Fix MediaCache asyncio Semaphore early import bug

TODO.md

@@ -90,7 +90,6 @@
 
 - Multiaccount aliases:
   - Warn when conflict with another alias
-  - Forbid spaces?
   - Add an explanation tooltip
   - Prevent sending messages with a user not in the current room
   - Support \ escaping

src/backend/pyotherside_events.py

@@ -1,15 +1,14 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
-from abc import ABC
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Optional
 
 import pyotherside
 
-from .models import SyncId
 from .utils import serialize_value_for_qml
 
 if TYPE_CHECKING:
+    from .models import SyncId
     from .models.model_item import ModelItem
 
 
@@ -66,7 +65,7 @@ class LoopException(PyOtherSideEvent):
 class ModelItemInserted(PyOtherSideEvent):
     """Indicate a `ModelItem` insertion into a `Backend` `Model`."""
 
-    sync_id: SyncId      = field()
+    sync_id: "SyncId"    = field()
     index:   int         = field()
     item:    "ModelItem" = field()
 
@@ -75,23 +74,23 @@ class ModelItemInserted(PyOtherSideEvent):
 class ModelItemFieldChanged(PyOtherSideEvent):
     """Indicate a `ModelItem`'s field value change in a `Backend` `Model`."""
 
-    sync_id:         SyncId = field()
-    item_index_then: int    = field()
-    item_index_now:  int    = field()
-    changed_field:   str    = field()
-    field_value:     Any    = field()
+    sync_id:         "SyncId" = field()
+    item_index_then: int      = field()
+    item_index_now:  int      = field()
+    changed_field:   str      = field()
+    field_value:     Any      = field()
 
 
 @dataclass
 class ModelItemDeleted(PyOtherSideEvent):
     """Indicate the removal of a `ModelItem` from a `Backend` `Model`."""
 
-    sync_id: SyncId = field()
-    index:   int    = field()
+    sync_id: "SyncId" = field()
+    index:   int      = field()
 
 
 @dataclass
 class ModelCleared(PyOtherSideEvent):
     """Indicate that a `Backend` `Model` was cleared."""
 
-    sync_id: SyncId = field()
+    sync_id: "SyncId" = field()

src/backend/qml_bridge.py

@@ -2,6 +2,10 @@
 
 """Install `uvloop` if possible and provide a `QMLBridge`."""
 
+# WARNING: make sure to not top-level import the media_cache module here,
+# directly or indirectly via another module import (e.g. backend).
+# See https://stackoverflow.com/a/55918049
+
 import asyncio
 import logging as log
 import signal
@@ -11,7 +15,6 @@ from operator import attrgetter
 from threading import Thread
 from typing import Coroutine, Sequence
 
-from .backend import Backend
 from .pyotherside_events import CoroutineDone, LoopException
 
 try:
@@ -36,6 +39,7 @@ class QMLBridge:
     """
 
     def __init__(self) -> None:
+        from .backend import Backend
         self.backend: Backend = Backend()
 
         self._loop = asyncio.get_event_loop()