Here are some guidelines for people who want to contribute their code to this software.

(0) Decide what to base your work on.

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

To find the tip of a topic branch, run "git log --first-parent master..pu" and look for the merge commit. The second parent of this commit is the tip of the topic branch.

(1) 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 summarise 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 behaviour when it should, and to show the feature does not trigger when it shouldn’t. Also make sure that the test suite passes after your commit. Do not forget to update the documentation to describe the updated behaviour.

Speaking of the documentation, it is currently 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.

(2) Describe your changes well.

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.

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

The body should provide a meaningful commit message, which:

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 behaviour. 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.

(3) 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 "master" branch head. If you are preparing a work based on "next" branch, that is fine, but please mark it as such.

(4) Sending your patches.

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 line) is not writable in ASCII, make sure that you send off a message in the correct encoding.

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 additional markers after PATCH and the closing bracket to mark the nature of the patch is also encouraged. E.g. [PATCH/RFC] is often used when the patch is not ready to be applied but it is for discussion, [PATCH v2], [PATCH v3] etc. are often seen when you are sending an update to what you have previously sent.

"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: lines, 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.

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, at least for now. 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.

Send your patch with "To:" set to the mailing list, with "cc:" listing people who are involved in the area you are touching (the output from "git blame $path" and "git shortlog --no-merges $path" would help to identify them), to solicit comments and reviews.

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 [1] and "cc:" the list [2] for inclusion.

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.

(5) Sign your work

To improve tracking of who did what, we’ve borrowed the "sign-off" procedure from the Linux kernel project on patches that are being emailed around. Although core Git is a lot smaller project it is a good discipline to follow it.

The sign-off is a simple line at the end of the explanation for the patch, which certifies that you wrote it or otherwise have the right to pass it on as a open-source patch. The rules are pretty simple: if you can certify the below:

then you just add a line saying

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

Notice that you can place your own Signed-off-by: line 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).

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

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

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:".

An ideal patch flow

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

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

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.