SubmittingPatches: explain why 'next' and above are inappropriate base

[thirdparty/git.git] / Documentation / SubmittingPatches

Submitting Patches
==================

== Guidelines

Here are some guidelines for contributing back to this
project. There is also a link:MyFirstContribution.html[step-by-step tutorial]
available which covers many of these same guidelines.

[[choose-starting-point]]
=== Choose a starting point.

As a preliminary step, you must first choose a starting point for your
work. Typically this means choosing a branch, although technically
speaking it is actually a particular commit (typically the HEAD, or tip,
of the branch).

There are several important branches to be aware of. Namely, there are
four integration branches as discussed in linkgit:gitworkflows[7]:

* maint
* master
* next
* seen

The branches lower on the list are typically descendants of the ones
that come before it. For example, `maint` is an "older" branch than
`master` because `master` usually has patches (commits) on top of
`maint`.

There are also "topic" branches, which contain work from other
contributors.  Topic branches are created by the Git maintainer (in
their fork) to organize the current set of incoming contributions on
the mailing list, and are itemized in the regular "What's cooking in
git.git" announcements.  To find the tip of a topic branch, run `git log
--first-parent master..seen` and look for the merge commit. The second
parent of this commit is the tip of the topic branch.

There is one guiding principle for choosing the right starting point: in
general, always base your work on the oldest integration branch that
your change is relevant to (see "Merge upwards" in
linkgit:gitworkflows[7]).  What this principle means is that for the
vast majority of cases, the starting point for new work should be the
latest HEAD commit of `maint` or `master` based on the following cases:

* If you are fixing bugs in the released version, use `maint` as the
  starting point (which may mean you have to fix things without using
  new API features on the cutting edge that recently appeared in
  `master` but were not available in the released version). If the bug
  exists in an older version (e.g., commit `X` introduced the bug, and
  `git describe --contains X` says `v2.30.0-rc2-gXXXXXX` has it), then
  use the tip of the maintenance branch for the 2.30.x versions in the
  `maint-2.30` branch in https://github.com/gitster/git[the maintainer's
  repo].

* Otherwise (such as if you are adding new features) use `master`.

This also means that `next` or `seen` are inappropriate starting points
for your work, if you want your work to have a realistic chance of
graduating to `master`.  They are simply not designed to be used as a
base for new work; they are only there to make sure that topics in
flight work well together. This is why both `next` and `seen` are
frequently re-integrated with incoming patches on the mailing list and
force-pushed to replace previous versions of themselves. A topic that is
literally built on top of `next` cannot be merged to `master` without
dragging in all the other topics in `next`, some of which may not be
ready.

For example, if you are making tree-wide changes, while somebody else is
also making their own tree-wide changes, your work may have severe
overlap with the other person's work.  This situation may tempt you to
use `next` as your starting point (because it would have the other
person's work included in it), but doing so would mean you'll not only
depend on the other person's work, but all the other random things from
other contributors that are already integrated into `next`.  And as soon
as `next` is updated with a new version, all of your work will need to
be rebased anyway in order for them to be cleanly applied by the
maintainer.

Under truly exceptional circumstances where you absolutely must depend
on a select few topic branches that are already in `next` but not in
`master`, you may want to create your own custom base-branch by forking
`master` and merging the required topic branches to it. You could then
work on top of this base-branch.  But keep in mind that this base-branch
would only be known privately to you.  So when you are ready to send
your patches to the list, be sure to communicate how you created it in
your cover letter.  This critical piece of information would allow
others to recreate your base-branch on their end in order for them to
try out your work.

Finally, note that some parts of the system have dedicated maintainers
with their own separate source code repositories (see the section
"Subsystems" below).

[[separate-commits]]
=== Make separate commits for logically separate changes.

Unless your patch is really trivial, you should not be sending
out a patch that was generated between your working tree and
your commit head.  Instead, always make a commit with complete
commit message and generate a series of patches from your
repository.  It is a good discipline.

Give an explanation for the change(s) that is detailed enough so
that people can judge if it is good thing to do, without reading
the actual patch text to determine how well the code does what
the explanation promises to do.

If your description starts to get too long, that's a sign that you
probably need to split up your commit to finer grained pieces.
That being said, patches which plainly describe the things that
help reviewers check the patch, and future maintainers understand
the code, are the most beautiful patches.  Descriptions that summarize
the point in the subject well, and describe the motivation for the
change, the approach taken by the change, and if relevant how this
differs substantially from the prior version, are all good things
to have.

Make sure that you have tests for the bug you are fixing.  See
`t/README` for guidance.

[[tests]]
When adding a new feature, make sure that you have new tests to show
the feature triggers the new behavior when it should, and to show the
feature does not trigger when it shouldn't.  After any code change,
make sure that the entire test suite passes.  When fixing a bug, make
sure you have new tests that break if somebody else breaks what you
fixed by accident to avoid regression.  Also, try merging your work to
'next' and 'seen' and make sure the tests still pass; topics by others
that are still in flight may have unexpected interactions with what
you are trying to do in your topic.

Pushing to a fork of https://github.com/git/git will use their CI
integration to test your changes on Linux, Mac and Windows. See the
<<GHCI,GitHub CI>> section for details.

Do not forget to update the documentation to describe the updated
behavior and make sure that the resulting documentation set formats
well (try the Documentation/doc-diff script).

We currently have a liberal mixture of US and UK English norms for
spelling and grammar, which is somewhat unfortunate.  A huge patch that
touches the files all over the place only to correct the inconsistency
is not welcome, though.  Potential clashes with other changes that can
result from such a patch are not worth it.  We prefer to gradually
reconcile the inconsistencies in favor of US English, with small and
easily digestible patches, as a side effect of doing some other real
work in the vicinity (e.g. rewriting a paragraph for clarity, while
turning en_UK spelling to en_US).  Obvious typographical fixes are much
more welcomed ("teh -> "the"), preferably submitted as independent
patches separate from other documentation changes.

[[whitespace-check]]
Oh, another thing.  We are picky about whitespaces.  Make sure your
changes do not trigger errors with the sample pre-commit hook shipped
in `templates/hooks--pre-commit`.  To help ensure this does not happen,
run `git diff --check` on your changes before you commit.

[[describe-changes]]
=== Describe your changes well.

The log message that explains your changes is just as important as the
changes themselves.  Your code may be clearly written with in-code
comment to sufficiently explain how it works with the surrounding
code, but those who need to fix or enhance your code in the future
will need to know _why_ your code does what it does, for a few
reasons:

. Your code may be doing something differently from what you wanted it
  to do.  Writing down what you actually wanted to achieve will help
  them fix your code and make it do what it should have been doing
  (also, you often discover your own bugs yourself, while writing the
  log message to summarize the thought behind it).

. Your code may be doing things that were only necessary for your
  immediate needs (e.g. "do X to directories" without implementing or
  even designing what is to be done on files).  Writing down why you
  excluded what the code does not do will help guide future developers.
  Writing down "we do X to directories, because directories have
  characteristic Y" would help them infer "oh, files also have the same
  characteristic Y, so perhaps doing X to them would also make sense?".
  Saying "we don't do the same X to files, because ..." will help them
  decide if the reasoning is sound (in which case they do not waste
  time extending your code to cover files), or reason differently (in
  which case, they can explain why they extend your code to cover
  files, too).

The goal of your log message is to convey the _why_ behind your
change to help future developers.

The first line of the commit message should be a short description (50
characters is the soft limit, see DISCUSSION in linkgit:git-commit[1]),
and should skip the full stop.  It is also conventional in most cases to
prefix the first line with "area: " where the area is a filename or
identifier for the general area of the code being modified, e.g.

* doc: clarify distinction between sign-off and pgp-signing
* githooks.txt: improve the intro section

If in doubt which identifier to use, run `git log --no-merges` on the
files you are modifying to see the current conventions.

[[summary-section]]
The title sentence after the "area:" prefix omits the full stop at the
end, and its first word is not capitalized (the omission
of capitalization applies only to the word after the "area:"
prefix of the title) unless there is a reason to
capitalize it other than because it is the first word in the sentence.
E.g. "doc: clarify...", not "doc: Clarify...", or "githooks.txt:
improve...", not "githooks.txt: Improve...".  But "refs: HEAD is also
treated as a ref" is correct, as we spell `HEAD` in all caps even when
it appears in the middle of a sentence.

[[meaningful-message]]
The body should provide a meaningful commit message, which:

. explains the problem the change tries to solve, i.e. what is wrong
  with the current code without the change.

. justifies the way the change solves the problem, i.e. why the
  result with the change is better.

. alternate solutions considered but discarded, if any.

[[present-tense]]
The problem statement that describes the status quo is written in the
present tense.  Write "The code does X when it is given input Y",
instead of "The code used to do Y when given input X".  You do not
have to say "Currently"---the status quo in the problem statement is
about the code _without_ your change, by project convention.

[[imperative-mood]]
Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behavior.  Try to make sure your explanation can be understood
without external resources. Instead of giving a URL to a mailing list
archive, summarize the relevant points of the discussion.

[[commit-reference]]

There are a few reasons why you may want to refer to another commit in
the "more stable" part of the history (i.e. on branches like `maint`,
`master`, and `next`):

. A commit that introduced the root cause of a bug you are fixing.

. A commit that introduced a feature that you are enhancing.

. A commit that conflicts with your work when you made a trial merge
  of your work into `next` and `seen` for testing.

When you reference a commit on a more stable branch (like `master`,
`maint` and `next`), use the format "abbreviated hash (subject,
date)", like this:

....
	Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30)
	noticed that ...
....

The "Copy commit summary" command of gitk can be used to obtain this
format (with the subject enclosed in a pair of double-quotes), or this
invocation of `git show`:

....
	git show -s --pretty=reference <commit>
....

or, on an older version of Git without support for --pretty=reference:

....
	git show -s --date=short --pretty='format:%h (%s, %ad)' <commit>
....

[[sign-off]]
=== Certify your work by adding your `Signed-off-by` trailer

To improve tracking of who did what, we ask you to certify that you
wrote the patch or have the right to pass it on under the same license
as ours, by "signing off" your patch.  Without sign-off, we cannot
accept your patches.

If (and only if) you certify the below D-C-O:

[[dco]]
.Developer's Certificate of Origin 1.1
____
By making a contribution to this project, I certify that:

a. The contribution was created in whole or in part by me and I
   have the right to submit it under the open source license
   indicated in the file; or

b. The contribution is based upon previous work that, to the best
   of my knowledge, is covered under an appropriate open source
   license and I have the right under that license to submit that
   work with modifications, whether created in whole or in part
   by me, under the same open source license (unless I am
   permitted to submit under a different license), as indicated
   in the file; or

c. The contribution was provided directly to me by some other
   person who certified (a), (b) or (c) and I have not modified
   it.

d. I understand and agree that this project and the contribution
   are public and that a record of the contribution (including all
   personal information I submit with it, including my sign-off) is
   maintained indefinitely and may be redistributed consistent with
   this project or the open source license(s) involved.
____

you add a "Signed-off-by" trailer to your commit, that looks like
this:

....
	Signed-off-by: Random J Developer <random@developer.example.org>
....

This line can be added by Git if you run the git-commit command with
the -s option.

Notice that you can place your own `Signed-off-by` trailer when
forwarding somebody else's patch with the above rules for
D-C-O.  Indeed you are encouraged to do so.  Do not forget to
place an in-body "From: " line at the beginning to properly attribute
the change to its true author (see (2) above).

This procedure originally came from the Linux kernel project, so our
rule is quite similar to theirs, but what exactly it means to sign-off
your patch differs from project to project, so it may be different
from that of the project you are accustomed to.

[[real-name]]
Also notice that a real name is used in the `Signed-off-by` trailer. Please
don't hide your real name.

[[commit-trailers]]
If you like, you can put extra tags at the end:

. `Reported-by:` is used to credit someone who found the bug that
  the patch attempts to fix.
. `Acked-by:` says that the person who is more familiar with the area
  the patch attempts to modify liked the patch.
. `Reviewed-by:`, unlike the other tags, can only be offered by the
  reviewers themselves when they are completely satisfied with the
  patch after a detailed analysis.
. `Tested-by:` is used to indicate that the person applied the patch
  and found it to have the desired effect.

You can also create your own tag or use one that's in common usage
such as "Thanks-to:", "Based-on-patch-by:", or "Mentored-by:".

[[git-tools]]
=== Generate your patch using Git tools out of your commits.

Git based diff tools generate unidiff which is the preferred format.

You do not have to be afraid to use `-M` option to `git diff` or
`git format-patch`, if your patch involves file renames.  The
receiving end can handle them just fine.

[[review-patch]]
Please make sure your patch does not add commented out debugging code,
or include any extra files which do not relate to what your patch
is trying to achieve. Make sure to review
your patch after generating it, to ensure accuracy.  Before
sending out, please make sure it cleanly applies to the starting point you
have chosen in the "Choose a starting point" section.

NOTE: From the perspective of those reviewing your patch, the `master`
branch is the default expected starting point.  So if you have chosen a
different starting point, please communicate this choice in your cover
letter.


[[send-patches]]
=== Sending your patches.

:security-ml: footnoteref:[security-ml,The Git Security mailing list: git-security@googlegroups.com]

Before sending any patches, please note that patches that may be
security relevant should be submitted privately to the Git Security
mailing list{security-ml}, instead of the public mailing list.

Learn to use format-patch and send-email if possible.  These commands
are optimized for the workflow of sending patches, avoiding many ways
your existing e-mail client that is optimized for "multipart/*" mime
type e-mails to corrupt and render your patches unusable.

People on the Git mailing list need to be able to read and
comment on the changes you are submitting.  It is important for
a developer to be able to "quote" your changes, using standard
e-mail tools, so that they may comment on specific portions of
your code.  For this reason, each patch should be submitted
"inline" in a separate message.

Multiple related patches should be grouped into their own e-mail
thread to help readers find all parts of the series.  To that end,
send them as replies to either an additional "cover letter" message
(see below), the first patch, or the respective preceding patch.

If your log message (including your name on the
`Signed-off-by` trailer) is not writable in ASCII, make sure that
you send off a message in the correct encoding.

WARNING: Be wary of your MUAs word-wrap
corrupting your patch.  Do not cut-n-paste your patch; you can
lose tabs that way if you are not careful.

It is a common convention to prefix your subject line with
[PATCH].  This lets people easily distinguish patches from other
e-mail discussions.  Use of markers in addition to PATCH within
the brackets to describe the nature of the patch is also
encouraged.  E.g. [RFC PATCH] (where RFC stands for "request for
comments") is often used to indicate a patch needs further
discussion before being accepted, [PATCH v2], [PATCH v3] etc.
are often seen when you are sending an update to what you have
previously sent.

The `git format-patch` command follows the best current practice to
format the body of an e-mail message.  At the beginning of the
patch should come your commit message, ending with the
`Signed-off-by` trailers, and a line that consists of three dashes,
followed by the diffstat information and the patch itself.  If
you are forwarding a patch from somebody else, optionally, at
the beginning of the e-mail message just before the commit
message starts, you can put a "From: " line to name that person.
To change the default "[PATCH]" in the subject to "[<text>]", use
`git format-patch --subject-prefix=<text>`.  As a shortcut, you
can use `--rfc` instead of `--subject-prefix="RFC PATCH"`, or
`-v <n>` instead of `--subject-prefix="PATCH v<n>"`.

You often want to add additional explanation about the patch,
other than the commit message itself.  Place such "cover letter"
material between the three-dash line and the diffstat.  For
patches requiring multiple iterations of review and discussion,
an explanation of changes between each iteration can be kept in
Git-notes and inserted automatically following the three-dash
line via `git format-patch --notes`.

[[attachment]]
Do not attach the patch as a MIME attachment, compressed or not.
Do not let your e-mail client send quoted-printable.  Do not let
your e-mail client send format=flowed which would destroy
whitespaces in your patches. Many
popular e-mail applications will not always transmit a MIME
attachment as plain text, making it impossible to comment on
your code.  A MIME attachment also takes a bit more time to
process.  This does not decrease the likelihood of your
MIME-attached change being accepted, but it makes it more likely
that it will be postponed.

Exception:  If your mailer is mangling patches then someone may ask
you to re-send them using MIME, that is OK.

[[pgp-signature]]
Do not PGP sign your patch. Most likely, your maintainer or other people on the
list would not have your PGP key and would not bother obtaining it anyway.
Your patch is not judged by who you are; a good patch from an unknown origin
has a far better chance of being accepted than a patch from a known, respected
origin that is done poorly or does incorrect things.

If you really really really really want to do a PGP signed
patch, format it as "multipart/signed", not a text/plain message
that starts with `-----BEGIN PGP SIGNED MESSAGE-----`.  That is
not a text/plain, it's something else.

:security-ml-ref: footnoteref:[security-ml]

As mentioned at the beginning of the section, patches that may be
security relevant should not be submitted to the public mailing list
mentioned below, but should instead be sent privately to the Git
Security mailing list{security-ml-ref}.

Send your patch with "To:" set to the mailing list, with "cc:" listing
people who are involved in the area you are touching (the `git
contacts` command in `contrib/contacts/` can help to
identify them), to solicit comments and reviews.  Also, when you made
trial merges of your topic to `next` and `seen`, you may have noticed
work by others conflicting with your changes.  There is a good possibility
that these people may know the area you are touching well.

:current-maintainer: footnote:[The current maintainer: gitster@pobox.com]
:git-ml: footnote:[The mailing list: git@vger.kernel.org]

After the list reached a consensus that it is a good idea to apply the
patch, re-send it with "To:" set to the maintainer{current-maintainer}
and "cc:" the list{git-ml} for inclusion.  This is especially relevant
when the maintainer did not heavily participate in the discussion and
instead left the review to trusted others.

Do not forget to add trailers such as `Acked-by:`, `Reviewed-by:` and
`Tested-by:` lines as necessary to credit people who helped your
patch, and "cc:" them when sending such a final version for inclusion.

== Subsystems with dedicated maintainers

Some parts of the system have dedicated maintainers with their own
repositories.

- `git-gui/` comes from git-gui project, maintained by Pratyush Yadav:

	https://github.com/prati0100/git-gui.git

- `gitk-git/` comes from Paul Mackerras's gitk project:

	git://git.ozlabs.org/~paulus/gitk

   Those who are interested in improve gitk can volunteer to help Paul
   in maintaining it cf. <YntxL/fTplFm8lr6@cleo>.

- `po/` comes from the localization coordinator, Jiang Xin:

	https://github.com/git-l10n/git-po/

Patches to these parts should be based on their trees.

[[patch-flow]]
== An ideal patch flow

Here is an ideal patch flow for this project the current maintainer
suggests to the contributors:

. You come up with an itch.  You code it up.

. Send it to the list and cc people who may need to know about
  the change.
+
The people who may need to know are the ones whose code you
are butchering.  These people happen to be the ones who are
most likely to be knowledgeable enough to help you, but
they have no obligation to help you (i.e. you ask for help,
don't demand).  +git log -p {litdd} _$area_you_are_modifying_+ would
help you find out who they are.

. You get comments and suggestions for improvements.  You may
  even get them in an "on top of your change" patch form.

. Polish, refine, and re-send to the list and the people who
  spend their time to improve your patch.  Go back to step (2).

. The list forms consensus that the last round of your patch is
  good.  Send it to the maintainer and cc the list.

. A topic branch is created with the patch and is merged to `next`,
  and cooked further and eventually graduates to `master`.

In any time between the (2)-(3) cycle, the maintainer may pick it up
from the list and queue it to `seen`, in order to make it easier for
people play with it without having to pick up and apply the patch to
their trees themselves.

[[patch-status]]
== Know the status of your patch after submission

* You can use Git itself to find out when your patch is merged in
  master. `git pull --rebase` will automatically skip already-applied
  patches, and will let you know. This works only if you rebase on top
  of the branch in which your patch has been merged (i.e. it will not
  tell you if your patch is merged in `seen` if you rebase on top of
  master).

* Read the Git mailing list, the maintainer regularly posts messages
  entitled "What's cooking in git.git" and "What's in git.git" giving
  the status of various proposed changes.

== GitHub CI[[GHCI]]

With an account at GitHub, you can use GitHub CI to test your changes
on Linux, Mac and Windows. See
https://github.com/git/git/actions/workflows/main.yml for examples of
recent CI runs.

Follow these steps for the initial setup:

. Fork https://github.com/git/git to your GitHub account.
  You can find detailed instructions how to fork here:
  https://help.github.com/articles/fork-a-repo/

After the initial setup, CI will run whenever you push new changes
to your fork of Git on GitHub.  You can monitor the test state of all your
branches here: `https://github.com/<Your GitHub handle>/git/actions/workflows/main.yml`

If a branch did not pass all test cases then it is marked with a red
cross. In that case you can click on the failing job and navigate to
"ci/run-build-and-tests.sh" and/or "ci/print-test-failures.sh". You
can also download "Artifacts" which are tarred (or zipped) archives
with test data relevant for debugging.

Then fix the problem and push your fix to your GitHub fork. This will
trigger a new CI build to ensure all tests pass.

[[mua]]
== MUA specific hints

Some of the patches I receive or pick up from the list share common
patterns of breakage.  Please make sure your MUA is set up
properly not to corrupt whitespaces.

See the DISCUSSION section of linkgit:git-format-patch[1] for hints on
checking your patch by mailing it to yourself and applying with
linkgit:git-am[1].

While you are at it, check the resulting commit log message from
a trial run of applying the patch.  If what is in the resulting
commit is not exactly what you would want to see, it is very
likely that your maintainer would end up hand editing the log
message when he applies your patch.  Things like "Hi, this is my
first patch.\n", if you really want to put in the patch e-mail,
should come after the three-dash line that signals the end of the
commit message.


=== Pine

(Johannes Schindelin)

....
I don't know how many people still use pine, but for those poor
souls it may be good to mention that the quell-flowed-text is
needed for recent versions.

... the "no-strip-whitespace-before-send" option, too. AFAIK it
was introduced in 4.60.
....

(Linus Torvalds)

....
And 4.58 needs at least this.

diff-tree 8326dd8350be64ac7fc805f6563a1d61ad10d32c (from e886a61f76edf5410573e92e38ce22974f9c40f1)
Author: Linus Torvalds <torvalds@g5.osdl.org>
Date:   Mon Aug 15 17:23:51 2005 -0700

    Fix pine whitespace-corruption bug

    There's no excuse for unconditionally removing whitespace from
    the pico buffers on close.

diff --git a/pico/pico.c b/pico/pico.c
--- a/pico/pico.c
+++ b/pico/pico.c
@@ -219,7 +219,9 @@ PICO *pm;
	    switch(pico_all_done){	/* prepare for/handle final events */
	      case COMP_EXIT :		/* already confirmed */
		packheader();
+#if 0
		stripwhitespace();
+#endif
		c |= COMP_EXIT;
		break;
....

(Daniel Barkalow)

....
> A patch to SubmittingPatches, MUA specific help section for
> users of Pine 4.63 would be very much appreciated.

Ah, it looks like a recent version changed the default behavior to do the
right thing, and inverted the sense of the configuration option. (Either
that or Gentoo did it.) So you need to set the
"no-strip-whitespace-before-send" option, unless the option you have is
"strip-whitespace-before-send", in which case you should avoid checking
it.
....

=== Thunderbird, KMail, GMail

See the MUA-SPECIFIC HINTS section of linkgit:git-format-patch[1].

=== Gnus

"|" in the `*Summary*` buffer can be used to pipe the current
message to an external program, and this is a handy way to drive
`git am`.  However, if the message is MIME encoded, what is
piped into the program is the representation you see in your
`*Article*` buffer after unwrapping MIME.  This is often not what
you would want for two reasons.  It tends to screw up non ASCII
characters (most notably in people's names), and also
whitespaces (fatal in patches).  Running "C-u g" to display the
message in raw form before using "|" to run the pipe can work
this problem around.