gitweb: Add git_merge sub

[git/gsoc2010-gitweb.git] / Documentation / gitworkflows.txt

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
linkgit:git-blame[1] and linkgit: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 linkgit:git-stash[1].


MANAGING BRANCHES
-----------------

There are two main tools that can be used to include changes from one
branch on another: linkgit:git-merge[1] and
linkgit: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:

.Merge upwards
[caption="Rule: "]
=====================================
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 linkgit: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:

.Topic branches
[caption="Rule: "]
=====================================
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 linkgit: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 linkgit: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:

.Merge to downstream only at well-defined points
[caption="Rule: "]
=====================================
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.

.Throw-away integration branches
[caption="Rule: "]
=====================================
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:

.Verify 'master' is a superset of 'maint'
[caption="Recipe: "]
=====================================
`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:

.Release tagging
[caption="Recipe: "]
=====================================
`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).

.Copy maint
[caption="Recipe: "]
=====================================
`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:

.Update maint to new release
[caption="Recipe: "]
=====================================
* `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':

.Rewind and rebuild next
[caption="Recipe: "]
=====================================
* `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:

* linkgit:git-push[1] copies your branches to a remote repository,
  usually to one that can be read by all involved parties;

* linkgit:git-fetch[1] that copies remote branches to your repository;
  and

* linkgit: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:

.Push/pull: Publishing branches/topics
[caption="Recipe: "]
=====================================
`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 linkgit: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:

.Push/pull: Staying up to date
[caption="Recipe: "]
=====================================
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.

.Push/pull: Merging remote topics
[caption="Recipe: "]
=====================================
`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
linkgit:git-format-patch[1] to generate the corresponding emails
(highly recommended over manually formatting them because it makes the
maintainer's life easier).

.format-patch/am: Publishing branches/topics
[caption="Recipe: "]
=====================================
* `git format-patch -M upstream..topic` to turn them into preformatted
  patch files
* `git send-email --to=<recipient> <patches>`
=====================================

See the linkgit:git-format-patch[1] and linkgit: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):

.format-patch/am: Keeping topics up to date
[caption="Recipe: "]
=====================================
`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:

.format-patch/am: Importing patches
[caption="Recipe: "]
=====================================
`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 linkgit:git-am[1] for
other options.


SEE ALSO
--------
linkgit:gittutorial[7],
linkgit:git-push[1],
linkgit:git-pull[1],
linkgit:git-merge[1],
linkgit:git-rebase[1],
linkgit:git-format-patch[1],
linkgit:git-send-email[1],
linkgit:git-am[1]

GIT
---
Part of the linkgit:git[1] suite.