Introduce, use a 'never' object

This can be used in cases where `await SOMETHING` is required,
but the task cannot be put back on the queue like `await sleep(0)` would
do.

This passes the test and the docs build (locally)

Author: jepler

asyncio/core.py

@@ -69,7 +69,6 @@ def __next__(self):
             self.exc.__traceback__ = None
             raise self.exc
 
-
 # Pause task execution for the given time (integer in milliseconds, uPy extension)
 # Use a SingletonGenerator to do it without allocating on the heap
 def sleep_ms(t, sgen=SingletonGenerator()):
@@ -93,6 +92,34 @@ def sleep(t):
     return sleep_ms(int(t * 1000))
 
 
+################################################################################
+# "Never schedule" object"
+# Don't re-schedule the object that awaits the _never singleton.
+# For internal use only. Some constructs, like `await event.wait()`,
+# work by NOT re-scheduling the task which calls wait(), but by
+# having some other task schedule it later.
+class _Never:
+    def __init__(self):
+        self.state = None
+        self.exc = StopIteration()
+
+    def __iter__(self):
+        return self
+
+    def __await__(self):
+        return self
+
+    def __next__(self):
+        if self.state is not None:
+            self.state = None
+            return None
+        else:
+           self.exc.__traceback__ = None
+           raise self.exc
+
+_never = _Never()
+
+
 ################################################################################
 # Queue and poller for stream IO
 

asyncio/event.py

@@ -62,7 +62,8 @@ async def wait(self):
             self.waiting.push_head(core.cur_task)
             # Set calling task's data to the event's queue so it can be removed if needed
             core.cur_task.data = self.waiting
-            yield
+            core._never.state = False
+            await core._never
         return True
 
 

asyncio/lock.py

@@ -69,7 +69,8 @@ async def acquire(self):
             # Set calling task's data to the lock's queue so it can be removed if needed
             core.cur_task.data = self.waiting
             try:
-                yield
+                core._never.state = False
+                await core._never
             except core.CancelledError as er:
                 if self.state == core.cur_task:
                     # Cancelled while pending on resume, schedule next waiting Task