wlroots/CONTRIBUTING.md

# Contributing to wlroots

Contributing just involves sending a pull request. You will probably be more
successful with your contribution if you visit
[#sway-devel](https://webchat.freenode.net/?channels=sway-devel) on
irc.freenode.net 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.

## Pull Requests

If you already have your own pull 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:

1. Fork wlroots
2. `git clone https://github.com/username/wlroots && cd wlroots`
3. `git remote add upstream https://github.com/swaywm/wlroots`

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:

1. `git fetch upstream`
2. `git checkout -b add-so-and-so-feature upstream/master`
3. Add and commit your changes
4. `git push -u origin add-so-and-so-feature`
5. Make a pull request from your feature branch

When you submit your pull 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 pull 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 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 *"Fix #742"* 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 Github
issues](https://help.github.com/articles/closing-issues-via-commit-messages/),
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 pull 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 pull request
description on Github is probably fair game for going into the extended commit
message as well.

See [here](https://chris.beams.io/posts/git-commit/) 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:

1. **Triage** the pull 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 GitHub feature, if you have the permissions, or
   with an @mention if necessary).
2. **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.
3. **Execute** the test plan, if present.
4. **Merge** the pull request when all reviewers approve.
5. **File** follow-up tickets if appropriate.

## Style Reference

wlroots is written in C with a style similar to the [kernel
style](https://www.kernel.org/doc/Documentation/process/coding-style.rst), 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.
```c
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.
```c
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

For functions that are responsible for constructing and destructing an object,
they should be written as a pair of one of two forms:
* `init`/`finish`: These initialize/deinitialize a type, but are **NOT**
responsible for allocating it. They should accept a pointer to some
pre-allocated memory (e.g. a member of a struct).
* `create`/`destroy`: These also initialize/deinitialize, but will return a
pointer to a `malloc`ed chunk of memory, and will `free` it in `destroy`.

A destruction function should always be able to accept a NULL pointer or a
zeroed value and exit cleanly; this simplifies error handling a lot.

### Error Codes

For functions not returning a value, they should return a (stdbool.h) bool to
indicated if 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.

### Example

```c
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:

```c
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:

```c
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`).
Destructor requests should only call `wl_resource_destroy`.

The compositor should not destroy resources on its own.

### 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:

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

Example:

```c
// 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);
}
```