Don't try to mock context manager; use a simple class instead

This one's related to #38. It turns out that while trying to mock a
context manager kind of works, it will do the wrong thing in edge cases
like when an exception is thrown from inside a `with` block, silently
swallowing it and causing a return that's completely wrong.

There may be some way to fix the mock to make it do the right thing, but
instead of getting fancier with these mocks that are already awful,
instead repair the problem by defining a plain class that implements
context manager and just use that.

Author: brandur

tests/client_test.py

@@ -16,17 +16,18 @@ def mock_driver() -> DriverProtocol:
 
 @pytest.fixture
 def mock_exec(mock_driver) -> ExecutorProtocol:
-    def mock_context_manager(val) -> Mock:
-        context_manager_mock = MagicMock()
-        context_manager_mock.__enter__.return_value = val
-        context_manager_mock.__exit__.return_value = Mock()
-        return context_manager_mock
+    class TrivialContextManager:
+        def __init__(self, with_val):
+            self.with_val = with_val
 
-    # def mock_context_manager(val) -> Mock:
-    #     return Mock(__enter__=val, __exit__=Mock())
+        def __enter__(self):
+            return self.with_val
+
+        def __exit__(self, exc_type, exc_val, exc_tb):
+            pass
 
     mock_exec = MagicMock(spec=ExecutorProtocol)
-    mock_driver.executor.return_value = mock_context_manager(mock_exec)
+    mock_driver.executor.return_value = TrivialContextManager(mock_exec)
 
     return mock_exec