NPE on sendEvent

- Adding null check for current state
- Remove synchronization from scheduleEventQueueProcessing
  method in executor. Looks like this sync is not really needed
  and indeed may cause jvm level deadlocks if threads are used
  for execution.
- Change how some internals in
  AbstractStateMachine are synchronized
  to limit changes for deadlock.
- Relates to #307
- Backport of #359 #360

spring-statemachine-build-tests/src/test/java/org/springframework/statemachine/buildtests/TimerSmokeTests.java

@@ -0,0 +1,143 @@
+/*
+ * Copyright 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.statemachine.buildtests;
+
+import org.junit.Test;
+import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
+import org.springframework.statemachine.StateMachine;
+import org.springframework.statemachine.config.StateMachineBuilder;
+import org.springframework.statemachine.test.StateMachineTestPlan;
+import org.springframework.statemachine.test.StateMachineTestPlanBuilder;
+
+public class TimerSmokeTests {
+
+	private static ThreadPoolTaskExecutor taskExecutor = new ThreadPoolTaskExecutor();
+	{
+		taskExecutor.initialize();
+	}
+
+	private StateMachine<String, String> buildMachine() throws Exception {
+
+		StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();
+
+		builder.configureConfiguration()
+			.withConfiguration()
+				.taskExecutor(taskExecutor);
+
+		builder.configureStates()
+			.withStates()
+				.initial("initial")
+				.end("end");
+
+		builder.configureTransitions()
+			.withExternal()
+				.source("initial")
+				.target("end")
+				.timerOnce(30)
+				.and()
+			.withLocal()
+				.source("initial")
+				.event("repeate");
+
+		return builder.build();
+	}
+
+	private StateMachine<String, String> buildMachine2() throws Exception {
+
+		StateMachineBuilder.Builder<String, String> builder = StateMachineBuilder.builder();
+
+		builder.configureConfiguration()
+			.withConfiguration()
+				.taskExecutor(taskExecutor);
+
+		builder.configureStates()
+			.withStates()
+				.initial("initial").end("end").and()
+				.withStates().parent("initial").initial("inner");
+
+		builder.configureTransitions()
+			.withExternal()
+				.source("initial")
+				.target("end")
+				.timerOnce(30)
+				.and()
+			.withExternal()
+				.source("inner")
+				.target("end")
+				.timerOnce(15)
+				.and()
+			.withLocal()
+				.source("inner")
+				.event("repeate");
+
+		return builder.build();
+	}
+
+	@Test
+	public void testNPE() throws Exception {
+		StateMachine<String, String> stateMachine;
+		for (int i = 0; i < 20; i++) {
+			stateMachine = buildMachine();
+			stateMachine.start();
+			while (!stateMachine.isComplete()) {
+				stateMachine.sendEvent("repeate");
+			}
+		}
+	}
+
+	@Test
+	public void testNPE2() throws Exception {
+
+		StateMachine<String, String> stateMachine;
+
+		for (int i = 0; i < 20; i++) {
+			stateMachine = buildMachine2();
+			stateMachine.start();
+			while(!stateMachine.isComplete()) {
+				stateMachine.sendEvent("repeate");
+			}
+			stateMachine.stop();
+		}
+	}
+
+	@Test
+	public void testDeadlock() throws Exception {
+		StateMachineTestPlan<String, String> plan;
+		for (int i = 0; i < 20; i++) {
+			plan = StateMachineTestPlanBuilder.<String, String> builder()
+					.defaultAwaitTime(1)
+					.stateMachine(buildMachine())
+					.step()
+						.expectStateMachineStarted(1)
+						.expectStateEntered(1)
+						.expectStateEntered("initial")
+						.and()
+					.step()
+						.sendEvent("repeate")
+						.expectStates("initial")
+						.and()
+					.step()
+						.expectStateEntered(1)
+						.expectStateEntered("end")
+						.and()
+					.step()
+						.expectStateMachineStopped(1)
+						.and()
+					.build();
+			plan.test();
+		}
+	}
+}