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Guidelines

Here are some guidelines for people who want to contribute their code to this software. There is also a step-by-step tutorial available which covers many of these same guidelines.

Decide what to base your work on.

In general, always base your work on the oldest branch that your change is relevant to.

  • A bugfix should be based on maint in general. If the bug is not present in maint, base it on master. For a bug that’s not yet in master, find the topic that introduces the regression, and base your work on the tip of the topic.

  • A new feature should be based on master in general. If the new feature depends on other topics that are in next, but not in master, fork a branch from the tip of master, merge these topics to the branch, and work on that branch. You can remind yourself of how you prepared the base with git log --first-parent master...

  • Corrections and enhancements to a topic not yet in master should be based on the tip of that topic. If the topic has not been merged to next, it’s alright to add a note to squash minor corrections into the series.

  • In the exceptional case that a new feature depends on several topics not in master, start working on next or seen privately and send out patches only for discussion. Once your new feature starts to stabilize, you would have to rebase it (see the "depends on other topics" above).

  • Some parts of the system have dedicated maintainers with their own repositories (see the section "Subsystems" below). Changes to these parts should be based on their trees.

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.

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.

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 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.

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 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:

  1. 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).

  2. 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 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.

The title sentence after the "area:" prefix omits the full stop at the end, and its first word is not capitalized 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.

The body should provide a meaningful commit message, which:

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

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

  3. alternate solutions considered but discarded, if any.

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.

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.

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):

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

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

  3. 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>

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:

Developer’s Certificate of Origin 1.1

By making a contribution to this project, I certify that:

  1. 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

  2. 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

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

  4. 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.

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

If you like, you can put extra tags at the end:

  1. Reported-by: is used to credit someone who found the bug that the patch attempts to fix.

  2. Acked-by: says that the person who is more familiar with the area the patch attempts to modify liked the patch.

  3. 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.

  4. 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:".

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.

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 base you have chosen in the "Decide what to base your work on" section, and unless it targets the master branch (which is the default), mark your patches as such.

Sending your patches.

Before sending any patches, please note that patches that may be security relevant should be submitted privately to the Git Security mailing list[1], 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.

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.

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.

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[1].

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.

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[2] and "cc:" the list[3] 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.

An ideal patch flow

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

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

  2. 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 -- $area_you_are_modifying would help you find out who they are.

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

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

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

  6. 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.

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

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:

  1. 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 specific hints

Some of 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 git-format-patch[1] for hints on checking your patch by mailing it to yourself and applying with 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 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.


1. The Git Security mailing list: git-security@googlegroups.com
2. The current maintainer: gitster@pobox.com
3. The mailing list: git@vger.kernel.org
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