5.6 Cherrypicking Changes

So far we've learned about elementary branches for maintaining changes apart from a primary development branch and development branches for coordinating asynchronous work on a single project (see Elementary Branches – Maintaining Private Changes and Development Branches – The star-merge Style of Cooperation).

In this chapter, we'll briefly describe a third kind of branch that's useful when a project consists of multiple "forks" – multiple, equally primary branches.

Let's suppose, somewhat abstractly, that Alice and Bob's mainline has grown quite large:

             mainline
             --------
             base-0
             patch-1
             ....
             patch-23
             patch-24
             patch-25
             ...
             patch-42

At some point, perhaps because some controversy has emerged over choices made in the mainline, a new developer, Derick, declares a fork and starts his own branch:

             mainline                derick
             --------                ------
             base-0          ------> base-0
             patch-1        '
             ....          '
             patch-23 ----'
             patch-24
             patch-25
             ...
             patch-42

We already know that Derick can use update or replay to keep current with the mainline, but what he doesn't want to? What if Derick wants the changes in patch-25 and patch-42, but none of the other post-patch-23 changes from the mainline?

Derick can apply specific changes from the mainline by specifying the exact revision he wants, rather than just specifying a version:

             % cd ~/wd

             % tla get hello-world--derick--0.1 derick

             % cd derick

             % tla replay -A lord@emf.net--2003-example \
                      hello-world--mainline--0.1--patch-23

             % tla replay -A lord@emf.net--2003-example \
                      hello-world--mainline--0.1--patch-42

             % tla missing -A lord@emf.net--2003-example \
                      hello-world--mainline--0.1
             patch-24
             patch-25
             ...
             patch-41

             % tla logs -A lord@emf.net--2003-example \
                      hello-world--mainline--0.1
             base-0
             patch-1
             ...
             patch-22
             patch-23
             patch-42

Cherrypicking changes in this manner isn't necessarily easy or even practical. It depends, for example, on the mainline changes being "clean changesets" (see Using commit Well – The Idea of a Clean Changeset in Exploring Changesets).

Nevertheless, for some projects, especially those characterized by lots of "forks", this technique can be useful.

Learning Note: Multiple revisions may be replayed with a single command, simply by giving all of them on the command line at once. The replay command also has a --list option which can useful for cherrypicking many changes at once. If you find yourself replaying specific revisions often, you should take a look at the --list option in tla replay --help.