Fixed race condition in get_async_backend (#714)

Fixes 425.

Co-authored-by: Alex Grönholm <alex.gronholm@nextday.fi>
Co-authored-by: Ganden Schaffner <github@xqzw.me>

docs/versionhistory.rst

@@ -5,6 +5,10 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
 **UNRELEASED**
 
+- Fixed a race condition that caused crashes when multiple event loops of the same
+  backend were running in separate threads and simultaneously attempted to use AnyIO for
+  their first time (`#425 <https://github.com/agronholm/anyio/issues/425>`_; PR by David
+  Jiricek and Ganden Schaffner)
 - Added the ``BlockingPortalProvider`` class to aid with constructing synchronous
   counterparts to asynchronous interfaces that would otherwise require multiple blocking
   portals

src/anyio/_core/_eventloop.py

@@ -25,6 +25,7 @@
 PosArgsT = TypeVarTuple("PosArgsT")
 
 threadlocals = threading.local()
+loaded_backends: dict[str, type[AsyncBackend]] = {}
 
 
 def run(
@@ -150,14 +151,16 @@ def claim_worker_thread(
         del threadlocals.current_token
 
 
-def get_async_backend(asynclib_name: str | None = None) -> AsyncBackend:
+def get_async_backend(asynclib_name: str | None = None) -> type[AsyncBackend]:
     if asynclib_name is None:
         asynclib_name = sniffio.current_async_library()
 
-    modulename = "anyio._backends._" + asynclib_name
+    # We use our own dict instead of sys.modules to get the already imported back-end
+    # class because the appropriate modules in sys.modules could potentially be only
+    # partially initialized
     try:
-        module = sys.modules[modulename]
+        return loaded_backends[asynclib_name]
     except KeyError:
-        module = import_module(modulename)
-
-    return getattr(module, "backend_class")
+        module = import_module(f"anyio._backends._{asynclib_name}")
+        loaded_backends[asynclib_name] = module.backend_class
+        return module.backend_class