Cherrypicking

At times, it can be useful to selectively merge some of the changes in a branch, but not all of them. This is commonly referred to as cherrypicking. Here are some examples of where cherrypicking is useful:

• selectively taking fixes from the main development branch into a release branch
• selectively taking improvements out of an experimental branch into a feature branch.

To merge only the changes made by revision X in branch foo, the command is:

bzr merge -c X foo

To merge only the changes up to revision X in branch foo, the command is:

bzr merge -r X foo

To merge only the changes since revision X in branch foo, the command is:

bzr merge -r X.. foo

To merge only the changes from revision X to revision Y in branch foo, the command is:

bzr merge -r X..Y foo

Like a normal merge, you must explicitly commit a cherrypick. You may wish to see the changes made using bzr diff, and run your test suite if any, before doing this.

Unlike a normal merge, Bazaar does not currently track cherrypicks. In particular, the changes look like a normal commit and the (internal) revision history of the changes from the other branch is lost. In many cases where they are useful (see above), this is not a major problem because there are good reasons why a full merge should never be done at a later time. In other cases, additional conflicts will need to be resolved when the changes are merged again.

Merging without parents

A related technique to cherrypicking, in that it makes changes without reference to the revisions that they came from is to perform a merge, but forget about the parent revisions before committing. This has the effect of making all of the changes that would have been in the merge happen in a single commit. After the merge and before the corresponding commit, you can do:

bzr revert --forget-merges

to keep the changes in the working tree, but remove the record of the revisions where the changes originated. The next commit would then record all of those changes without any record of the merged revisions.

This is desired by some users to make their history “cleaner”, but you should be careful that the loss of history does not outweigh the value of cleanliness, particularly given Bazaar’s capabilities for progressively disclosing merged revisions. In particular, because this will include the changes from the source branch, but without attribution to that branch, it can lead to additional conflicts on later merges that involve the same source and target branches.

Reverse cherrypicking

Cherrypicking can be used to reverse a set of changes made by giving an upper bound in the revision range which is below the lower bound. For example, to back-out changes made in revision 10, the command is:

bzr merge -r 10..9

If you want to take most changes, but not all, from somewhere else, you may wish to do a normal merge followed by a few reverse cherrypicks.

Merging uncommitted changes

If you have several branches and you accidentally start making changes in the wrong one, here are the steps to take to correct this. Assuming you began working in branch foo when you meant to work in branch bar:

1. Change into branch bar.
2. Run bzr merge --uncommitted foo
3. Check the changes came across (bzr diff)
4. Change into branch foo
5. Run bzr revert.

Rebasing

Another option to normal merging is rebasing, i.e. making it look like the current branch originated from a different point than it did. Rebasing is supported in Bazaar by the rebase command provided by the rebase plugin.

The rebase command takes the location of another branch on which the branch in the current working directory will be rebased. If a branch is not specified then the parent branch is used, and this is usually the desired result.

The first step identifies the revisions that are in the current branch that are not in the parent branch. The current branch is then set to be at the same revision as the target branch, and each revision is replayed on top of the branch. At the end of the process it will appear as though your current branch was branched off the current last revision of the target.

Each revision that is replayed may cause conflicts in the tree. If this happens the command will stop and allow you to fix them up. Resolve the commits as you would for a merge, and then run bzr resolve to marked them as resolved. Once you have resolved all the conflicts, you should run bzr rebase-continue to continue the rebase operation. If conflicts are encountered and you decide not to continue, you can run bzr rebase-abort. You can also use rebase-todo to show the list of commits still to be replayed.

Note: Some users coming from central VCS tools with poor merge tracking like rebasing because it’s similar to how they are use to working in older tools, or because “perfectly clean” history seems important. Before rebasing in Bazaar, think about whether a normal merge is a better choice. In particular, rebasing a private branch before sharing it is OK but rebasing after sharing a branch with someone else is strongly discouraged.