Software Release Practice HOWTO
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# 2. Good patching practice

Most people get involved in open-source software by writing patches for other peoples' software before releasing projects of their own. Suppose you've written a set of source-code changes for someone else's baseline code. Now put yourself in that person's shoes. How is he to judge whether to include the patch?

The truth is that it is very difficult to judge the quality of code. So developers tend to evaluate patches by the quality of the submission. They look for clues in the submitter's style and communications behavior instead — indications that the person has been in their shoes and understands what it's like to have to evaluate and merge an incoming patch.

This is actually a pretty reliable proxy for code quality. In many years of dealing with patches from many hundreds of strangers, I have only seldom seen a patch that was thoughtfully presented and respectful of my time but technically bogus. On the other hand, experience teaches me that patches which look careless or are packaged in a lazy and inconsiderate way are very likely to actually be bogus.

Here are some tips on how to get your patch accepted:

## 2.1. Do send patches, don't send whole archives or files

If your change includes a new file that doesn't exist in the code, then of course you have to send the whole file. But if you're modifying an already-existing file, don't send the whole file. Send a diff instead; specifically, send the output of the diff(1) command run to compare the baseline distributed version against your modified version.

The diff command and its dual, patch(1) (which automatically applies a diff to a baseline file) are the most basic tools of open-source development. Diffs are better than whole files because the developer you're sending a patch to may have changed the baseline version since you got your copy. By sending him a diff you save him the effort of separating your changes from his; you show respect for his time.

## 2.2. Send patches against the current version of the code.

It is both counterproductive and rude to send a maintainer patches against the code as it existed several releases ago, and expect him to do all the work of determining which changes duplicate things he have since done, versus which things are actually novel in your patch.

As a patch submitter, it is your responsibility to track the state of the source and send the maintainer a minimal patch that expresses what you want done to the main-line codebase. That means sending a patch against the current version.

## 2.3. Don't include patches for generated files.

Before you send your patch, walk through it and delete any patch bands for files in it that are going to be automatically regenerated once he applies the patch and remakes. The classic examples of this error are C files generated by Bison or Flex.

These days the most common mistake of this kind is sending a diff with a huge band that is nothing but changebars between your configure script and his. This file is generated by autoconf.

This is inconsiderate. It means your recipient is put to the trouble of separating the real content of the patch from a lot of bulky noise. It's a minor error, not as important as some of the things we'll get to further on — but it will count against you.

## 2.4. Don't send patch bands that just tweak version-control $-symbols.

Some people put special tokens in their source files that are expanded by the version-control system when the file is checked in: the $Id$ construct used by RCS, CVS, and Subversion, for example.

If you're using a local version-control system yourself, your changes may alter these tokens. This isn't really harmful, because when your recipient checks his code back in after applying your patch they'll get re-expanded based on his version-control status. But those extra patch bands are noise. They're distracting. It's more considerate not to send them.

This is another minor error. You'll be forgiven for it if you get the big things right. But you want to avoid it anyway.

## 2.5. Do use -c or -u format, don't use the default (-e) format

The default (-e) format of diff(1) is very brittle. It doesn't include any context, so the patch tool can't cope if any lines have been inserted or deleted in the baseline code since you took the copy you modified.

Getting an -e diff is annoying, and suggests that the sender is either an extreme newbie, careless, or clueless. Most such patches get tossed out without a second thought.

## 2.6. Do include documentation with your patch

This is very important. If your patch makes a user-visible addition or change to the software's features, include changes to the appropriate man pages and other documentation files in your patch. Do not assume that the recipient will be happy to document your code for you, or else to have undocumented features lurking in the code.

Documenting your changes well demonstrates some good things. First, it's considerate to the person you are trying to persuade. Second, it shows that you understand the ramifications of your change well enough to explain it to somebody who can't see the code. Third, it demonstrates that you care about the people who will ultimately use the software.

Good documentation is usually the most visible sign of what separates a solid contribution from a quick and dirty hack. If you take the time and care necessary to produce it, you'll find you're already 85% of the way to having your patch accepted with most developers.

## 2.7. Do include an explanation with your patch

Your patch should include cover notes explaining why you think the patch is necessary or useful. This is explanation directed not to the users of the software but to the maintainer to whom you are sending the patch.

The note can be short — in fact, some of the most effective cover notes I've ever seen just said "See the documentation updates in this patch". But it should show the right attitude.

The right attitude is helpful, respectful of the maintainer's time, quietly confident but unassuming. It's good to display understanding of the code you're patching. It's good to show that you can identify with the maintainer's problems. It's also good to be up front about any risks you perceive in applying the patch. Here are some examples of the sorts of explanatory comments that I like to see in cover notes:

" I've seen two problems with this code, X and Y. I fixed problem X, but I didn't try addressing problem Y because I don't think I understand the part of the code that I believe is involved. "

" Fixed a core dump that can happen when one of the foo inputs is too long. While I was at it, I went looking for similar overflows elsewhere. I found a possible one in blarg.c, near line 666. Are you sure the sender can't generate more than 80 characters per transmission? "

" Have you considered using the Foonly algorithm for this problem? There is a good implementation at <http://www.somesite.com/~jsmith/foonly.html>. "

" This patch solves the immediate problem, but I realize it complicates the memory allocation in an unpleasant way. Works for me, but you should probably test it under heavy load before shipping. "

" This may be featurititis, but I'm sending it anyway. Maybe you'll know a cleaner way to implement the feature. "

## 2.8. Do include useful comments in your code

Usually as a maintainer, I will want to have strong confidence that I understand your changes before merging them in. This isn't an invariable rule; if you have a track record of good work, with me I may just run a casual eyeball over the changes before checking them in semi-automatically. But everything you can do to help me understand your code and decrease my uncertainty increases your chances that I will accept your patch.

Good comments in your code help me understand it. Bad comments don't.

Here's an example of a bad comment:

    /* norman newbie fixed this 13 Aug 2009 */

This conveys no information. It's nothing but a muddy territorial bootprint you're planting in the middle of my code. If I take your patch (which you've made less likely) I'll almost certainly strip out this comment. If you want a credit, include a patch band for the project `NEWS` or `HISTORY` file. I'll probably take that.

Here's an example of a good comment:

    /*
     * This conditional needs to be guarded so that crunch_data()
     * never gets passed a NULL pointer.  <norman_newbie@foosite.com>
     */

This comment shows that you understand not only my code but the kind of information that I need to have confidence in your changes. This kind of comment gives me confidence in your changes.

## 2.9. Just one bugfix or new feature per patch.

Don't gather various bugfixes, new features or other stuff and send them as one single diff file. Instead, build a single diff for every bugfix or feature. Every single diff should be generated using the current version of the code and should not depend on any other patch you or someone else sent before.

This helps the maintainer to read and understand your code, and he can decide whether he wants to accept or abadon this single bugfix or feature. For example, if you add a feature garanting full root access for everyone because it's useful in your special setting, the maintainer will probably not accept this part of your patch. When you send a single diff file confounding this root access feature along with some bugfixes and other useful things, you have a good chance the maintainer will ignore your complete patch.

With each bugfixes and new feature as a single diff file, the maintainer could e.g. include your bugfixes in a few minutes and take later a closer look to your new features or security issues.

Patches depending on other patches raise similar problems. If the maintainer rejects your basic patch, he won't be able to apply the others. If you can't avoid such dependencies, offer to the maintainer that you will bundle a patch with the sections or features he wants to add.

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