aiorpcx: monkeypatch TimeoutAfter internals

see kyuupichan/aiorpcX#44

electrum/util.py

@@ -1246,6 +1246,37 @@ async def join(self):
             if self.completed:
                 self.completed.result()
 
+# We monkey-patch aiorpcx TimeoutAfter (used by timeout_after and ignore_after API),
+# to fix a timing issue present in asyncio as a whole re timing out tasks.
+# To see the issue we are trying to fix, consider example:
+#     async def outer_task():
+#         async with timeout_after(0.1):
+#             await inner_task()
+# When the 0.1 sec timeout expires, inner_task will get cancelled by timeout_after (=internal cancellation).
+# If around the same time (in terms of event loop iterations) another coroutine
+# cancels outer_task (=external cancellation), there will be a race.
+# Both cancellations work by propagating a CancelledError out to timeout_after, which then
+# needs to decide (in TimeoutAfter.__aexit__) whether it's due to an internal or external cancellation.
+# AFAICT asyncio provides no reliable way of distinguishing between the two.
+# This patch tries to always give priority to external cancellations.
+# see https://github.com/kyuupichan/aiorpcX/issues/44
+# see https://github.com/aio-libs/async-timeout/issues/229
+# see https://bugs.python.org/issue42130 and https://bugs.python.org/issue45098
+def _aiorpcx_monkeypatched_set_new_deadline(task, deadline):
+    def timeout_task():
+        task._orig_cancel()
+        task._timed_out = None if getattr(task, "_externally_cancelled", False) else deadline
+    def mycancel():
+        task._orig_cancel()
+        task._externally_cancelled = True
+        task._timed_out = None
+    if not hasattr(task, "_orig_cancel"):
+        task._orig_cancel = task.cancel
+        task.cancel = mycancel
+    task._deadline_handle = task._loop.call_at(deadline, timeout_task)
+
+aiorpcx.curio._set_new_deadline = _aiorpcx_monkeypatched_set_new_deadline
+
 
 class NetworkJobOnDefaultServer(Logger, ABC):
     """An abstract base class for a job that runs on the main network