Introduce noop backend

The noop backend is similar to headless, but it doesn't contain a
renderer. It can be used as a place to stash views for when there's no
physical outputs connected.

backend/backend.c

@@ -11,6 +11,7 @@
 #include <wlr/backend/interface.h>
 #include <wlr/backend/libinput.h>
 #include <wlr/backend/multi.h>
+#include <wlr/backend/noop.h>
 #include <wlr/backend/session.h>
 #include <wlr/backend/wayland.h>
 #include <wlr/config.h>
@@ -133,6 +134,20 @@ static struct wlr_backend *attempt_headless_backend(
 	return backend;
 }
 
+static struct wlr_backend *attempt_noop_backend(struct wl_display *display) {
+	struct wlr_backend *backend = wlr_noop_backend_create(display);
+	if (backend == NULL) {
+		return NULL;
+	}
+
+	size_t outputs = parse_outputs_env("WLR_NOOP_OUTPUTS");
+	for (size_t i = 0; i < outputs; ++i) {
+		wlr_noop_add_output(backend);
+	}
+
+	return backend;
+}
+
 static struct wlr_backend *attempt_drm_backend(struct wl_display *display,
 		struct wlr_backend *backend, struct wlr_session *session,
 		wlr_renderer_create_func_t create_renderer_func) {
@@ -170,6 +185,8 @@ static struct wlr_backend *attempt_backend_by_name(struct wl_display *display,
 #endif
 	} else if (strcmp(name, "headless") == 0) {
 		return attempt_headless_backend(display, create_renderer_func);
+	} else if (strcmp(name, "noop") == 0) {
+		return attempt_noop_backend(display);
 	} else if (strcmp(name, "drm") == 0 || strcmp(name, "libinput") == 0) {
 		// DRM and libinput need a session
 		if (!*session) {

backend/meson.build

@@ -20,6 +20,8 @@ backend_files = files(
 	'libinput/tablet_tool.c',
 	'libinput/touch.c',
 	'multi/backend.c',
+	'noop/backend.c',
+	'noop/output.c',
 	'session/direct-ipc.c',
 	'session/session.c',
 	'wayland/backend.c',

backend/noop/backend.c

@@ -0,0 +1,68 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <wlr/interfaces/wlr_output.h>
+#include <wlr/util/log.h>
+#include "backend/noop.h"
+#include "util/signal.h"
+
+struct wlr_noop_backend *noop_backend_from_backend(
+		struct wlr_backend *wlr_backend) {
+	assert(wlr_backend_is_noop(wlr_backend));
+	return (struct wlr_noop_backend *)wlr_backend;
+}
+
+static bool backend_start(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+	wlr_log(WLR_INFO, "Starting noop backend");
+
+	struct wlr_noop_output *output;
+	wl_list_for_each(output, &backend->outputs, link) {
+		wlr_output_update_enabled(&output->wlr_output, true);
+		wlr_signal_emit_safe(&backend->backend.events.new_output,
+			&output->wlr_output);
+	}
+
+	backend->started = true;
+	return true;
+}
+
+static void backend_destroy(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+	if (!wlr_backend) {
+		return;
+	}
+
+	struct wlr_noop_output *output, *output_tmp;
+	wl_list_for_each_safe(output, output_tmp, &backend->outputs, link) {
+		wlr_output_destroy(&output->wlr_output);
+	}
+
+	wlr_signal_emit_safe(&wlr_backend->events.destroy, backend);
+
+	free(backend);
+}
+
+static const struct wlr_backend_impl backend_impl = {
+	.start = backend_start,
+	.destroy = backend_destroy,
+};
+
+struct wlr_backend *wlr_noop_backend_create(struct wl_display *display) {
+	wlr_log(WLR_INFO, "Creating noop backend");
+
+	struct wlr_noop_backend *backend =
+		calloc(1, sizeof(struct wlr_noop_backend));
+	if (!backend) {
+		wlr_log(WLR_ERROR, "Failed to allocate wlr_noop_backend");
+		return NULL;
+	}
+	wlr_backend_init(&backend->backend, &backend_impl);
+	backend->display = display;
+	wl_list_init(&backend->outputs);
+
+	return &backend->backend;
+}
+
+bool wlr_backend_is_noop(struct wlr_backend *backend) {
+	return backend->impl == &backend_impl;
+}

backend/noop/output.c

@@ -0,0 +1,75 @@
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <wlr/interfaces/wlr_output.h>
+#include <wlr/util/log.h>
+#include "backend/noop.h"
+#include "util/signal.h"
+
+static struct wlr_noop_output *noop_output_from_output(
+		struct wlr_output *wlr_output) {
+	assert(wlr_output_is_noop(wlr_output));
+	return (struct wlr_noop_output *)wlr_output;
+}
+
+static void output_transform(struct wlr_output *wlr_output,
+		enum wl_output_transform transform) {
+	// empty
+}
+
+static bool output_make_current(struct wlr_output *wlr_output, int *buffer_age) {
+	return true;
+}
+
+static bool output_swap_buffers(struct wlr_output *wlr_output,
+		pixman_region32_t *damage) {
+	return true;
+}
+
+static void output_destroy(struct wlr_output *wlr_output) {
+	struct wlr_noop_output *output =
+		noop_output_from_output(wlr_output);
+
+	wl_list_remove(&output->link);
+
+	free(output);
+}
+
+static const struct wlr_output_impl output_impl = {
+	.transform = output_transform,
+	.destroy = output_destroy,
+	.make_current = output_make_current,
+	.swap_buffers = output_swap_buffers,
+};
+
+bool wlr_output_is_noop(struct wlr_output *wlr_output) {
+	return wlr_output->impl == &output_impl;
+}
+
+struct wlr_output *wlr_noop_add_output(struct wlr_backend *wlr_backend) {
+	struct wlr_noop_backend *backend = noop_backend_from_backend(wlr_backend);
+
+	struct wlr_noop_output *output = calloc(1, sizeof(struct wlr_noop_output));
+	if (output == NULL) {
+		wlr_log(WLR_ERROR, "Failed to allocate wlr_noop_output");
+		return NULL;
+	}
+	output->backend = backend;
+	wlr_output_init(&output->wlr_output, &backend->backend, &output_impl,
+			backend->display);
+	struct wlr_output *wlr_output = &output->wlr_output;
+
+	strncpy(wlr_output->make, "noop", sizeof(wlr_output->make));
+	strncpy(wlr_output->model, "noop", sizeof(wlr_output->model));
+	snprintf(wlr_output->name, sizeof(wlr_output->name), "NOOP-%d",
+		wl_list_length(&backend->outputs) + 1);
+
+	wl_list_insert(&backend->outputs, &output->link);
+
+	if (backend->started) {
+		wlr_output_update_enabled(wlr_output, true);
+		wlr_signal_emit_safe(&backend->backend.events.new_output, wlr_output);
+	}
+
+	return wlr_output;
+}

docs/env_vars.md

@@ -11,7 +11,7 @@ wlroots specific
   control instead of the atomic interface
 * *WLR_LIBINPUT_NO_DEVICES*: set to 1 to not fail without any input devices
 * *WLR_BACKENDS*: comma-separated list of backends to use (available backends:
-  wayland, x11, headless)
+  wayland, x11, headless, noop)
 * *WLR_WL_OUTPUTS*: when using the wayland backend specifies the number of outputs
 * *WLR_X11_OUTPUTS*: when using the X11 backend specifies the number of outputs
 * *WLR_HEADLESS_OUTPUTS*: when using the headless backend specifies the number

include/backend/noop.h

@@ -0,0 +1,24 @@
+#ifndef BACKEND_NOOP_H
+#define BACKEND_NOOP_H
+
+#include <wlr/backend/noop.h>
+#include <wlr/backend/interface.h>
+
+struct wlr_noop_backend {
+	struct wlr_backend backend;
+	struct wl_display *display;
+	struct wl_list outputs;
+	bool started;
+};
+
+struct wlr_noop_output {
+	struct wlr_output wlr_output;
+
+	struct wlr_noop_backend *backend;
+	struct wl_list link;
+};
+
+struct wlr_noop_backend *noop_backend_from_backend(
+	struct wlr_backend *wlr_backend);
+
+#endif

include/wlr/backend/noop.h

@@ -0,0 +1,31 @@
+/*
+ * This an unstable interface of wlroots. No guarantees are made regarding the
+ * future consistency of this API.
+ */
+#ifndef WLR_USE_UNSTABLE
+#error "Add -DWLR_USE_UNSTABLE to enable unstable wlroots features"
+#endif
+
+#ifndef WLR_BACKEND_NOOP_H
+#define WLR_BACKEND_NOOP_H
+
+#include <wlr/backend.h>
+#include <wlr/types/wlr_output.h>
+
+/**
+ * Creates a noop backend. Noop backends do not have a framebuffer and are not
+ * capable of rendering anything. They are useful for when there's no real
+ * outputs connected; you can stash your views on a noop output until an output
+ * is connected.
+ */
+struct wlr_backend *wlr_noop_backend_create(struct wl_display *display);
+
+/**
+ * Create a new noop output.
+ */
+struct wlr_output *wlr_noop_add_output(struct wlr_backend *backend);
+
+bool wlr_backend_is_noop(struct wlr_backend *backend);
+bool wlr_output_is_noop(struct wlr_output *output);
+
+#endif