Issue #15064: Make BaseManager.__enter__() start server if necessary.
diff --git a/Lib/multiprocessing/managers.py b/Lib/multiprocessing/managers.py
index f6611af..1ab147e 100644
--- a/Lib/multiprocessing/managers.py
+++ b/Lib/multiprocessing/managers.py
@@ -561,6 +561,9 @@
             conn.close()
 
     def __enter__(self):
+        if self._state.value == State.INITIAL:
+            self.start()
+        assert self._state.value == State.STARTED
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):