Fix infinite loop after resetting state machine

[radmirr]

• Fixes Resetting spring state machine results in infinite loop #1011

Glossary

Description of unit test state names:
S8_1 - itintial state
S8_2 - custom start state that we pass with default context
S8_5 - end state
S8_3, S8_4 - transit states

Problem description

In some situations (e.g. app crash) we need to start state machine from custom state in order to resume execution. To do that we are following next steps:

1. Stop state machine with stopReactively;
2. Reset via accessor with default context and specified state (S8_2);
3. Start machine with startReactively;

Expected behavior

Transitions from S8_2 to S8_5 with state machine stop

Actual behavior

Infinite loop with transitions S8_2 -> S8_3 -> S8_4 -> S8_5 -> S8_2 -> S8_3 -> ... as decribed in #1011

What causes the problem

1. Resetting state machine leads to lastState field being filled with custom value that we passed in context (S8_2)
2. After exiting from last state (S8_5) ReactiveStateMachineExecutor checks triggerless transitions.

spring-statemachine/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/ReactiveStateMachineExecutor.java

Line 141 in 4495c7d

mono = mono.then(handleTriggerlessTransitions(null, null));

3. In normal scenarios this filter must return false and state machine must stop.

spring-statemachine/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/ReactiveStateMachineExecutor.java

Lines 364 to 385 in 4495c7d

	.filter(t -> {
	State<S,E> source = t.getSource();
	if (source == null) {
	return false;
	}
	State<S,E> currentState = stateMachine.getState();
	if (currentState == null) {
	return false;
	}
	if (!StateMachineUtils.containsAtleastOne(source.getIds(), currentState.getIds())) {
	return false;
	}
	if (transitionConflictPolicy != TransitionConflictPolicy.PARENT && completion != null
	&& !source.getId().equals(completion.getId())) {
	if (source.isOrthogonal()) {
	return false;
	} else if (!StateMachineUtils.isSubstate(source, completion)) {
	return false;
	}
	}
	return true;
	})

4. With not null lastState field we'll get its value instead of currentState because of isComplete() returning true.
   

   spring-statemachine/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/AbstractStateMachine.java

   Lines 184 to 194 in 4495c7d

   	@Override
   	public State<S,E> getState() {
   	// if we're complete assume we're stopped
   	// and state was stashed into lastState
   	State<S, E> s = lastState;
   	if (s != null && isComplete()) {
   	return s;
   	} else {
   	return currentState;
   	}
   	}

5. Comparing S8_2 as source with S8_2 as currentState returns true, causing this check to be skipped and transition t now passing to flatMap. This transition will be called again and again.
   

   spring-statemachine/spring-statemachine-core/src/main/java/org/springframework/statemachine/support/ReactiveStateMachineExecutor.java

   Lines 373 to 375 in 4495c7d

   	if (!StateMachineUtils.containsAtleastOne(source.getIds(), currentState.getIds())) {
   	return false;
   	}

Fix description

Main goal of this fix is guaranteed returning of currentState while state machine is in running life cycle.
Triggerless state machines do all work inside STARTING state during startReactively call, so we can count it as a part of running life cycle.

Existing tests was not affected by this change and passed successfully.

Commenting running check from getState will cause added test to run in infinite loop.

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