PYTHON-4695 Fix test event loop policy and improve error traceback for ClientBulkWriteException (#1828)

pymongo/_client_bulk_shared.py

@@ -74,4 +74,6 @@ def _throw_client_bulk_write_exception(
                 "to your connection string."
             )
             raise OperationFailure(errmsg, code, full_result)
+    if isinstance(full_result["error"], BaseException):
+        raise ClientBulkWriteException(full_result, verbose_results) from full_result["error"]
     raise ClientBulkWriteException(full_result, verbose_results)

pyproject.toml

@@ -74,6 +74,7 @@ addopts = ["-ra", "--strict-config", "--strict-markers", "--junitxml=xunit-resul
 testpaths = ["test"]
 log_cli_level = "INFO"
 faulthandler_timeout = 1500
+asyncio_default_fixture_loop_scope = "session"
 xfail_strict = true
 filterwarnings = [
     "error",

test/asynchronous/conftest.py

@@ -17,6 +17,8 @@ def event_loop_policy():
     # has issues with sharing sockets across loops (https://github.com/python/cpython/issues/122240)
     # We explicitly use a different loop implementation here to prevent that issue
     if sys.platform == "win32":
+        # Needed for Python 3.8.
+        asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
         return asyncio.WindowsSelectorEventLoopPolicy()  # type: ignore[attr-defined]
 
     return asyncio.get_event_loop_policy()

test/conftest.py

@@ -15,6 +15,8 @@ def event_loop_policy():
     # has issues with sharing sockets across loops (https://github.com/python/cpython/issues/122240)
     # We explicitly use a different loop implementation here to prevent that issue
     if sys.platform == "win32":
+        # Needed for Python 3.8.
+        asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
         return asyncio.WindowsSelectorEventLoopPolicy()  # type: ignore[attr-defined]
 
     return asyncio.get_event_loop_policy()