GITWORKFLOWS(7) Git Manual GITWORKFLOWS(7)
NAME
gitworkflows - An overview of recommended workflows with Git
SYNOPSIS
git *
DESCRIPTION
This document attempts to write down and motivate some of the workflow elements used for git.git itself. Many
ideas apply in general, though the full workflow is rarely required for smaller projects with fewer people
involved.
We formulate a set of rules for quick reference, while the prose tries to motivate each of them. Do not always
take them literally; you should value good reasons for your actions higher than manpages such as this one.
SEPARATE CHANGES
As a general rule, you should try to split your changes into small logical steps, and commit each of them.
They should be consistent, working independently of any later commits, pass the test suite, etc. This makes
the review process much easier, and the history much more useful for later inspection and analysis, for
example with git-blame(1) and git-bisect(1).
To achieve this, try to split your work into small steps from the very beginning. It is always easier to
squash a few commits together than to split one big commit into several. Don’t be afraid of making too small
or imperfect steps along the way. You can always go back later and edit the commits with git rebase
--interactive before you publish them. You can use git stash save --keep-index to run the test suite
independent of other uncommitted changes; see the EXAMPLES section of git-stash(1).
MANAGING BRANCHES
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:
· pu (proposed updates) is an integration branch for things that are not quite ready for inclusion yet (see
Always commit your fixes to the oldest supported branch that require 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.
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 pu.
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:
master using the surviving topics on next:
Example 9. Rewind and rebuild next
· git checkout next
· git reset --hard 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 pu. A public announcement is not necessary since pu is
a throw-away branch, as described above.
DISTRIBUTED WORKFLOWS
After the last section, you should know how to manage topics. In general, you will not be the only person
working on the project, so you will have to share your work.
Roughly speaking, there are two important workflows: merge and patch. The important difference is that the
merge workflow can propagate full history, including merges, while patches cannot. Both workflows can be used
in parallel: in git.git, only subsystem maintainers use the merge workflow, while everyone else sends patches.
Note that the maintainer(s) may impose restrictions, such as "Signed-off-by" requirements, that all
commits/patches submitted for inclusion must adhere to. Consult your project’s documentation for more
information.
Merge workflow
The merge workflow works by copying branches between upstream and downstream. Upstream can merge contributions
into the official history; downstream base their work on the official history.
There are three main tools that can be used for this:
· git-push(1) copies your branches to a remote repository, usually to one that can be read by all involved
parties;
· git-fetch(1) that copies remote branches to your repository; and
· git-pull(1) that does fetch and merge in one go.
Note the last point. Do not use git pull unless you actually want to merge the remote branch.
Getting changes out is easy:
Then simply fork your topic branches from the stable remotes as explained earlier.
If you are a maintainer and would like to merge other people’s topic branches to the integration branches,
they will typically send a request to do so by mail. Such a request looks like
Please pull from
<url> <branch>
In that case, git pull can do the fetch and merge in one go, as follows.
Example 12. Push/pull: Merging remote topics
git pull <url> <branch>
Occasionally, the maintainer may get merge conflicts when he tries to pull changes from downstream. In this
case, he can ask downstream to do the merge and resolve the conflicts themselves (perhaps they will know
better how to resolve them). It is one of the rare cases where downstream should merge from upstream.
Patch workflow
If you are a contributor that sends changes upstream in the form of emails, you should use topic branches as
usual (see above). Then use git-format-patch(1) to generate the corresponding emails (highly recommended over
manually formatting them because it makes the maintainer’s life easier).
Example 13. format-patch/am: Publishing branches/topics
· git format-patch -M upstream..topic to turn them into preformatted patch files
· git send-email --to=<recipient> <patches>
See the git-format-patch(1) and git-send-email(1) manpages for further usage notes.
If the maintainer tells you that your patch no longer applies to the current upstream, you will have to rebase
your topic (you cannot use a merge because you cannot format-patch merges):
Example 14. format-patch/am: Keeping topics up to date
git pull --rebase <url> <branch>
You can then fix the conflicts during the rebase. Presumably you have not published your topic other than by
mail, so rebasing it is not a problem.
If you receive such a patch series (as maintainer, or perhaps as a reader of the mailing list it was sent to),
save the mails to files, create a new topic branch and use git am to import the commits:
Example 15. format-patch/am: Importing patches
git am < patch
One feature worth pointing out is the three-way merge, which can help if you get conflicts: git am -3 will use
index information contained in patches to figure out the merge base. See git-am(1) for other options.
SEE ALSO
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