There are two main tools that can be used to include changes from
one branch on another: git-merge(1) and git-cherry-pick(1).
Merges have many advantages, so we try to solve as many problems
as possible with merges alone. Cherry-picking is still
occasionally useful; see "Merging upwards" below for an example.
Most importantly, merging works at the branch level, while
cherry-picking works at the commit level. This means that a merge
can carry over the changes from 1, 10, or 1000 commits with equal
ease, which in turn means the workflow scales much better to a
large number of contributors (and contributions). Merges are also
easier to understand because a merge commit is a "promise" that
all changes from all its parents are now included.
There is a tradeoff of course: merges require a more careful
branch management. The following subsections discuss the
important points.
Graduation
As a given feature goes from experimental to stable, it also
"graduates" between the corresponding branches of the software.
git.git uses the following integration branches:
• maint tracks the commits that should go into the next
"maintenance release", i.e., update of the last released
stable version;
• master tracks the commits that should go into the next
release;
• next is intended as a testing branch for topics being tested
for stability for master.
There is a fourth official branch that is used slightly
differently:
• seen (patches seen by the maintainer) is an integration
branch for things that are not quite ready for inclusion yet
(see "Integration Branches" below).
Each of the four branches is usually a direct descendant of the
one above it.
Conceptually, the feature enters at an unstable branch (usually
next or seen), and "graduates" to master for the next release
once it is considered stable enough.
Merging upwards
The "downwards graduation" discussed above cannot be done by
actually merging downwards, however, since that would merge all
changes on the unstable branch into the stable one. Hence the
following:
Example 1. Merge upwards
Always commit your fixes to the oldest supported branch that
requires them. Then (periodically) merge the integration branches
upwards into each other.
This gives a very controlled flow of fixes. If you notice that
you have applied a fix to e.g. master that is also required in
maint, you will need to cherry-pick it (using git-cherry-pick(1))
downwards. This will happen a few times and is nothing to worry
about unless you do it very frequently.
Topic branches
Any nontrivial feature will require several patches to implement,
and may get extra bugfixes or improvements during its lifetime.
Committing everything directly on the integration branches leads
to many problems: Bad commits cannot be undone, so they must be
reverted one by one, which creates confusing histories and
further error potential when you forget to revert part of a group
of changes. Working in parallel mixes up the changes, creating
further confusion.
Use of "topic branches" solves these problems. The name is pretty
self explanatory, with a caveat that comes from the "merge
upwards" rule above:
Example 2. Topic branches
Make a side branch for every topic (feature, bugfix, ...). Fork
it off at the oldest integration branch that you will eventually
want to merge it into.
Many things can then be done very naturally:
• To get the feature/bugfix into an integration branch, simply
merge it. If the topic has evolved further in the meantime,
merge again. (Note that you do not necessarily have to merge
it to the oldest integration branch first. For example, you
can first merge a bugfix to next, give it some testing time,
and merge to maint when you know it is stable.)
• If you find you need new features from the branch other to
continue working on your topic, merge other to topic.
(However, do not do this "just habitually", see below.)
• If you find you forked off the wrong branch and want to move
it "back in time", use git-rebase(1).
Note that the last point clashes with the other two: a topic that
has been merged elsewhere should not be rebased. See the section
on RECOVERING FROM UPSTREAM REBASE in git-rebase(1).
We should point out that "habitually" (regularly for no real
reason) merging an integration branch into your topics — and by
extension, merging anything upstream into anything downstream on
a regular basis — is frowned upon:
Example 3. Merge to downstream only at well-defined points
Do not merge to downstream except with a good reason: upstream
API changes affect your branch; your branch no longer merges to
upstream cleanly; etc.
Otherwise, the topic that was merged to suddenly contains more
than a single (well-separated) change. The many resulting small
merges will greatly clutter up history. Anyone who later
investigates the history of a file will have to find out whether
that merge affected the topic in development. An upstream might
even inadvertently be merged into a "more stable" branch. And so
on.
Throw-away integration
If you followed the last paragraph, you will now have many small
topic branches, and occasionally wonder how they interact.
Perhaps the result of merging them does not even work? But on the
other hand, we want to avoid merging them anywhere "stable"
because such merges cannot easily be undone.
The solution, of course, is to make a merge that we can undo:
merge into a throw-away branch.
Example 4. Throw-away integration branches
To test the interaction of several topics, merge them into a
throw-away branch. You must never base any work on such a branch!
If you make it (very) clear that this branch is going to be
deleted right after the testing, you can even publish this
branch, for example to give the testers a chance to work with it,
or other developers a chance to see if their in-progress work
will be compatible. git.git has such an official throw-away
integration branch called seen.
Branch management for a release
Assuming you are using the merge approach discussed above, when
you are releasing your project you will need to do some
additional branch management work.
A feature release is created from the master branch, since master
tracks the commits that should go into the next feature release.
The master branch is supposed to be a superset of maint. If this
condition does not hold, then maint contains some commits that
are not included on master. The fixes represented by those
commits will therefore not be included in your feature release.
To verify that master is indeed a superset of maint, use git log:
Example 5. Verify master is a superset of maint
git log master..maint
This command should not list any commits. Otherwise, check out
master and merge maint into it.
Now you can proceed with the creation of the feature release.
Apply a tag to the tip of master indicating the release version:
Example 6. Release tagging
git tag -s -m "Git X.Y.Z" vX.Y.Z master
You need to push the new tag to a public Git server (see
"DISTRIBUTED WORKFLOWS" below). This makes the tag available to
others tracking your project. The push could also trigger a
post-update hook to perform release-related items such as
building release tarballs and preformatted documentation pages.
Similarly, for a maintenance release, maint is tracking the
commits to be released. Therefore, in the steps above simply tag
and push maint rather than master.
Maintenance branch management after a feature release
After a feature release, you need to manage your maintenance
branches.
First, if you wish to continue to release maintenance fixes for
the feature release made before the recent one, then you must
create another branch to track commits for that previous release.
To do this, the current maintenance branch is copied to another
branch named with the previous release version number (e.g.
maint-X.Y.(Z-1) where X.Y.Z is the current release).
Example 7. Copy maint
git branch maint-X.Y.(Z-1) maint
The maint branch should now be fast-forwarded to the newly
released code so that maintenance fixes can be tracked for the
current release:
Example 8. Update maint to new release
• git checkout maint
• git merge --ff-only master
If the merge fails because it is not a fast-forward, then it is
possible some fixes on maint were missed in the feature release.
This will not happen if the content of the branches was verified
as described in the previous section.
Branch management for next and seen after a feature release
After a feature release, the integration branch next may
optionally be rewound and rebuilt from the tip of master using
the surviving topics on next:
Example 9. Rewind and rebuild next
• git switch -C next master
• git merge ai/topic_in_next1
• git merge ai/topic_in_next2
• ...
The advantage of doing this is that the history of next will be
clean. For example, some topics merged into next may have
initially looked promising, but were later found to be
undesirable or premature. In such a case, the topic is reverted
out of next but the fact remains in the history that it was once
merged and reverted. By recreating next, you give another
incarnation of such topics a clean slate to retry, and a feature
release is a good point in history to do so.
If you do this, then you should make a public announcement
indicating that next was rewound and rebuilt.
The same rewind and rebuild process may be followed for seen. A
public announcement is not necessary since seen is a throw-away
branch, as described above.
