Fix issue when resetting config after new value has been added

CHANGELOG.asciidoc

@@ -49,6 +49,7 @@ endif::[]
 * Fix for Starlette/FastAPI to correctly capture request bodies as strings {pull}1042[#1041]
 * Fix transaction names for Starlette Mount routes {pull}1037[#1037]
 * Fix for elastic excepthook arguments {pull}1050[#1050]
+* Fix issue with remote configuration when resetting config values {pull}1068[#1068]
 
 
 [[release-notes-6.x]]

elasticapm/conf/__init__.py

@@ -689,7 +689,7 @@ def reset(self):
         """
         callbacks = []
         for key in compat.iterkeys(self._config.values):
-            if self._config.values[key] != self._first_config.values[key]:
+            if key in self._first_config.values and self._config.values[key] != self._first_config.values[key]:
                 callbacks.append((key, self._config.values[key], self._first_config.values[key]))
 
         with self._lock:

tests/config/tests.py

@@ -396,6 +396,19 @@ class MyConfig(_ConfigBase):
     assert test_var["foo"] == 3
 
 
+def test_reset_after_adding_config():
+    class MyConfig(_ConfigBase):
+        foo = _ConfigValue("foo")
+        bar = _ConfigValue("bar")
+
+    c = VersionedConfig(MyConfig({"foo": "baz"}), version=1)
+
+    c.update(version=2, bar="bazzinga")
+
+    c.reset()
+    assert c.bar is None
+
+
 def test_valid_values_validator():
     # Case sensitive
     v = EnumerationValidator(["foo", "Bar", "baz"], case_sensitive=False)