Merging Structural Changes

In 2008, a project I was working on set out to reinvent their build process, -migrating from a mass of poorly-written Ant scripts to Maven and reorganizing -their source tree in the process. The development process was based on having -a branch per client, so there was a lot of ongoing development on the original -layout for clients that hadn't been migrated yet. We discovered that our -version control tool, Subversion, was unable -to merge the changes between client branches on the old structure and the -trunk on the new structure automatically.

Curiousity piqued, I cooked up a script that reproduces the problem and -performs the merge from various directions to examine the results. Subversion, -sadly, performed dismally: none of the merge scenarios tested retained content -changes when merging structural changes to the same files.

The Preferred Outcome

Both changes survive the
-merge.

The diagram above shows a very simple source tree with one directory, dir-a, -containing one file with two lines in it. On one branch, the file is modified -to have a third line; on another branch, the directory is renamed to dir-b. -Then, both branches are merged, and the resulting tree contains both sets of -changes: the file has three lines, and the directory has a new name.

This is the preferred outcome, as no changes are lost or require manual -merging.

Subversion

Subversion loses the content
-change.

There are two merge scenarios in this diagram, with almost the same outcome. -On the left, a working copy of the branch where the file's content changed is -checked out, then the changes from the branch where the structure changed are -merged in. On the right, a working copy of the branch where the structure -changed is checked out, then the changes from the branch where the content -changed are merged in. In both cases, the result of the merge has the new -directory name, and the original file contents. In one case, the merge -triggers a rather opaque warning about a “missing file”; in the other, the -merge silently ignores the content changes.

This is a consequence of the way Subversion implements renames and copies. -When Subversion assembles a changeset for committing to the repository, it -comes up with a list of primitive operations that reproduce the change. There -is no primitive that says “this object was moved,” only primitives which say -“this object was deleted” or “this object was added, as a copy of that -object.” When you move a file in Subversion, those two operations are -scheduled. Later, when Subversion goes to merge content changes to the -original file, all it sees is that the file has been deleted; it's completely -unaware that there is a new name for the same file.

This would be fairly easy to remedy by adding a “this object was moved to that -object” primitive to the changeset language, and a bug report for just such a -feature was filed in -2002. However, by that time Subversion's repository and changeset formats had -essentially frozen, as Subversion was approaching a 1.0 release and more -important bugs without workarounds were a priority.

There is some work going on in Subversion 1.6 to handle tree conflicts (the -kind of conflicts that come from this kind of structural change) more -sensibly, which will cause the two merges above to generate a Conflict result, -which is not as good as automatically merging it but far better than silently -ignoring changes.

Mercurial

Mercurial preserves the content
-change.

Interestingly, there are tools which get this merge scenario right: the -diagram above shows how Mercurial handles -the same two tests. Since its changeset language does include an “object -moved” primitive, it's able to take a content change for dir-a/file and -apply it to dir-b/file if appropriate.

Git

Git also gets this scenario right, usually. Unlike Mercurial, Git does not -track file copies or renames in its commits at all, prefering to infer them by -content comparison every time it performs a move-aware operation, such as a -merge.

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