backend/session: add noop session

This is the first step towards being able to run via DRM leasing and on render
nodes.

Test with:

    export WLR_BACKENDS=drm
    export WLR_SESSION=noop
    export WLR_DRM_DEVICES=/dev/dri/renderD128

backend/meson.build

@@ -23,6 +23,7 @@ backend_files = files(
 	'noop/backend.c',
 	'noop/output.c',
 	'session/direct-ipc.c',
+	'session/noop.c',
 	'session/session.c',
 	'wayland/backend.c',
 	'wayland/output.c',

backend/session/noop.c

@@ -0,0 +1,48 @@
+#define _POSIX_C_SOURCE 200809L
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <wayland-server.h>
+#include <wlr/backend/session/interface.h>
+#include <wlr/util/log.h>
+#include "util/signal.h"
+
+const struct session_impl session_noop;
+
+static int noop_session_open(struct wlr_session *base, const char *path) {
+	return open(path, O_RDWR | O_CLOEXEC);
+}
+
+static void noop_session_close(struct wlr_session *base, int fd) {
+	close(fd);
+}
+
+static bool noop_change_vt(struct wlr_session *base, unsigned vt) {
+	return false;
+}
+
+static void noop_session_destroy(struct wlr_session *base) {
+	free(base);
+}
+
+static struct wlr_session *noop_session_create(struct wl_display *disp) {
+	struct wlr_session *session = calloc(1, sizeof(*session));
+	if (!session) {
+		wlr_log_errno(WLR_ERROR, "Allocation failed");
+		return NULL;
+	}
+
+	session->impl = &session_noop;
+
+	wlr_log(WLR_INFO, "Successfully initialized noop session");
+	return session;
+}
+
+const struct session_impl session_noop = {
+	.create = noop_session_create,
+	.destroy = noop_session_destroy,
+	.open = noop_session_open,
+	.close = noop_session_close,
+	.change_vt = noop_change_vt,
+};

backend/session/session.c

@@ -17,6 +17,7 @@
 
 extern const struct session_impl session_logind;
 extern const struct session_impl session_direct;
+extern const struct session_impl session_noop;
 
 static const struct session_impl *impls[] = {
 #if WLR_HAS_SYSTEMD || WLR_HAS_ELOGIND
@@ -69,16 +70,20 @@ struct wlr_session *wlr_session_create(struct wl_display *disp) {
 
 	const char *env_wlr_session = getenv("WLR_SESSION");
 	if (env_wlr_session) {
-		if (!strcmp(env_wlr_session, "logind") || !strcmp(env_wlr_session, "systemd")) {
+		if (strcmp(env_wlr_session, "logind") == 0 ||
+				strcmp(env_wlr_session, "systemd") == 0) {
 #if WLR_HAS_SYSTEMD || WLR_HAS_ELOGIND
 			session = session_logind.create(disp);
 #else
 			wlr_log(WLR_ERROR, "wlroots is not compiled with logind support");
 #endif
-		} else if (!strcmp(env_wlr_session, "direct")) {
+		} else if (strcmp(env_wlr_session, "direct") == 0) {
 			session = session_direct.create(disp);
+		} else if (strcmp(env_wlr_session, "noop") == 0) {
+			session = session_noop.create(disp);
 		} else {
-			wlr_log(WLR_ERROR, "WLR_SESSION has an invalid value: %s", env_wlr_session);
+			wlr_log(WLR_ERROR, "Unsupported WLR_SESSION: %s",
+				env_wlr_session);
 		}
 	} else {
 		const struct session_impl **iter;
@@ -220,24 +225,23 @@ bool wlr_session_change_vt(struct wlr_session *session, unsigned vt) {
 /* Tests if 'path' is KMS compatible by trying to open it.
  * It leaves the open device in *fd_out it it succeeds.
  */
-static int open_if_kms(struct wlr_session *restrict session, const char *restrict path) {
-	int fd;
-
+static int open_if_kms(struct wlr_session *restrict session,
+		const char *restrict path) {
 	if (!path) {
 		return -1;
 	}
 
-	fd = wlr_session_open_file(session, path);
+	int fd = wlr_session_open_file(session, path);
 	if (fd < 0) {
 		return -1;
 	}
 
-	drmModeRes *res = drmModeGetResources(fd);
-	if (!res) {
+	drmVersion *ver = drmGetVersion(fd);
+	if (!ver) {
 		goto out_fd;
 	}
 
-	drmModeFreeResources(res);
+	drmFreeVersion(ver);
 	return fd;
 
 out_fd: