extmod/uasyncio: Add ThreadSafeFlag.

This is a MicroPython-extension that allows for code running in IRQ
(hard or soft) or scheduler context to sequence asyncio code.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

extmod/uasyncio/__init__.py

@@ -10,6 +10,7 @@ _attrs = {
     "wait_for_ms": "funcs",
     "gather": "funcs",
     "Event": "event",
+    "ThreadSafeFlag": "event",
     "Lock": "lock",
     "open_connection": "stream",
     "start_server": "stream",

extmod/uasyncio/event.py

@@ -14,6 +14,8 @@ class Event:
 
     def set(self):
         # Event becomes set, schedule any tasks waiting on it
+        # Note: This must not be called from anything except the thread running
+        # the asyncio loop (i.e. neither hard or soft IRQ, or a different thread).
         while self.waiting.peek():
             core._task_queue.push_head(self.waiting.pop_head())
         self.state = True
@@ -29,3 +31,32 @@ class Event:
             core.cur_task.data = self.waiting
             yield
         return True
+
+
+# MicroPython-extension: This can be set from outside the asyncio event loop,
+# such as other threads, IRQs or scheduler context. Implementation is a stream
+# that asyncio will poll until a flag is set.
+# Note: Unlike Event, this is self-clearing.
+try:
+    import uio
+
+    class ThreadSafeFlag(uio.IOBase):
+        def __init__(self):
+            self._flag = 0
+
+        def ioctl(self, req, flags):
+            if req == 3:  # MP_STREAM_POLL
+                return self._flag * flags
+            return None
+
+        def set(self):
+            self._flag = 1
+
+        async def wait(self):
+            if not self._flag:
+                yield core._io_queue.queue_read(self)
+            self._flag = 0
+
+
+except ImportError:
+    pass