extmod/uasyncio: Allow task state to be a callable.

This implements a form of CPython's "add_done_callback()", but at this
stage it is a hidden feature and only intended to be used internally.

Signed-off-by: Damien George <damien@micropython.org>

extmod/moduasyncio.c

@@ -265,6 +265,9 @@ STATIC mp_obj_t task_getiter(mp_obj_t self_in, mp_obj_iter_buf_t *iter_buf) {
     } else if (self->state == TASK_STATE_RUNNING_NOT_WAITED_ON) {
         // Allocate the waiting queue.
         self->state = task_queue_make_new(&task_queue_type, 0, 0, NULL);
+    } else if (mp_obj_get_type(self->state) != &task_queue_type) {
+        // Task has state used for another purpose, so can't also wait on it.
+        mp_raise_msg(&mp_type_RuntimeError, MP_ERROR_TEXT("can't wait"));
     }
     return self_in;
 }

extmod/uasyncio/core.py

@@ -195,6 +195,11 @@ def run_until_complete(main_task=None):
                 if t.state is True:
                     # "None" indicates that the task is complete and not await'ed on (yet).
                     t.state = None
+                elif callable(t.state):
+                    # The task has a callback registered to be called on completion.
+                    t.state(t, er)
+                    t.state = False
+                    waiting = True
                 else:
                     # Schedule any other tasks waiting on the completion of this task.
                     while t.state.peek():

extmod/uasyncio/task.py

@@ -123,7 +123,7 @@ class Task:
     def __init__(self, coro, globals=None):
         self.coro = coro  # Coroutine of this Task
         self.data = None  # General data for queue it is waiting on
-        self.state = True  # None, False, True or a TaskQueue instance
+        self.state = True  # None, False, True, a callable, or a TaskQueue instance
         self.ph_key = 0  # Pairing heap
         self.ph_child = None  # Paring heap
         self.ph_child_last = None  # Paring heap
@@ -137,6 +137,9 @@ class Task:
         elif self.state is True:
             # Allocated head of linked list of Tasks waiting on completion of this task.
             self.state = TaskQueue()
+        elif type(self.state) is not TaskQueue:
+            # Task has state used for another purpose, so can't also wait on it.
+            raise RuntimeError("can't wait")
         return self
 
     def __next__(self):