backend/session: make optional

Some compositors are not interested in wlr_session, for instance
nested compositors.

Disabling wlr_session removes the udev dependency.

backend/backend.c

@@ -9,7 +9,6 @@
 #include <wlr/backend/headless.h>
 #include <wlr/backend/interface.h>
 #include <wlr/backend/multi.h>
-#include <wlr/backend/session.h>
 #include <wlr/backend/wayland.h>
 #include <wlr/config.h>
 #include <wlr/render/wlr_renderer.h>
@@ -20,6 +19,10 @@
 #include "util/env.h"
 #include "util/time.h"
 
+#if WLR_HAS_SESSION
+#include <wlr/backend/session.h>
+#endif
+
 #if WLR_HAS_DRM_BACKEND
 #include <wlr/backend/drm.h>
 #include "backend/drm/monitor.h"
@@ -67,6 +70,7 @@ void wlr_backend_destroy(struct wlr_backend *backend) {
 	}
 }
 
+#if WLR_HAS_SESSION
 static struct wlr_session *session_create_and_wait(struct wl_display *disp) {
 	struct wlr_session *session = wlr_session_create(disp);
 
@@ -106,6 +110,7 @@ static struct wlr_session *session_create_and_wait(struct wl_display *disp) {
 
 	return session;
 }
+#endif
 
 clockid_t wlr_backend_get_presentation_clock(struct wlr_backend *backend) {
 	if (backend->impl->get_presentation_clock) {
@@ -247,7 +252,9 @@ static bool attempt_backend_by_name(struct wl_display *display,
 	} else if (strcmp(name, "drm") == 0 || strcmp(name, "libinput") == 0) {
 		// DRM and libinput need a session
 		if (*session_ptr == NULL) {
+#if WLR_HAS_SESSION
 			*session_ptr = session_create_and_wait(display);
+#endif
 			if (*session_ptr == NULL) {
 				wlr_log(WLR_ERROR, "failed to start a session");
 				return false;
@@ -343,7 +350,9 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 #endif
 
 	// Attempt DRM+libinput
+#if WLR_HAS_SESSION
 	session = session_create_and_wait(display);
+#endif
 	if (!session) {
 		wlr_log(WLR_ERROR, "Failed to start a DRM session");
 		goto error;
@@ -388,6 +397,8 @@ success:
 
 error:
 	wlr_backend_destroy(multi);
+#if WLR_HAS_SESSION
 	wlr_session_destroy(session);
+#endif
 	return NULL;
 }

backend/drm/meson.build

@@ -1,3 +1,7 @@
+if not features['session']
+	subdir_done()
+endif
+
 hwdata = dependency('hwdata', required: false, native: true)
 if hwdata.found()
 	hwdata_dir = hwdata.get_variable(pkgconfig: 'pkgdatadir')

backend/libinput/meson.build

@@ -10,7 +10,7 @@ libinput = dependency(
 	not_found_message: '\n'.join(msg),
 )
 
-if not libinput.found()
+if not (libinput.found() and features['session'])
 	subdir_done()
 endif
 

backend/meson.build

@@ -1,6 +1,3 @@
-udev = dependency('libudev')
-wlr_deps += udev
-
 wlr_files += files('backend.c')
 
 all_backends = ['drm', 'libinput', 'x11']
@@ -11,6 +8,16 @@ elif 'auto' in backends and get_option('auto_features').disabled()
 	backends = []
 endif
 
+session_required = 'drm' in backends or 'libinput' in backends or get_option('session').enabled()
+if get_option('session').disabled()
+	if session_required
+		error('Session support is required for the DRM or libinput backends')
+	endif
+	session_required = disabler()
+endif
+
+subdir('session')
+
 foreach backend : all_backends
 	if backend in backends or 'auto' in backends
 		subdir(backend)
@@ -20,5 +27,3 @@ endforeach
 subdir('multi')
 subdir('wayland')
 subdir('headless')
-
-subdir('session')

backend/session/meson.build

@@ -1,8 +1,17 @@
+msg = 'Required for session support.'
+udev = dependency('libudev', required: session_required, not_found_message: msg)
 libseat = dependency(
 	'libseat',
 	version: '>=0.2.0',
 	fallback: 'seatd',
 	default_options: ['server=disabled', 'man-pages=disabled', 'examples=disabled'],
+	required: session_required,
+	not_found_message: msg,
 )
+if not (udev.found() and libseat.found())
+	subdir_done()
+endif
+
 wlr_files += files('session.c')
-wlr_deps += libseat
+wlr_deps += [udev, libseat]
+features += { 'session': true }