asyncio: hold our own strong refs for tasks and futures

see https://docs.python.org/3.13/library/asyncio-task.html#asyncio.create_task :

> Important
>
> Save a reference to the result of this function, to avoid a task
> disappearing mid-execution. The event loop only keeps weak references
> to tasks. A task that isn’t referenced elsewhere may get garbage
> collected at any time, even before it’s done. For reliable
> “fire-and-forget” background tasks, gather them in a collection

ref python/cpython#91887
ref beeware/toga#2814

Author: SomberNight

electrum/network.py

@@ -755,7 +755,6 @@ async def tor_probe_task(p):
 
         proxy = self.proxy
         if proxy and proxy.enabled and proxy.mode == 'socks5':
-            # FIXME GC issues? do we need to store the Future?
             asyncio.run_coroutine_threadsafe(tor_probe_task(proxy), self.asyncio_loop)
 
     @log_exceptions

electrum/util.py

@@ -1652,6 +1652,7 @@ def run_event_loop():
             _asyncio_event_loop = None
 
     loop.set_exception_handler(on_exception)
+    _set_custom_task_factory(loop)
     # loop.set_debug(True)
     stopping_fut = loop.create_future()
     loop_thread = threading.Thread(
@@ -1670,6 +1671,42 @@ def run_event_loop():
     return loop, stopping_fut, loop_thread
 
 
+_running_asyncio_tasks = set()  # type: Set[asyncio.Future]
+def _set_custom_task_factory(loop: asyncio.AbstractEventLoop):
+    """Wrap task creation to track pending and running tasks.
+    When tasks are created, asyncio only maintains a weak reference to them.
+    Hence, the garbage collector might destroy the task mid-execution.
+    To avoid this, we store a strong reference for the task until it completes.
+
+    Without this, a lot of APIs are basically Heisenbug-generators... e.g.:
+    - "asyncio.create_task"
+    - "loop.create_task"
+    - "asyncio.ensure_future"
+    - what about "asyncio.run_coroutine_threadsafe"? not sure if that is safe.
+
+    related:
+        - https://bugs.python.org/issue44665
+        - https://github.com/python/cpython/issues/88831
+        - https://github.com/python/cpython/issues/91887
+        - https://textual.textualize.io/blog/2023/02/11/the-heisenbug-lurking-in-your-async-code/
+        - https://github.com/python/cpython/issues/91887#issuecomment-1434816045
+        - "Task was destroyed but it is pending!"
+    """
+
+    platform_task_factory = loop.get_task_factory()
+
+    def factory(loop_, coro, **kwargs):
+        if platform_task_factory is not None:
+            task = platform_task_factory(loop_, coro, **kwargs)
+        else:
+            task = asyncio.Task(coro, loop=loop_, **kwargs)
+        _running_asyncio_tasks.add(task)
+        task.add_done_callback(_running_asyncio_tasks.discard)
+        return task
+
+    loop.set_task_factory(factory)
+
+
 class OrderedDictWithIndex(OrderedDict):
     """An OrderedDict that keeps track of the positions of keys.
 

tests/test_util.py

@@ -1,3 +1,4 @@
+import asyncio
 from datetime import datetime
 from decimal import Decimal
 
@@ -472,3 +473,30 @@ def test_shortchannelid(self):
         self.assertTrue(ShortID.from_components(3, 30, 300) > ShortID.from_components(3, 1, 999))
         self.assertTrue(ShortID.from_components(3, 30, 300) < ShortID.from_components(3, 999, 1))
 
+    async def test_custom_task_factory(self):
+        loop = util.get_running_loop()
+        # set our factory.  note: this does not leak into other unit tests
+        util._set_custom_task_factory(loop)
+
+        evt = asyncio.Event()
+        async def foo():
+            await evt.wait()
+
+        fut = asyncio.ensure_future(foo())
+        self.assertTrue(fut in util._running_asyncio_tasks)
+        fut = asyncio.create_task(foo())
+        self.assertTrue(fut in util._running_asyncio_tasks)
+        fut = loop.create_task(foo())
+        self.assertTrue(fut in util._running_asyncio_tasks)
+        #fut = asyncio.run_coroutine_threadsafe(foobar(), loop=loop)
+        #self.assertTrue(fut in util._running_asyncio_tasks)
+
+        # we should have stored one ref for each above.
+        # (though what if test framework is doing stuff ~concurrently?)
+        self.assertEqual(3, len(util._running_asyncio_tasks))
+        evt.set()
+        for _ in range(10):  # wait a few event loop iterations
+            await asyncio.sleep(0)
+        # refs should be cleaned up by now:
+        self.assertEqual(0, len(util._running_asyncio_tasks))
+