Fix a bug where bootstrap code wasn't called (#1894)

betty/__init__.py

@@ -3,5 +3,8 @@
 from pathlib import Path
 
 
+from betty import _bootstrap  # noqa: F401
+
+
 # This lives here so it can be imported before any third-party Python modules are available.
 _ROOT_DIRECTORY_PATH = Path(__file__).resolve().parents[1]

betty/_bootstrap.py

@@ -0,0 +1,29 @@
+from asyncio import BaseEventLoop
+from multiprocessing import set_start_method
+from typing import Any
+
+
+async def _shutdown_default_executor(
+    loop: BaseEventLoop, *args: Any, **kwargs: Any
+) -> None:
+    try:
+        await _original_shutdown_default_executor(loop, *args, **kwargs)
+    except RuntimeError as error:
+        # Work around a bug in Python 3.12 that will randomly cause a RuntimeError with the
+        # following message to be raised.
+        if "can't create new thread at interpreter shutdown" not in str(error):
+            raise
+
+
+_bootstrapped = False
+
+
+if not _bootstrapped:
+    _bootstrapped = True
+
+    _original_shutdown_default_executor = BaseEventLoop.shutdown_default_executor
+    BaseEventLoop.shutdown_default_executor = _shutdown_default_executor  # type: ignore[assignment, callable-functiontype, method-assign, misc]
+
+    # Avoid `fork` so as not to start worker processes with unneeded resources.
+    # Use `spawn`, which is the Python 3.14 default.
+    set_start_method("spawn", force=True)