feat: SERVER_TIMESTAMP should survive deep copies (googleapis#820)

* Add dunder copy and deepcopy to Sentinel

google/cloud/firestore_v1/transforms.py

@@ -26,6 +26,15 @@ def __init__(self, description) -> None:
     def __repr__(self):
         return "Sentinel: {}".format(self.description)
 
+    def __copy__(self):
+        # Sentinel identity should be preserved across copies.
+        return self
+
+    def __deepcopy__(self, memo):
+        # Sentinel identity should be preserved across deep copies.
+        return self
+
+
 
 DELETE_FIELD = Sentinel("Value used to delete a field in a document.")
 

tests/unit/v1/test_transforms.py

@@ -114,3 +114,23 @@ def test__numericvalue___eq___same_value():
     inst = _make_numeric_value(value)
     other = _make_numeric_value(value)
     assert inst == other
+
+
+def test__server_timestamp_is_same_after_copy():
+    from google.cloud.firestore_v1.transforms import SERVER_TIMESTAMP
+    import copy
+
+    value = SERVER_TIMESTAMP
+
+    value_copy = copy.copy(value)
+    assert value_copy is value
+
+
+def test__server_timestamp_is_same_after_deepcopy():
+    from google.cloud.firestore_v1.transforms import SERVER_TIMESTAMP
+    import copy
+
+    value = SERVER_TIMESTAMP
+
+    value_copy = copy.deepcopy(value)
+    assert value_copy is value