pbio/light/animation: Fix lockup on restarting animation during animation frame.

Fixes pybricks/support#1295

Author: laurensvalk

CHANGELOG.md

@@ -24,7 +24,10 @@
   loops ([support#1668]).
 - Fixed program restarting if the stop button was held to end the program
   without an exception ([support#1863]).
+- Fixed program lockup when restarting a hub light or light matrix animation
+  at exact multiples of its animation interval ([support#1295]).
 
+[support#1295]: https://github.com/pybricks/support/issues/1295
 [support#1623]: https://github.com/pybricks/support/issues/1623
 [support#1661]: https://github.com/pybricks/support/issues/1661
 [support#1668]: https://github.com/pybricks/support/issues/1668

lib/pbio/src/light/animation.c

@@ -50,10 +50,10 @@ void pbio_light_animation_start(pbio_light_animation_t *animation) {
     pbio_light_animation_list_head = animation;
 
     process_start(&pbio_light_animation_process);
-    // HACK: init timer since we don't call etimer_set()
-    timer_set(&animation->timer.timer, 0);
-    // fake a timer event to load the first cell and start the timer
-    process_post_synch(&pbio_light_animation_process, PROCESS_EVENT_TIMER, &animation->timer);
+    // Fake a timer event to load the first cell.
+    PROCESS_CONTEXT_BEGIN(&pbio_light_animation_process);
+    etimer_set(&animation->timer, 0);
+    PROCESS_CONTEXT_END(&pbio_light_animation_process);
 
     assert(animation->next_animation != PBIO_LIGHT_ANIMATION_STOPPED);
 }
@@ -117,10 +117,15 @@ PROCESS_THREAD(pbio_light_animation_process, ev, data) {
         PROCESS_WAIT_EVENT_UNTIL(ev == PROCESS_EVENT_TIMER);
         struct etimer *timer = data;
         pbio_light_animation_t *animation = PBIO_CONTAINER_OF(timer, pbio_light_animation_t, timer);
-        clock_time_t interval = animation->next(animation);
-        if (pbio_light_animation_is_started(animation)) {
-            etimer_reset_with_new_interval(&animation->timer, interval);
+
+        // Like all etimer events, only handle expired etimers. Need this check
+        // in case the timer was stopped or reset before this event was handled.
+        if (!etimer_expired(&animation->timer) || !pbio_light_animation_is_started(animation)) {
+            continue;
         }
+
+        clock_time_t interval = animation->next(animation);
+        etimer_reset_with_new_interval(&animation->timer, interval);
     }
 
     PROCESS_END();