Fix segfault on coroutine cancelling from QML

Since 87fcb0a773f4855cdae7212fa9448a05de57be56, it was possible to call
a Python coroutine, but cancel it (due to parent component destruction)
before Python even has time to start it.
The registred QML callbacks for this coro would then be called,
potentially causing a segfault if that callback tried to access the
parent component or its properties.

src/backend/qml_bridge.py

@@ -21,7 +21,7 @@ import traceback
 from concurrent.futures import Future
 from operator import attrgetter
 from threading import Thread
-from typing import Coroutine, Dict, Sequence
+from typing import Coroutine, Dict, Sequence, Set
 
 import pyotherside
 
@@ -53,6 +53,7 @@ class QMLBridge:
         self.backend: Backend = Backend()
 
         self._running_futures: Dict[str, Future] = {}
+        self._cancelled_early: Set[str]          = set()
 
         Thread(target=self._start_asyncio_loop).start()
 
@@ -78,6 +79,10 @@ class QMLBridge:
     def _call_coro(self, coro: Coroutine, uuid: str) -> None:
         """Schedule a coroutine to run in our thread and return a `Future`."""
 
+        if uuid in self._cancelled_early:
+            self._cancelled_early.remove(uuid)
+            return
+
         def on_done(future: Future) -> None:
             """Send a PyOtherSide event with the coro's result/exception."""
             result = exception = trace = None
@@ -101,6 +106,9 @@ class QMLBridge:
     ) -> None:
         """Schedule a coroutine from the `QMLBridge.backend` object."""
 
+        if uuid in self._cancelled_early:
+            self._cancelled_early.remove(uuid)
+        else:
             self._call_coro(attrgetter(name)(self.backend)(*args), uuid)
 
 
@@ -109,6 +117,9 @@ class QMLBridge:
     ) -> None:
         """Schedule a coroutine from a `QMLBridge.backend.clients` client."""
 
+        if uuid in self._cancelled_early:
+            self._cancelled_early.remove(uuid)
+        else:
             client = self.backend.clients[user_id]
             self._call_coro(attrgetter(name)(client)(*args), uuid)
 
@@ -116,10 +127,10 @@ class QMLBridge:
     def cancel_coro(self, uuid: str) -> None:
         """Cancel a couroutine scheduled by the `QMLBridge` methods."""
 
-        try:
+        if uuid in self._running_futures:
             self._running_futures[uuid].cancel()
-        except KeyError:
-            log.warning("Couldn't cancel coroutine %s, future not found", uuid)
+        else:
+            self._cancelled_early.add(uuid)
 
 
     def pdb(self, additional_data: Sequence = ()) -> None:

src/gui/PythonBridge/EventHandlers.qml

@@ -35,6 +35,8 @@ QtObject {
     }
 
     function onCoroutineDone(uuid, result, error, traceback) {
+        if (! Globals.pendingCoroutines[uuid]) return
+
         const onSuccess = Globals.pendingCoroutines[uuid].onSuccess
         const onError   = Globals.pendingCoroutines[uuid].onError
 

src/gui/PythonBridge/PythonBridge.qml

@@ -43,6 +43,7 @@ Python {
     }
 
     function cancelCoro(uuid) {
+        delete Globals.pendingCoroutines[uuid]
         call("BRIDGE.cancel_coro", [uuid])
     }