backend/session: add a timeout waiting for GPUs

If a GPU doesn't show up in 10s, bail out and return zero GPUs.

backend/session/session.c

@@ -7,6 +7,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
+#include <time.h>
 #include <wayland-server-core.h>
 #include <wlr/backend/session.h>
 #include <wlr/backend/session/interface.h>
@@ -17,6 +18,8 @@
 #include "backend/session/session.h"
 #include "util/signal.h"
 
+#define WAIT_GPU_TIMEOUT 10000 // ms
+
 extern const struct session_impl session_libseat;
 extern const struct session_impl session_logind;
 extern const struct session_impl session_direct;
@@ -322,6 +325,12 @@ static struct udev_enumerate *enumerate_drm_cards(struct udev *udev) {
 	return en;
 }
 
+static uint64_t get_current_time_ms(void) {
+	struct timespec ts = {0};
+	clock_gettime(CLOCK_MONOTONIC, &ts);
+	return (uint64_t)ts.tv_sec * 1000 + (uint64_t)ts.tv_nsec / 1000000;
+}
+
 struct find_gpus_add_handler {
 	bool added;
 	struct wl_listener listener;
@@ -356,16 +365,24 @@ size_t wlr_session_find_gpus(struct wlr_session *session,
 		handler.listener.notify = find_gpus_handle_add;
 		wl_signal_add(&session->events.add_drm_card, &handler.listener);
 
+		uint64_t started_at = get_current_time_ms();
+		uint64_t timeout = WAIT_GPU_TIMEOUT;
 		struct wl_event_loop *event_loop =
 			wl_display_get_event_loop(session->display);
 		while (!handler.added) {
-			int ret = wl_event_loop_dispatch(event_loop, -1);
+			int ret = wl_event_loop_dispatch(event_loop, (int)timeout);
 			if (ret < 0) {
 				wlr_log_errno(WLR_ERROR, "Failed to wait for DRM card device: "
 					"wl_event_loop_dispatch failed");
 				udev_enumerate_unref(en);
 				return -1;
 			}
+
+			uint64_t now = get_current_time_ms();
+			if (now >= started_at + WAIT_GPU_TIMEOUT) {
+				break;
+			}
+			timeout = started_at + WAIT_GPU_TIMEOUT - now;
 		}
 
 		wl_list_remove(&handler.listener.link);