fix: Configuration.add()/discard() write through model setter for persistence (#596)

In-place mutation of the OrderedSet returned by self.value lost state changes
when the model uses a deserializing property (e.g., Django model field backed
by a DB column). Each getter call returned a fresh object, the in-place
mutation modified a temporary, and the model was never updated.

Changed add() and discard() to create a new OrderedSet and assign via
self.value = new, which calls setattr on the model. This only affects
atomic_configuration_update=False (StateChart default); the atomic path
was never affected.

Benchmark impact: ~4-5% on parallel region events (~2µs per event),
negligible vs. callback execution in the same microstep.

Signed-off-by: Fernando Macedo <fernando.macedo@jusbrasil.com.br>

Author: fgmacedo

statemachine/configuration.py

@@ -106,21 +106,23 @@ def add(self, state: "State"):
         if csv is None:
             self.value = state.value
         elif isinstance(csv, MutableSet):
-            csv.add(state.value)
-            self._invalidate()
+            new = OrderedSet(csv)
+            new.add(state.value)
+            self.value = new
         else:
             self.value = OrderedSet([csv, state.value])
 
     def discard(self, state: "State"):
         """Remove *state* from the configuration, normalizing back to scalar."""
         csv = self.value
         if isinstance(csv, MutableSet):
-            csv.discard(state.value)
-            self._invalidate()
-            if len(csv) == 1:
-                self.value = next(iter(csv))
-            elif len(csv) == 0:
+            new = OrderedSet(v for v in csv if v != state.value)
+            if len(new) == 0:
                 self.value = None
+            elif len(new) == 1:
+                self.value = next(iter(new))
+            else:
+                self.value = new
         elif csv == state.value:
             self.value = None