Defensive access to volatile ScheduledFuture field

Includes defensive test arrangement for isInThePast() with at least 1 ms having passed.

See gh-24560

spring-context/src/main/java/org/springframework/scheduling/config/ScheduledTask.java

@@ -86,8 +86,9 @@ public void cancel(boolean mayInterruptIfRunning) {
 	 */
 	@Nullable
 	public Instant nextExecution() {
-		if (this.future != null && !this.future.isCancelled()) {
-			long delay = this.future.getDelay(TimeUnit.MILLISECONDS);
+		ScheduledFuture<?> future = this.future;
+		if (future != null && !future.isCancelled()) {
+			long delay = future.getDelay(TimeUnit.MILLISECONDS);
 			if (delay > 0) {
 				return Instant.now().plusMillis(delay);
 			}

spring-context/src/test/java/org/springframework/scheduling/config/TaskTests.java

@@ -24,7 +24,9 @@
 
 /**
  * Tests for {@link Task}.
+ *
  * @author Brian Clozel
+ * @since 6.2
  */
 class TaskTests {
 
@@ -77,20 +79,32 @@ static class TestRunnable implements Runnable {
 
 		@Override
 		public void run() {
+			try {
+				Thread.sleep(1);
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
 			this.hasRun = true;
 		}
 	}
 
+
 	static class FailingTestRunnable implements Runnable {
 
 		boolean hasRun;
 
 		@Override
 		public void run() {
+			try {
+				Thread.sleep(1);
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
 			this.hasRun = true;
 			throw new IllegalStateException("test exception");
 		}
 	}
 
-
 }