extmod/asyncio: Fix early exit of asyncio scheduler.

This commit fixes three open issues related to the asyncio scheduler
exiting prematurely when the main task queue is empty, in cases where
CPython would not exit (for example, because the main task is not done
because it's on a different queue).

In the first case, the scheduler exits because running a task via
`run_until_complete` did not schedule any dependent tasks.

In the other two cases, the scheduler exits because the tasks are queued in
an event queue.

Tests have been added which reproduce the original issues.  These test
cases document the unauthorized use of `Event.set()` from a soft IRQ, and
are skipped in unsupported environments (webassembly and native emitter).

Fixes issues #16759, #16569 and #16318.

Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>

extmod/asyncio/core.py

@@ -163,9 +163,16 @@ def run_until_complete(main_task=None):
                 # A task waiting on _task_queue; "ph_key" is time to schedule task at
                 dt = max(0, ticks_diff(t.ph_key, ticks()))
             elif not _io_queue.map:
-                # No tasks can be woken so finished running
+                # No tasks can be woken
                 cur_task = None
-                return
+                if not main_task or not main_task.state:
+                    # no main_task, or main_task is done so finished running
+                    return
+                # At this point, there is theoretically nothing that could wake the
+                # scheduler, but it is not allowed to exit either. We keep the code
+                # running so that a hypothetical debugger (or other such meta-process)
+                # can get a view of what is happening and possibly abort.
+                dt = 3
             # print('(poll {})'.format(dt), len(_io_queue.map))
             _io_queue.wait_io_event(dt)
 
@@ -187,31 +194,33 @@ def run_until_complete(main_task=None):
         except excs_all as er:
             # Check the task is not on any event queue
             assert t.data is None
-            # This task is done, check if it's the main task and then loop should stop
-            if t is main_task:
+            # If it's the main task, it is considered as awaited by the caller
+            awaited = t is main_task
+            if awaited:
                 cur_task = None
-                if isinstance(er, StopIteration):
-                    return er.value
-                raise er
+                if not isinstance(er, StopIteration):
+                    t.state = False
+                    raise er
+                if t.state is None:
+                    t.state = False
             if t.state:
                 # Task was running but is now finished.
-                waiting = False
                 if t.state is True:
                     # "None" indicates that the task is complete and not await'ed on (yet).
-                    t.state = None
+                    t.state = False if awaited else 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
+                    awaited = True
                 else:
                     # Schedule any other tasks waiting on the completion of this task.
                     while t.state.peek():
                         _task_queue.push(t.state.pop())
-                        waiting = True
+                        awaited = True
                     # "False" indicates that the task is complete and has been await'ed on.
                     t.state = False
-                if not waiting and not isinstance(er, excs_stop):
+                if not awaited and not isinstance(er, excs_stop):
                     # An exception ended this detached task, so queue it for later
                     # execution to handle the uncaught exception if no other task retrieves
                     # the exception in the meantime (this is handled by Task.throw).
@@ -229,6 +238,9 @@ def run_until_complete(main_task=None):
                 _exc_context["exception"] = exc
                 _exc_context["future"] = t
                 Loop.call_exception_handler(_exc_context)
+            # If it's the main task then the loop should stop
+            if t is main_task:
+                return er.value
 
 
 # Create a new task from a coroutine and run it until it finishes

tests/extmod/asyncio_event_queue.py

@@ -0,0 +1,64 @@
+# Ensure that an asyncio task can wait on an Event when the
+# _task_queue is empty
+# https://github.com/micropython/micropython/issues/16569
+
+try:
+    import asyncio
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# This test requires checking that the asyncio scheduler
+# remains active "indefinitely" when the task queue is empty.
+#
+# To check this, we need another independent scheduler that
+# can wait for a certain amount of time. So we have to
+# create one using micropython.schedule() and time.ticks_ms()
+#
+# Technically, this code breaks the rules, as it is clearly
+# documented that Event.set() should _NOT_ be called from a
+# schedule (soft IRQ) because in some cases, a race condition
+# can occur, resulting in a crash. However:
+# - since the risk of a race condition in that specific
+#   case has been analysed and excluded
+# - given that there is no other simple alternative to
+#   write this test case,
+# an exception to the rule was deemed acceptable. See
+# https://github.com/micropython/micropython/pull/16772
+
+import micropython, time
+
+try:
+    micropython.schedule
+except AttributeError:
+    print("SKIP")
+    raise SystemExit
+
+
+evt = asyncio.Event()
+
+
+def schedule_watchdog(end_ticks):
+    if time.ticks_diff(end_ticks, time.ticks_ms()) <= 0:
+        print("asyncio still pending, unlocking event")
+        # Caution: about to call Event.set() from a schedule
+        #          (see the note in the comment above)
+        evt.set()
+        return
+    micropython.schedule(schedule_watchdog, end_ticks)
+
+
+async def foo():
+    print("foo waiting")
+    schedule_watchdog(time.ticks_add(time.ticks_ms(), 100))
+    await evt.wait()
+    print("foo done")
+
+
+async def main():
+    print("main started")
+    await foo()
+    print("main done")
+
+
+asyncio.run(main())

tests/extmod/asyncio_event_queue.py.exp

@@ -0,0 +1,5 @@
+main started
+foo waiting
+asyncio still pending, unlocking event
+foo done
+main done

tests/extmod/asyncio_iterator_event.py

@@ -0,0 +1,86 @@
+# Ensure that an asyncio task can wait on an Event when the
+# _task_queue is empty, in the context of an async iterator
+# https://github.com/micropython/micropython/issues/16318
+
+try:
+    import asyncio
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# This test requires checking that the asyncio scheduler
+# remains active "indefinitely" when the task queue is empty.
+#
+# To check this, we need another independent scheduler that
+# can wait for a certain amount of time. So we have to
+# create one using micropython.schedule() and time.ticks_ms()
+#
+# Technically, this code breaks the rules, as it is clearly
+# documented that Event.set() should _NOT_ be called from a
+# schedule (soft IRQ) because in some cases, a race condition
+# can occur, resulting in a crash. However:
+# - since the risk of a race condition in that specific
+#   case has been analysed and excluded
+# - given that there is no other simple alternative to
+#   write this test case,
+# an exception to the rule was deemed acceptable. See
+# https://github.com/micropython/micropython/pull/16772
+
+import micropython, time
+
+try:
+    micropython.schedule
+except AttributeError:
+    print("SKIP")
+    raise SystemExit
+
+ai = None
+
+
+def schedule_watchdog(end_ticks):
+    if time.ticks_diff(end_ticks, time.ticks_ms()) <= 0:
+        print("good: asyncio iterator is still pending, exiting")
+        # Caution: ai.fetch_data() will invoke Event.set()
+        #          (see the note in the comment above)
+        ai.fetch_data(None)
+        return
+    micropython.schedule(schedule_watchdog, end_ticks)
+
+
+async def test(ai):
+    for x in range(3):
+        await asyncio.sleep(0.1)
+        ai.fetch_data(f"bar {x}")
+
+
+class AsyncIterable:
+    def __init__(self):
+        self.message = None
+        self.evt = asyncio.Event()
+
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self):
+        await self.evt.wait()
+        self.evt.clear()
+        if self.message is None:
+            raise StopAsyncIteration
+        return self.message
+
+    def fetch_data(self, message):
+        self.message = message
+        self.evt.set()
+
+
+async def main():
+    global ai
+    ai = AsyncIterable()
+    asyncio.create_task(test(ai))
+    schedule_watchdog(time.ticks_add(time.ticks_ms(), 500))
+    async for message in ai:
+        print(message)
+    print("end main")
+
+
+asyncio.run(main())

tests/extmod/asyncio_iterator_event.py.exp

@@ -0,0 +1,5 @@
+bar 0
+bar 1
+bar 2
+good: asyncio iterator is still pending, exiting
+end main

tests/extmod/asyncio_wait_for_linked_task.py

@@ -0,0 +1,66 @@
+# Test asyncio.wait_for, with dependent tasks
+# https://github.com/micropython/micropython/issues/16759
+
+try:
+    import asyncio
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+# CPython 3.12 deprecated calling get_event_loop() when there is no current event
+# loop, so to make this test run on CPython requires setting the event loop.
+if hasattr(asyncio, "set_event_loop"):
+    asyncio.set_event_loop(asyncio.new_event_loop())
+
+
+class Worker:
+    def __init__(self):
+        self._eventLoop = None
+        self._tasks = []
+
+    def launchTask(self, asyncJob):
+        if self._eventLoop is None:
+            self._eventLoop = asyncio.get_event_loop()
+        return self._eventLoop.create_task(asyncJob)
+
+    async def job(self, prerequisite, taskName):
+        if prerequisite:
+            await prerequisite
+        await asyncio.sleep(0.1)
+        print(taskName, "work completed")
+
+    def planTasks(self):
+        self._tasks.append(self.launchTask(self.job(None, "task0")))
+        self._tasks.append(self.launchTask(self.job(self._tasks[0], "task1")))
+        self._tasks.append(self.launchTask(self.job(self._tasks[1], "task2")))
+
+    async def waitForTask(self, taskIdx):
+        return await self._tasks[taskIdx]
+
+    def syncWaitForTask(self, taskIdx):
+        return self._eventLoop.run_until_complete(self._tasks[taskIdx])
+
+
+async def async_test():
+    print("--- async test")
+    worker = Worker()
+    worker.planTasks()
+    await worker.waitForTask(0)
+    print("-> task0 done")
+    await worker.waitForTask(2)
+    print("-> task2 done")
+
+
+def sync_test():
+    print("--- sync test")
+    worker = Worker()
+    worker.planTasks()
+    worker.syncWaitForTask(0)
+    print("-> task0 done")
+    worker.syncWaitForTask(2)
+    print("-> task2 done")
+
+
+asyncio.get_event_loop().run_until_complete(async_test())
+sync_test()

tests/run-tests.py

@@ -162,6 +162,9 @@ platform_tests_to_skip = {
         "extmod/asyncio_new_event_loop.py",
         "extmod/asyncio_threadsafeflag.py",
         "extmod/asyncio_wait_for_fwd.py",
+        "extmod/asyncio_event_queue.py",
+        "extmod/asyncio_iterator_event.py",
+        "extmod/asyncio_wait_for_linked_task.py",
         "extmod/binascii_a2b_base64.py",
         "extmod/deflate_compress_memory_error.py",  # tries to allocate unlimited memory
         "extmod/re_stack_overflow.py",
@@ -843,6 +846,8 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
         )  # native doesn't have proper traceback info
         skip_tests.add("micropython/schedule.py")  # native code doesn't check pending events
         skip_tests.add("stress/bytecode_limit.py")  # bytecode specific test
+        skip_tests.add("extmod/asyncio_event_queue.py")  # native can't run schedule
+        skip_tests.add("extmod/asyncio_iterator_event.py")  # native can't run schedule
 
     def run_one_test(test_file):
         test_file = test_file.replace("\\", "/")