py/scheduler: Add comment warning about C callbacks scheduling new tasks.

Signed-off-by: Andrew Leech <andrew.leech@planetinnovation.com.au>

Author: pi-anl

py/scheduler.c

@@ -88,7 +88,9 @@ static inline void mp_sched_run_pending(void) {
 
     #if MICROPY_SCHEDULER_STATIC_NODES
     // Run all pending C callbacks.
-    while (MP_STATE_VM(sched_head) != NULL) {
+    // Warning: if C callbacks themserlves schedule new C callbacks this
+    // loop will never exit.
+    while (MP_STATE_VM(sched_head) != NULL && !pass_finished) {
         mp_sched_node_t *node = MP_STATE_VM(sched_head);
         MP_STATE_VM(sched_head) = node->next;
         if (MP_STATE_VM(sched_head) == NULL) {