Make FlightRecorderApplicationStartup thread safe

Prior to this commit, in some cases application context startup steps
could be created concurrently, which could cause issues with the current
implementation tracking the parent/child relationship between steps.

This commit ensures that the flight recorder implementation is using
thread safe collection implementations for that.

Fixes gh-26941

spring-core/src/main/java/org/springframework/core/metrics/jfr/FlightRecorderApplicationStartup.java

 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -16,8 +16,9 @@
 
 package org.springframework.core.metrics.jfr;
 
-import java.util.ArrayDeque;
 import java.util.Deque;
+import java.util.concurrent.ConcurrentLinkedDeque;
+import java.util.concurrent.atomic.AtomicLong;
 
 import org.springframework.core.metrics.ApplicationStartup;
 import org.springframework.core.metrics.StartupStep;
@@ -37,24 +38,23 @@ import org.springframework.core.metrics.StartupStep;
  */
 public class FlightRecorderApplicationStartup implements ApplicationStartup {
 
-	private long currentSequenceId;
+	private final AtomicLong currentSequenceId = new AtomicLong(0);
 
 	private final Deque<Long> currentSteps;
 
 
 	public FlightRecorderApplicationStartup() {
-		this.currentSequenceId = 0;
-		this.currentSteps = new ArrayDeque<>();
-		this.currentSteps.offerFirst(0L);
+		this.currentSteps = new ConcurrentLinkedDeque<>();
+		this.currentSteps.offerFirst(this.currentSequenceId.get());
 	}
 
 
 	@Override
 	public StartupStep start(String name) {
-		FlightRecorderStartupStep step = new FlightRecorderStartupStep(++this.currentSequenceId, name,
-				this.currentSteps.getFirst(), committedStep -> this.currentSteps.removeFirst());
-		this.currentSteps.offerFirst(this.currentSequenceId);
-		return step;
+		long sequenceId = this.currentSequenceId.incrementAndGet();
+		this.currentSteps.offerFirst(sequenceId);
+		return new FlightRecorderStartupStep(sequenceId, name,
+				this.currentSteps.getFirst(), committedStep -> this.currentSteps.removeFirstOccurrence(sequenceId));
 	}
 
 }