extmod/uasyncio: Fix edge case for cancellation of wait_for.

This fixes the cases where the task being waited on finishes just before or
just after the wait_for itself is cancelled.

Fixes issue #8717.

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

extmod/uasyncio/funcs.py

@@ -1,49 +1,51 @@
 # MicroPython uasyncio module
-# MIT license; Copyright (c) 2019-2020 Damien P. George
+# MIT license; Copyright (c) 2019-2022 Damien P. George
 
 from . import core
 
 
+def _run(waiter, aw):
+    try:
+        result = await aw
+        status = True
+    except BaseException as er:
+        result = None
+        status = er
+    if waiter.data is None:
+        # The waiter is still waiting, cancel it.
+        if waiter.cancel():
+            # Waiter was cancelled by us, change its CancelledError to an instance of
+            # CancelledError that contains the status and result of waiting on aw.
+            # If the wait_for task subsequently gets cancelled externally then this
+            # instance will be reset to a CancelledError instance without arguments.
+            waiter.data = core.CancelledError(status, result)
+
+
 async def wait_for(aw, timeout, sleep=core.sleep):
     aw = core._promote_to_task(aw)
     if timeout is None:
         return await aw
 
-    def runner(waiter, aw):
-        nonlocal status, result
-        try:
-            result = await aw
-            s = True
-        except BaseException as er:
-            s = er
-        if status is None:
-            # The waiter is still waiting, set status for it and cancel it.
-            status = s
-            waiter.cancel()
-
     # Run aw in a separate runner task that manages its exceptions.
-    status = None
-    result = None
-    runner_task = core.create_task(runner(core.cur_task, aw))
+    runner_task = core.create_task(_run(core.cur_task, aw))
 
     try:
         # Wait for the timeout to elapse.
         await sleep(timeout)
     except core.CancelledError as er:
-        if status is True:
-            # aw completed successfully and cancelled the sleep, so return aw's result.
-            return result
-        elif status is None:
+        status = er.value
+        if status is None:
             # This wait_for was cancelled externally, so cancel aw and re-raise.
-            status = True
             runner_task.cancel()
             raise er
+        elif status is True:
+            # aw completed successfully and cancelled the sleep, so return aw's result.
+            return er.args[1]
         else:
             # aw raised an exception, propagate it out to the caller.
             raise status
 
     # The sleep finished before aw, so cancel aw and raise TimeoutError.
-    status = True
     runner_task.cancel()
     await runner_task
     raise core.TimeoutError

tests/extmod/uasyncio_wait_for.py

@@ -111,6 +111,21 @@ async def main():
     await asyncio.sleep(0.01)
     print(sep)
 
+    # When wait_for gets cancelled and the task it's waiting on finishes around the
+    # same time as the cancellation of the wait_for
+    for num_sleep in range(1, 5):
+        t = asyncio.create_task(task_wait_for_cancel(4 + num_sleep, 0, 2))
+        for _ in range(num_sleep):
+            await asyncio.sleep(0)
+        assert not t.done()
+        print("cancel wait_for")
+        t.cancel()
+        try:
+            await t
+        except asyncio.CancelledError as er:
+            print(repr(er))
+        print(sep)
+
     print("finish")
 
 

tests/extmod/uasyncio_wait_for.py.exp

@@ -32,4 +32,31 @@ task_wait_for_cancel_ignore cancelled
 ignore cancel
 task_catch done
 ----------
+task_wait_for_cancel start
+cancel wait_for
+task start 5
+task_wait_for_cancel cancelled
+CancelledError()
+----------
+task_wait_for_cancel start
+task start 6
+cancel wait_for
+task end 6
+task_wait_for_cancel cancelled
+CancelledError()
+----------
+task_wait_for_cancel start
+task start 7
+task end 7
+cancel wait_for
+task_wait_for_cancel cancelled
+CancelledError()
+----------
+task_wait_for_cancel start
+task start 8
+task end 8
+cancel wait_for
+task_wait_for_cancel cancelled
+CancelledError()
+----------
 finish