[V3 Config] Fix for unnecessary writes on context mgr exit (#1859)

* Fix for #1857

* And copy it so we don't have mutability issues

* Add a test

redbot/core/config.py

@@ -29,12 +29,15 @@ class _ValueCtxManager:
     def __init__(self, value_obj, coro):
         self.value_obj = value_obj
         self.coro = coro
+        self.raw_value = None
+        self.__original_value = None
 
     def __await__(self):
         return self.coro.__await__()
 
     async def __aenter__(self):
         self.raw_value = await self
+        self.__original_value = deepcopy(self.raw_value)
         if not isinstance(self.raw_value, (list, dict)):
             raise TypeError(
                 "Type of retrieved value must be mutable (i.e. "
@@ -44,7 +47,8 @@ class _ValueCtxManager:
         return self.raw_value
 
     async def __aexit__(self, *exc_info):
-        await self.value_obj.set(self.raw_value)
+        if self.raw_value != self.__original_value:
+            await self.value_obj.set(self.raw_value)
 
 
 class Value:

tests/core/test_config.py

@@ -1,3 +1,4 @@
+from unittest.mock import patch
 import pytest
 
 
@@ -399,6 +400,16 @@ async def test_ctxmgr_no_shared_default(config, member_factory):
     assert 1 not in await config.member(m2).foo()
 
 
+@pytest.mark.asyncio
+async def test_ctxmgr_no_unnecessary_write(config):
+    config.register_global(foo=[])
+    foo_value_obj = config.foo
+    with patch.object(foo_value_obj, "set") as set_method:
+        async with foo_value_obj() as foo:
+            pass
+        set_method.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_get_then_mutate(config):
     """Tests that mutating an object after getting it as a value doesn't mutate the data store."""