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Contributing to wlroots

Contributing just involves sending a merge request. You will probably be more successful with your contribution if you visit #sway-devel on Libera Chat upfront and discuss your plans.

Note: rules are made to be broken. Adjust or ignore any/all of these as you see fit, but be prepared to justify it to your peers.

Merge Requests

If you already have your own merge request habits, feel free to use them. If you don't, however, allow me to make a suggestion: feature branches pulled from upstream. Try this:

Fork wlroots (make the fork public to allow the CI to run)
git clone git@gitlab.freedesktop.org:<username>/wlroots.git && cd wlroots
git remote add upstream https://gitlab.freedesktop.org/wlroots/wlroots.git

You only need to do this once. You're never going to use your fork's master branch. Instead, when you start working on a feature, do this:

git fetch upstream
git checkout -b add-so-and-so-feature upstream/master
Add and commit your changes
git push -u origin add-so-and-so-feature
Make a merge request from your feature branch

When you submit your merge request, your commit log should do most of the talking when it comes to describing your changes and their motivation. In addition to this, your merge request's comments will ideally include a test plan that the reviewers can use to (1) demonstrate the problem on master, if applicable and (2) verify that the problem no longer exists with your changes applied (or that your new features work correctly). Document all of the edge cases you're aware of so we can adequately test them - then verify the test plan yourself before submitting.

Commit Log

Unlike many projects using GitHub and GitLab, wlroots 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 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. A linear history eases reviewing, cherry-picking and reverting changes.

If you aren't comfortable with manipulating the Git history, have a look at git-rebase.io.

Commit Messages

Please strive to write good commit messages. Here's some guidelines to follow:

The first line should be limited to 50 characters and should be a sentence that completes the thought [When applied, this commit will...] "Implement cmd_move" or "Improve performance of arrange_windows on ARM" or similar.

The subsequent lines should be separated from the subject line by a single blank line, and include optional details. In this you can give justification for the change, reference issues, or explain some of the subtler details of your patch. This is important because when someone finds a line of code they don't understand later, they can use the git blame command to find out what the author was thinking when they wrote it. It's also easier to review your merge requests if they're separated into logical commits that have good commit messages and justify themselves in the extended commit description.

As a good rule of thumb, anything you might put into the merge request description on GitLab is probably fair game for going into the extended commit message as well.

See How to Write a Git Commit Message for more details.

Code Review

When your changes are submitted for review, one or more core committers will look over them. Smaller changes might be merged with little fanfare, but larger changes will typically see review from several people. Be prepared to receive some feedback - you may be asked to make changes to your work. Our code review process is:

Triage the merge request. Do the commit messages make sense? Is a test plan necessary and/or present? Add anyone as reviewers that you think should be there (using the relevant GitLab feature, if you have the permissions, or with an @mention if necessary).
Review the code. Look for code style violations, naming convention violations, buffer overflows, memory leaks, logic errors, non-portable code (including GNU-isms), etc. For significant changes to the public API, loop in a couple more people for discussion.
Execute the test plan, if present.
Merge the merge request when all reviewers approve.
File follow-up tickets if appropriate.

Code of Conduct

Note that as a project hosted on freedesktop.org, wlroots follows its Code of Conduct, based on the Contributor Covenant. Please conduct yourself in a respectful and civilized manner when communicating with community members on IRC and bug tracker.

Style Reference

wlroots is written in C with a style similar to the kernel style, but with a few notable differences.

Try to keep your code conforming to C11 and POSIX as much as possible, and do not use GNU extensions.

Brackets

Brackets always go on the same line, including in functions. Always include brackets for if/while/for, even if it's a single statement.

void function(void) {
	if (condition1) {
		do_thing1();
	}

	if (condition2) {
		do_thing2();
	} else {
		do_thing3();
	}
}

Indentation

Indentations are a single tab.

For long lines that need to be broken, the continuation line should be indented with an additional tab. If the line being broken is opening a new block (functions, if, while, etc.), the continuation line should be indented with two tabs, so they can't be misread as being part of the block.

really_long_function(argument1, argument2, ...,
	argument3, argument4);

if (condition1 && condition2 && ...
		condition3 && condition4) {
	do_thing();
}

Try to break the line in the place which you think is the most appropriate.

Line Length

Try to keep your lines under 80 columns, but you can go up to 100 if it improves readability. Don't break lines indiscriminately, try to find nice breaking points so your code is easy to read.

Names

Global function and type names should be prefixed with wlr_submodule_ (e.g. struct wlr_output, wlr_output_set_cursor). For static functions and types local to a file, the names chosen aren't as important. Local function names shouldn't have a wlr_ prefix.

For include guards, use the header's filename relative to include. Uppercase all of the characters, and replace any invalid characters with an underscore.

Construction/Destruction Functions

Functions that are responsible for constructing objects should take one of the two following forms:

init: for functions which accept a pointer to a pre-allocated object (e.g. a member of a struct) and initialize it. Such functions must call memset() to zero out the memory before initializing it to avoid leaving unset fields.
create: for functions which allocate the memory for an object, initialize it, and return a pointer. Such functions should allocate the memory with calloc() to avoid leaving unset fields.

Likewise, functions that are responsible for destructing objects should take one of the two following forms:

finish: for functions which accept a pointer to an object and deinitialize it. If a finished object isn't destroyed but kept for future use, it must be reinitialized to be used again.
destroy: for functions which accept a pointer to an object, deinitialize it, and free the memory. Such functions should always be able to accept a NULL pointer.

Error Codes

For functions not returning a value, they should return a (stdbool.h) bool to indicate whether they succeeded or not.

Macros

Try to keep the use of macros to a minimum, especially if a function can do the job. If you do need to use them, try to keep them close to where they're being used and #undef them after.

Documentation

Documentation comments for declarations start with /** and end with */.
Cross-reference other declarations by ending function names with (), and writing struct, union, enum or typedef before types.
Document fields which can be NULL with a // may be NULL comment, optionally with more details describing when this can happen.
Document the bits of a bitfield with a // enum bar comment.
Document the data argument of a struct wl_signal with a // struct foo comment.
Document the contents and container of a struct wl_list with a // content.link and // container.list comment.

Example

struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
	struct wlr_backend *backend;
	if (getenv("WAYLAND_DISPLAY") || getenv("_WAYLAND_DISPLAY")) {
		backend = attempt_wl_backend(display);
		if (backend) {
			return backend;
		}
	}

	const char *x11_display = getenv("DISPLAY");
	if (x11_display) {
		return wlr_x11_backend_create(display, x11_display);
	}

	// Attempt DRM+libinput

	struct wlr_session *session = wlr_session_create(display);
	if (!session) {
		wlr_log(WLR_ERROR, "Failed to start a DRM session");
		return NULL;
	}

	int gpu = wlr_session_find_gpu(session);
	if (gpu == -1) {
		wlr_log(WLR_ERROR, "Failed to open DRM device");
		goto error_session;
	}

	backend = wlr_multi_backend_create(session);
	if (!backend) {
		goto error_gpu;
	}

	struct wlr_backend *libinput = wlr_libinput_backend_create(display, session);
	if (!libinput) {
		goto error_multi;
	}

	struct wlr_backend *drm = wlr_drm_backend_create(display, session, gpu);
	if (!drm) {
		goto error_libinput;
	}

	wlr_multi_backend_add(backend, libinput);
	wlr_multi_backend_add(backend, drm);
	return backend;

error_libinput:
	wlr_backend_destroy(libinput);
error_multi:
	wlr_backend_destroy(backend);
error_gpu:
	wlr_session_close_file(session, gpu);
error_session:
	wlr_session_destroy(session);
	return NULL;
}

Wayland protocol implementation

Each protocol generally lives in a file with the same name, usually containing at least one struct for each interface in the protocol. For instance, xdg_shell lives in types/wlr_xdg_shell.h and has a wlr_xdg_surface struct.

Globals

Global interfaces generally have public constructors and destructors. Their struct has a field holding the wl_global itself, a destroy signal and a wl_display destroy listener. Example:

struct wlr_compositor {
	struct wl_global *global;
	…

	struct wl_listener display_destroy;

	struct {
		struct wl_signal new_surface;
		struct wl_signal destroy;
	} events;
};

When the destructor is called, it should emit the destroy signal, remove the display destroy listener, destroy the wl_global and then destroy the struct. The destructor can assume all clients and resources have been already destroyed.

Resources

Resources are the representation of Wayland objects on the compositor side. They generally have an associated struct, called the object struct, stored in their user_data field.

Object structs can be retrieved from resources via wl_resource_get_data. To prevent bad casts, a safe helper function checking the type of the resource is used:

static const struct wl_surface_interface surface_impl;

struct wlr_surface *wlr_surface_from_resource(struct wl_resource *resource) {
	assert(wl_resource_instance_of(resource, &wl_surface_interface,
		&surface_impl));
	return wl_resource_get_user_data(resource);
}

If a pointer to a wl_resource is stored, a resource destroy handler needs to be registered to clean it up. libwayland will automatically destroy resources in an arbitrary order when a client is disconnected, the compositor must handle this correctly.

Destroying resources

Object structs should only be destroyed when their resource is destroyed, ie. in the resource destroy handler (set with wl_resource_set_implementation).

If the object has a destructor request: the request handler should just call wl_resource_destroy and do nothing else. The compositor must not destroy resources on its own outside the destructor request handler.
If the protocol specifies that an object is destroyed when an event is sent: it's the only case where the compositor is allowed to send the event and then call wl_resource_destroy. An example of this is wl_callback.

Inert resources

Some resources can become inert in situations described in the protocol or when the compositor decides to get rid of them. All requests made to inert resources should be ignored, except the destructor. This is achieved by:

When the resource becomes inert: destroy the object struct and call wl_resource_set_user_data(resource, NULL). Do not destroy the resource.
For each request made to a resource that can be inert: add a NULL check to ignore the request if the resource is inert.
When the client calls the destructor request on the resource: call wl_resource_destroy(resource) as usual.
When the resource is destroyed, if the resource isn't inert, destroy the object struct.

Example:

// Handles the destroy request
static void subsurface_handle_destroy(struct wl_client *client,
		struct wl_resource *resource) {
	wl_resource_destroy(resource);
}

// Handles a regular request
static void subsurface_set_position(struct wl_client *client,
		struct wl_resource *resource, int32_t x, int32_t y) {
	struct wlr_subsurface *subsurface = subsurface_from_resource(resource);
	if (subsurface == NULL) {
		return;
	}

	…
}

// Destroys the wlr_subsurface struct
static void subsurface_destroy(struct wlr_subsurface *subsurface) {
	if (subsurface == NULL) {
		return;
	}

	…

	wl_resource_set_user_data(subsurface->resource, NULL);
	free(subsurface);
}

// Resource destroy listener
static void subsurface_handle_resource_destroy(struct wl_resource *resource) {
	struct wlr_subsurface *subsurface = subsurface_from_resource(resource);
	subsurface_destroy(subsurface);
}

// Makes the resource inert
static void subsurface_handle_surface_destroy(struct wl_listener *listener,
		void *data) {
	struct wlr_subsurface *subsurface =
		wl_container_of(listener, subsurface, surface_destroy);
	subsurface_destroy(subsurface);
}

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