tests: clear util.callback_mgr between test cases

util.callback_mgr.callbacks was not getting properly cleared between tests.
Every time an Abstract_Wallet or an LNWorker (or many other subclasses of EventListener) is instantiated,
self.register_callbacks() is called in __init__, which puts callbacks into util.callback_mgr.callbacks.
These are only cleaned up if we explicitly call Abstract_Wallet.stop() or LNWorker.stop() later, which we usually do not do in the tests.

As a result, when running multiple unit tests in a row, lots of objects created in a given testcase are never GC-ed and leak into subsequent tests. This is not only a memory leak, but wastes compute too: when events are triggered and cbs get called, these old objects also have their cbs called.

After running all (~1061) unit tests, I observe util.callback_mgr.callbacks had 30 events with a total of 3156 callbacks stored.

On my laptop, running all unit tests previously took ~115 sec, and now it takes ~73 sec.

electrum/util.py

@@ -1953,20 +1953,24 @@ class CallbackManager(Logger):
     def __init__(self):
         Logger.__init__(self)
         self.callback_lock = threading.Lock()
-        self.callbacks = defaultdict(list)      # note: needs self.callback_lock
+        self.callbacks = defaultdict(list)  # type: Dict[str, List[Callable]]  # note: needs self.callback_lock
 
-    def register_callback(self, func, events):
+    def register_callback(self, func: Callable, events: Sequence[str]) -> None:
         with self.callback_lock:
             for event in events:
                 self.callbacks[event].append(func)
 
-    def unregister_callback(self, callback):
+    def unregister_callback(self, callback: Callable) -> None:
         with self.callback_lock:
             for callbacks in self.callbacks.values():
                 if callback in callbacks:
                     callbacks.remove(callback)
 
-    def trigger_callback(self, event, *args):
+    def clear_all_callbacks(self) -> None:
+        with self.callback_lock:
+            self.callbacks.clear()
+
+    def trigger_callback(self, event: str, *args) -> None:
         """Trigger a callback with given arguments.
         Can be called from any thread. The callback itself will get scheduled
         on the event loop.

tests/__init__.py

@@ -75,6 +75,7 @@ class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
         util._asyncio_event_loop = loop
 
     def tearDown(self):
+        util.callback_mgr.clear_all_callbacks()
         shutil.rmtree(self.electrum_path)
         super().tearDown()
         util._asyncio_event_loop = None  # cleared here, at the ~last possible moment. asyncTearDown is too early.

tests/test_lnpeer.py

@@ -1117,12 +1117,8 @@ class TestPeerDirect(TestPeer):
         util.register_callback(on_htlc_fulfilled, ["htlc_fulfilled"])
         util.register_callback(on_htlc_failed, ["htlc_failed"])
 
-        try:
-            with self.assertRaises(SuccessfulTest):
-                await f()
-        finally:
-            util.unregister_callback(on_htlc_fulfilled)
-            util.unregister_callback(on_htlc_failed)
+        with self.assertRaises(SuccessfulTest):
+            await f()
 
     async def test_payment_recv_mpp_confusion2(self):
         """Regression test for https://github.com/spesmilo/electrum/security/advisories/GHSA-8r85-vp7r-hjxf"""
@@ -1191,12 +1187,8 @@ class TestPeerDirect(TestPeer):
         util.register_callback(on_htlc_fulfilled, ["htlc_fulfilled"])
         util.register_callback(on_htlc_failed, ["htlc_failed"])
 
-        try:
-            with self.assertRaises(SuccessfulTest):
-                await f()
-        finally:
-            util.unregister_callback(on_htlc_fulfilled)
-            util.unregister_callback(on_htlc_failed)
+        with self.assertRaises(SuccessfulTest):
+            await f()
 
     async def test_legacy_shutdown_low(self):
         await self._test_shutdown(alice_fee=100, bob_fee=150)