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Contributing to Weston

Finding something to work on

Weston's development is tracked on GitLab. In addition to reviewing code submissions (see below), we use the issue tracker to discuss both bugfixes and development of new features.

The 'good for new contributors' label is used for issues the development team thinks are a good place to begin working on Weston. These issues cover features or bugfixes which are small, self-contained, don't require much specific background knowledge, and aren't blocked by more complex work.

If you have picked an issue you would like to work on, you may want to mention in the issue tracker that you would like to pick it up. You can also discuss it with the developers in the issue tracker, or on the mailing list. Many developers also use IRC through OFTC's #wayland channel; however you may need to wait some time for a response on IRC, which requires keeping your client connected. If you cannot stay for a long time (potentially some hours due to timezone differences), then you may want to send your question to the list or issue tracker instead.

Sending patches

Patches should be sent via GitLab merge requests. Weston is hosted on freedesktop.org's GitLab: in order to submit code, you should create an account on this GitLab instance, fork the core Weston repository, push your changes to a branch in your new repository, and then submit these patches for review through a merge request.

Weston formerly accepted patches via git-send-email, sent to [email protected]; these were tracked using Patchwork. New email patches are no longer accepted.

Formatting and separating commits

Unlike many projects using GitHub and GitLab, Weston has a linear, 'recipe' style history. This means that every commit should be small, digestible, stand-alone, and functional. Rather than a purely chronological commit history like this:

doc: final docs for view transforms
fix tests when disabled, redo broken doc formatting
better transformed-view iteration (thanks Hannah!)
try to catch more cases in tests
tests: add new spline test
fix compilation on splines
doc: notes on reticulating splines
compositor: add spline reticulation for view transforms

we aim to have a clean history which only reflects the final state, broken up into functional groupings:

compositor: add spline reticulation for view transforms
compositor: new iterator for view transforms
tests: add view-transform correctness tests
doc: fix Doxygen formatting for view transforms

This ensures that the final patch series only contains the final state, without the changes and missteps taken along the development process.

The first line of a commit message should contain a prefix indicating what part is affected by the patch followed by one sentence that describes the change. For examples:

compositor-drm: Support modifiers for drm_fb

and

input: do not forward unmatched touch-ups

If in doubt what prefix to use, look at other commits that change the same file(s) as the patch being sent.

The body of the commit message should describe what the patch changes and why, and also note any particular side effects. This shouldn't be empty on most of the cases. It shouldn't take a lot of effort to write a commit message for an obvious change, so an empty commit message body is only acceptable if the questions "What?" and "Why?" are already answered on the one-line summary.

The lines of the commit message should have at most 76 characters, to cope with the way git log presents them.

See notes on commit messages for a recommended reading on writing commit messages.

Your patches must also include a Signed-off-by line with your name (or pseudonym) and email address which indicates that you agree to the Developer's Certificate of Origin 1.1. If you're not the patch's original author, you should also gather S-o-b's from them (and/or whomever gave the patch to you) in addition to your own S-o-b. The significance of this is that it certifies that you created the patch, that it was created under an appropriate open source license, or provided to you under those terms. This lets us indicate a chain of responsibility for the copyright status of the code.

Agreeing to DCO 1.1 is mandatory. Patches without a Signed-off-by cannot be accepted, but using a pseudonym is fine as long as the email address is yours personally.

When you re-send patches, revised or not, it would be very good to document the changes compared to the previous revision in the commit message and/or the merge request. If you have already received Reviewed-by or Acked-by tags, you should evaluate whether they still apply and include them in the respective commit messages. Otherwise the tags may be lost, reviewers miss the credit they deserve, and the patches may cause redundant review effort.

Tracking patches and following up

Once submitted to GitLab, your patches will be reviewed by the Weston development team on GitLab. Review may be entirely positive and result in your code landing instantly, in which case, great! You're done. However, we may ask you to make some revisions: fixing some bugs we've noticed, working to a slightly different design, or adding documentation and tests.

If you do get asked to revise the patches, please bear in mind the notes above. You should use git rebase -i to make revisions, so that your patches follow the clear linear split documented above. Following that split makes it easier for reviewers to understand your work, and to verify that the code you're submitting is correct.

A common request is to split single large patch into multiple patches. This can happen, for example, if when adding a new feature you notice a bug in Weston's core which you need to fix to progress. Separating these changes into separate commits will allow us to verify and land the bugfix quickly, pushing part of your work for the good of everyone, whilst revision and discussion continues on the larger feature part. It also allows us to direct you towards reviewers who best understand the different areas you are working on.

When you have made any requested changes, please rebase the commits, verify that they still individually look good, then force-push your new branch to GitLab. This will update the merge request and notify everyone subscribed to your merge request, so they can review it again.

There are also many GitLab CLI clients, if you prefer to avoid the web interface. It may be difficult to follow review comments without using the web interface though, so we do recommend using this to go through the review process, even if you use other clients to track the list of available patches.

Coding style

You should follow the style of the file you're editing. In general, we try to follow the rules below.

Note: this file uses spaces due to markdown rendering issues for tabs. Code must be indented using tabs.

  • indent with tabs, and a tab is always 8 characters wide
  • opening braces are on the same line as the if statement;
  • no braces in an if-body with just one statement;
  • if one of the branches of an if-else condition has braces, then the other branch should also have braces;
  • there is always an empty line between variable declarations and the code;
static int
my_function(void)
{
        int a = 0;

        if (a)
                b();
        else
                c();

        if (a) {
                b();
                c();
        } else {
                d();
        }
}
  • lines should be less than 80 characters wide;
  • when breaking lines with functions calls, the parameters are aligned with the opening parentheses;
  • when assigning a variable with the result of a function call, if the line would be longer we break it around the equal '=' sign if it makes sense;
        long_variable_name =
                function_with_a_really_long_name(parameter1, parameter2,
                                                 parameter3, parameter4);

        x = function_with_a_really_long_name(parameter1, parameter2,
                                             parameter3, parameter4);

Conduct

As a freedesktop.org project, Wayland follows the Contributor Covenant, found at: https://www.freedesktop.org/wiki/CodeOfConduct

Please conduct yourself in a respectful and civilised manner when interacting with community members on mailing lists, IRC, or bug trackers. The community represents the project as a whole, and abusive or bullying behaviour is not tolerated by the project.

Licensing

Weston is licensed with the intention to be usable anywhere X.org is. Originally, X.org was covered under the MIT X11 license, but changed to the MIT Expat license. Similarly, Weston was covered initially as MIT X11 licensed, but changed to the MIT Expat license, following in X.org's footsteps. Other than wording, the two licenses are substantially the same, with the exception of a no-advertising clause in X11 not included in Expat.

New source code files should specify the MIT Expat license in their boilerplate, as part of the copyright statement.

Review

All patches, even trivial ones, require at least one positive review (Reviewed-by). Additionally, if no Reviewed-by's have been given by people with commit access, there needs to be at least one Acked-by from someone with commit access. A person with commit access is expected to be able to evaluate the patch with respect to the project scope and architecture.

The below review guidelines are intended to be interpreted in spirit, not by the letter. There may be circumstances where some guidelines are better ignored. We rely very much on the judgement of reviewers and commit rights holders.

During review, the following matters should be checked:

  • The commit message explains why the change is being made.

  • The code fits the project's scope.

  • The code license is the same MIT licence the project generally uses.

  • Stable ABI or API is not broken.

  • Stable ABI or API additions must be justified by actual use cases, not only by speculation. They must also be documented, and it is strongly recommended to include tests exercising the additions in the test suite.

  • The code fits the existing software architecture, e.g. no layering violations.

  • The code is correct and does not introduce new failures for existing users, does not add new corner-case bugs, and does not introduce new compiler warnings.

  • The patch does what it says in the commit message and changes nothing else.

  • The patch is a single logical change. If the commit message addresses multiple points, it is a hint that the commit might need splitting up.

  • A bug fix should target the underlying root cause instead of hiding symptoms. If a complete fix is not practical, partial fixes are acceptable if they come with code comments and filed Gitlab issues for the remaining bugs.

  • The bug root cause rule applies to external software components as well, e.g. do not work around kernel driver issues in userspace.

  • The test suite passes.

  • The code does not depend on API or ABI which has no working free open source implementation.

  • The code is not dead or untestable. E.g. if there are no free open source software users for it then it is effectively dead code.

  • The code is written to be easy to understand, or if code cannot be clear enough on its own there are code comments to explain it.

  • The code is minimal, i.e. prefer refactor and re-use when possible unless clarity suffers.

  • The code adheres to the style guidelines.

  • In a patch series, every intermediate step adheres to the above guidelines.

Commit rights

Commit rights will be granted to anyone who requests them and fulfills the below criteria:

  • Submitted some (10 as a rule of thumb) non-trivial (not just simple spelling fixes and whitespace adjustment) patches that have been merged already.

  • Are actively participating in public discussions about their work (on the mailing list or IRC). This should not be interpreted as a requirement to review other peoples patches but just make sure that patch submission isn't one-way communication. Cross-review is still highly encouraged.

  • Will be regularly contributing further patches. This includes regular contributors to other parts of the open source graphics stack who only do the occasional development in this project.

  • Agrees to use their commit rights in accordance with the documented merge criteria, tools, and processes.

To apply for commit rights, create a new issue in gitlab for the respective project and give it the "accounts" label.

Committers are encouraged to request their commit rights get removed when they no longer contribute to the project. Commit rights will be reinstated when they come back to the project.

Maintainers and committers should encourage contributors to request commit rights, especially junior contributors tend to underestimate their skills.

Stabilising for releases

A release cycle ends with a stable release which also starts a new cycle and lifts any code freezes. Gradual code freezing towards a stable release starts with an alpha release. The release stages of a cycle are:

  • Alpha release: Signified by version number #.#.91. Major features must have landed before this. Major features include invasive code motion and refactoring, high risk changes, and new stable library ABI.

  • Beta release: Signified by version number #.#.92. Minor features must have landed before this. Minor features include all new features that are not major, low risk changes, clean-ups, and documentation. Stable ABI that was new in the alpha release can be removed before a beta release if necessary.

  • Release candidates (RC): Signified by version number #.#.93 and up to #.#.99. Bug fixes that are not release critical must have landed before this. Release critical bug fixes can still be landed after this, but they may call for another RC.

  • Stable release: Signified by version number #.#.0. Ideally no changes since the last RC.

Mind that version #.#.90 is never released. It is used during development when no code freeze is in effect. Stable branches and point releases are not covered by the above.