BaseRestartWorkChain: fix handler_overrides ignoring `enabled=Fal…

…se` (#5598)

In commit `7b8c61d0869751455993891cfe5cf6c637593344`, the input that
allows overriding handlers, `handler_overrides`, was updated to include
the possibility of overriding the priority. The changes broke the
original behavior though where a handler that was disabled by default
could not be enabled by specifying `enabled=False` in the overrides.

aiida/engine/processes/workchains/restart.py

@@ -372,7 +372,7 @@ def get_process_handlers_by_priority(self) -> List[Tuple[int, FunctionType]]:
                 enabled = overrides.pop('enabled', None)
                 priority = overrides.pop('priority', None)
 
-            if enabled is False or not handler.enabled:  # type: ignore[attr-defined]
+            if enabled is False or (enabled is None and not handler.enabled):  # type: ignore[attr-defined]
                 continue
 
             handlers.append((priority or handler.priority, handler))  # type: ignore[attr-defined]

tests/engine/processes/workchains/test_restart.py

@@ -33,6 +33,10 @@ def handler_a(self, node):
     def handler_b(self, _):
         return
 
+    @engine.process_handler(priority=0, enabled=False)
+    def handler_c(self, _):
+        return
+
     def not_a_handler(self, _):
         pass
 
@@ -46,12 +50,14 @@ def test_is_process_handler():
 
 def test_get_process_handlers():
     """Test the `BaseRestartWorkChain.get_process_handlers` class method."""
-    assert [handler.__name__ for handler in SomeWorkChain.get_process_handlers()] == ['handler_a', 'handler_b']
+    assert [handler.__name__ for handler in SomeWorkChain.get_process_handlers()
+            ] == ['handler_a', 'handler_b', 'handler_c']
 
 
 # yapf: disable
 @pytest.mark.parametrize('inputs, priorities', (
     ({}, [100, 200]),
+    ({'handler_overrides': {'handler_c': {'enabled': True}}}, [0, 100, 200]),
     ({'handler_overrides': {'handler_a': {'priority': 50}}}, [50, 100]),
     ({'handler_overrides': {'handler_a': {'enabled': False}}}, [100]),
     ({'handler_overrides': {'handler_a': False}}, [100]),  # This notation is deprecated
@@ -67,8 +73,10 @@ def test_get_process_handlers_by_priority(generate_work_chain, inputs, prioritie
     # Verify the actual handlers on the class haven't been modified
     assert getattr(SomeWorkChain, 'handler_a').priority == 200
     assert getattr(SomeWorkChain, 'handler_b').priority == 100
+    assert getattr(SomeWorkChain, 'handler_c').priority == 0
     assert getattr(SomeWorkChain, 'handler_a').enabled
     assert getattr(SomeWorkChain, 'handler_b').enabled
+    assert not getattr(SomeWorkChain, 'handler_c').enabled
 
 
 @pytest.mark.requires_rmq