Ubuntu Manpage: gitworkflows - An overview of recommended workflows with Git

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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 "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 pu), 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 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.

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 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:

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 pu 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 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:

Example 10. Push/pull: Publishing branches/topics

git push <remote> <branch> and tell everyone where they can fetch from.

You will still have to tell people by other means, such as mail. (Git provides the git-
request-pull(1) to send preformatted pull requests to upstream maintainers to simplify
this task.)

If you just want to get the newest copies of the integration branches, staying up to date
is easy too:

Example 11. Push/pull: Staying up to date

Use git fetch <remote> or git remote update to stay up to date.

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.

GIT

       Part of the git(1) suite.