fix(ThreadWorker): don't unload modules without __loader__

DuckDB (and likely other packages) manually add their submodules (e.g. duckdb.duckdb.functional) during side-effects when the "parent" (e.g. duckdb.duckdb) is loaded. Since extension modules can't be reloaded, it is impossible to re-run these side-effects to recreate these modules. This commit preserves modules without the __loader__ attribute (which is added by Python's normal import machinery), sacrificing isolation for compatibility.

questionpy_server/worker/impl/thread.py

@@ -39,6 +39,7 @@ def run(self) -> None:
         # is only for testing anyway, it'll do for now.
         original_path = sys.path.copy()
         original_module_names = set(sys.modules.keys())
+        original_path_importer_cache = sys.path_importer_cache.copy()
 
         connection = WorkerToServerConnection(self._pipe.right, self._pipe.right)
         manager = WorkerManager(connection)
@@ -50,7 +51,15 @@ def run(self) -> None:
             self._loop.call_soon_threadsafe(self._end_event.set)
 
             sys.path = original_path
+            sys.path_importer_cache = original_path_importer_cache
             for module_name in sys.modules.keys() - original_module_names:
+                if getattr(sys.modules[module_name], "__loader__", None) is None:
+                    log.warning(
+                        "Not unloading '%s', as it doesn't have a '__loader__' attribute and was probably added "
+                        "manually.",
+                        sys.modules[module_name].__name__,
+                    )
+                    continue
                 # Having reset the path, this forces questionpy and any package modules to be reloaded upon next import.
                 del sys.modules[module_name]