Add no-op session

This is useful for headless testing, for instance with VKMS:

    modprobe vkms
    export WLR_DRM_DEVICES=/dev/dri/card1
    export WLR_BACKENDS=drm
    export LIBSEAT_BACKEND=noop
    sway

We don't need any of the VT handling in this case.

libseat/backend/noop.c

@@ -0,0 +1,135 @@
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#include "backend.h"
+#include "log.h"
+
+struct backend_noop {
+	struct libseat base;
+	struct libseat_seat_listener *seat_listener;
+	void *seat_listener_data;
+
+	bool initial_setup;
+	int sockets[2];
+};
+
+extern const struct seat_impl noop_impl;
+
+static struct backend_noop *backend_noop_from_libseat_backend(struct libseat *base) {
+	assert(base->impl == &noop_impl);
+	return (struct backend_noop *)base;
+}
+
+static void destroy(struct backend_noop *backend) {
+	close(backend->sockets[0]);
+	close(backend->sockets[1]);
+	free(backend);
+}
+
+static int close_seat(struct libseat *base) {
+	struct backend_noop *backend = backend_noop_from_libseat_backend(base);
+	destroy(backend);
+	return 0;
+}
+
+static int disable_seat(struct libseat *base) {
+	(void)base;
+	return 0;
+}
+
+static const char *seat_name(struct libseat *base) {
+	(void)base;
+	return "noop";
+}
+
+static int open_device(struct libseat *base, const char *path, int *fd) {
+	(void)base;
+
+	int tmpfd = open(path, O_RDWR | O_CLOEXEC);
+	if (tmpfd < 0) {
+		log_errorf("Failed to open device: %s", strerror(errno));
+		return -1;
+	}
+
+	*fd = tmpfd;
+	return tmpfd;
+}
+
+static int close_device(struct libseat *base, int device_id) {
+	(void)base;
+	(void)device_id;
+	return 0;
+}
+
+static int switch_session(struct libseat *base, int s) {
+	(void)base;
+	(void)s;
+	log_errorf("No-op backend cannot switch to session %d", s);
+	return -1;
+}
+
+static int get_fd(struct libseat *base) {
+	struct backend_noop *backend = backend_noop_from_libseat_backend(base);
+	return backend->sockets[0];
+}
+
+static int dispatch_background(struct libseat *base, int timeout) {
+	struct backend_noop *backend = backend_noop_from_libseat_backend(base);
+
+	if (backend->initial_setup) {
+		backend->initial_setup = false;
+		backend->seat_listener->enable_seat(&backend->base, backend->seat_listener_data);
+	}
+
+	struct pollfd fd = {
+		.fd = backend->sockets[0],
+		.events = POLLIN,
+	};
+	if (poll(&fd, 1, timeout) < 0) {
+		if (errno == EAGAIN || errno == EINTR) {
+			return 0;
+		} else {
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
+static struct libseat *noop_open_seat(struct libseat_seat_listener *listener, void *data) {
+	struct backend_noop *backend = calloc(1, sizeof(struct backend_noop));
+	if (backend == NULL) {
+		return NULL;
+	}
+
+	if (socketpair(AF_UNIX, SOCK_STREAM, 0, backend->sockets) != 0) {
+		log_errorf("socketpair() failed: %s", strerror(errno));
+		free(backend);
+		return NULL;
+	}
+
+	backend->seat_listener = listener;
+	backend->seat_listener_data = data;
+	backend->base.impl = &noop_impl;
+
+	return &backend->base;
+}
+
+const struct seat_impl noop_impl = {
+	.open_seat = noop_open_seat,
+	.disable_seat = disable_seat,
+	.close_seat = close_seat,
+	.seat_name = seat_name,
+	.open_device = open_device,
+	.close_device = close_device,
+	.switch_session = switch_session,
+	.get_fd = get_fd,
+	.dispatch = dispatch_background,
+};