lib/utils: Lock the scheduler when executing hard callback functions.

Otherwise scheduled functions may execute during the hard callback and then
fail if they try to allocate heap memory.

lib/utils/mpirq.c

@@ -62,8 +62,10 @@ mp_irq_obj_t *mp_irq_new(const mp_irq_methods_t *methods, mp_obj_t parent) {
 void mp_irq_handler(mp_irq_obj_t *self) {
     if (self->handler != mp_const_none) {
         if (self->ishard) {
-            // When executing code within a handler we must lock the GC to prevent
-            // any memory allocations.
+            // When executing code within a handler we must lock the scheduler to
+            // prevent any scheduled callbacks from running, and lock the GC to
+            // prevent any memory allocations.
+            mp_sched_lock();
             gc_lock();
             nlr_buf_t nlr;
             if (nlr_push(&nlr) == 0) {
@@ -77,6 +79,7 @@ void mp_irq_handler(mp_irq_obj_t *self) {
                 mp_obj_print_exception(&mp_plat_print, MP_OBJ_FROM_PTR(nlr.ret_val));
             }
             gc_unlock();
+            mp_sched_unlock();
         } else {
             // Schedule call to user function
             mp_sched_schedule(self->handler, self->parent);