backend/session: make udev dependency optional

udev is now optional at build-time but will fail at runtime because
we don't have an alternative implementation yet.

backend/libinput/meson.build

@@ -1,16 +1,21 @@
 msg = ['Required for libinput backend support.']
 if 'libinput' in backends
-	msg += 'Install "libinput" or disable the libinput backend.'
+	msg += 'Install "@0@" or disable the libinput backend.'
 endif
 
 libinput = dependency(
 	'libinput',
 	version: '>=1.19.0',
 	required: 'libinput' in backends,
-	not_found_message: '\n'.join(msg),
+	not_found_message: '\n'.join(msg).format('libinput'),
+)
+udev = dependency(
+	'libudev',
+	required: 'libinput' in backends,
+	not_found_message: '\n'.join(msg).format('libudev'),
 )
 
-if not (libinput.found() and features['session'])
+if not (libinput.found() and udev.found() and features['session'])
 	subdir_done()
 endif
 

backend/session/meson.build

@@ -1,17 +1,22 @@
-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,
+	not_found_message: 'Required for session support.',
 )
-if not (udev.found() and libseat.found())
+if not libseat.found()
 	subdir_done()
 endif
 
-wlr_files += files('session.c', 'udev.c')
+wlr_files += files('session.c')
-wlr_deps += [udev, libseat]
+wlr_deps += [libseat]
 features += { 'session': true }
+
+udev = dependency('libudev', required: get_option('udev'))
+if udev.found()
+	wlr_files += files('udev.c')
+	wlr_deps += [udev]
+	internal_features += { 'udev': true }
+endif

backend/session/session.c

@@ -15,9 +15,13 @@
 #include <xf86drm.h>
 #include <xf86drmMode.h>
 #include "backend/session/session.h"
-#include "backend/session/udev.h"
+#include "config.h"
 #include "util/time.h"
 
+#if HAVE_UDEV
+#include "backend/session/udev.h"
+#endif
+
 #define WAIT_GPU_TIMEOUT 10000 // ms
 
 struct wlr_device *session_find_device_by_devid(struct wlr_session *session, dev_t devid) {
@@ -188,11 +192,16 @@ struct wlr_session *wlr_session_create(struct wl_event_loop *event_loop) {
 		goto error_open;
 	}
 
+#if HAVE_UDEV
 	session->device_manager = wlr_udev_device_manager_create(session);
 	if (session->device_manager == NULL) {
 		wlr_log(WLR_ERROR, "Failed to create udev device manager");
 		goto error_session;
 	}
+#else
+	wlr_log(WLR_ERROR, "Session requires udev");
+	goto error_session;
+#endif
 
 	session->event_loop_destroy.notify = handle_event_loop_destroy;
 	wl_event_loop_add_destroy_listener(event_loop, &session->event_loop_destroy);

meson.build

@@ -82,6 +82,7 @@ internal_features = {
 	'xcb-errors': false,
 	'egl': false,
 	'libliftoff': false,
+	'udev': false,
 }
 internal_config = configuration_data()
 

meson.options

@@ -7,6 +7,7 @@ option('backends', type: 'array', choices: ['auto', 'drm', 'libinput', 'x11'], v
 option('allocators', type: 'array', choices: ['auto', 'gbm', 'udmabuf'], value: ['auto'],
 	description: 'Select built-in allocators')
 option('session', type: 'feature', value: 'auto', description: 'Enable session support')
+option('udev', type: 'feature', value: 'auto', description: 'Use udev for device management')
 option('tests', type: 'boolean', value: true, description: 'Build tests and benchmarks')
 option('color-management', type: 'feature', value: 'auto', description: 'Enable support for color management')
 option('libliftoff', type: 'feature', value: 'auto', description: 'Enable support for libliftoff')