control specification is `break` (which means that no more property changes should be applied to that path). If the control specification is `cont`—an abbreviation for `continue`—matching will continue with the next line of the configuration file.

Any whitespace in the regular expression, property name, or property value must be surrounded by either single or double quotes. You can escape quotes that are not used for wrapping whitespace by preceding them with a backslash (`\`) character. The backslash escapes only quotes when parsing the configuration file, so do not protect any other characters beyond what is necessary for the regular expression.

[25] And is entirely bug-free, of course!

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