Just like the svn update command, svn merge applies changes to your working copy. And therefore it’s also capable of creating conflicts. The conflicts produced by svn merge, however, are sometimes different, and this section explains those differences.

To begin with, assume that your working copy has no local edits. When you svn update to a particular revision, the changes sent by the server will always apply “cleanly” to your working copy. The server produces the delta by comparing two trees: a virtual snapshot of your working copy, and the revision tree you’re interested in. Because the left hand side of the comparison is exactly equal to what you already have, the delta is guaranteed to correctly convert your working copy into the righthand tree.

But svn merge has no such guarantees and can be much more chaotic: the advanced user can ask the server to compare any two trees at all, even ones that are unrelated to the working copy! This means there’s large potential for human error. Users will sometimes compare the wrong two trees, creating a delta that doesn’t apply cleanly. svn merge will do its best to apply as much of the delta as possible, but some parts may be impossible. Just as the Unix patch command sometimes complains about “failed hunks,” svn merge will similarly complain about “skipped targets”:

$ svn merge -r 1288:1351 http://svn.example.com/repos/branch
U    foo.c
U    bar.c
Skipped missing target: 'baz.c'
U    glub.c
U    sputter.h

Conflict discovered in 'glorb.h'.
Select: (p) postpone, (df) diff-full, (e) edit,
        (h) help for more options:

In the preceding example, it might be the case that baz.c exists in both snapshots of the branch being compared, and the resultant delta wants to change the file’s contents, but the file doesn’t exist in the working copy. Whatever the case, the “skipped” message means that the user is most likely comparing the wrong two trees; it’s the classic sign of user error. When this happens, it’s easy to recursively revert all the changes created by the merge (svn revert . --recursive), delete any unversioned files or directories left behind after the revert, and rerun svn merge with different arguments.

Also notice that the preceding example shows a conflict happening on glorb.h. We already stated that the working copy has no local edits: how can a conflict possibly happen? Again, because the user can use svn merge to define and apply any old delta to the working copy, that delta may contain textual changes that don’t cleanly apply to a working file, even if the file has no local modifications.

Another small difference between svn update and svn merge is the names of the full-text files created when a conflict happens. In Resolve Conflicts (Merging Others’ Changes), we saw that an update produces files named filename.mine, filename.rOLDREV, and filename.rNEWREV. When svn merge produces a conflict, however, it creates three files that are named filename.working, filename.left, and filename.right. In this case, the terms “left” and “right” are describing which side of the double-tree comparison the file came from. In any case, these differing names will help you distinguish between conflicts that happened as a result of an update and ones that happened as a result of a merge.