Merge pull request #46685 from schmidti159

* pr/46685:
  Polish 'Fix race condition in OutputCapture'
  Fix race condition in OutputCapture

Closes gh-46685

Author: philwebb

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/system/OutputCapture.java

@@ -40,6 +40,7 @@
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @author Sam Brannen
+ * @author Daniel Schmidt
  * @see OutputCaptureExtension
  * @see OutputCaptureRule
  */
@@ -49,11 +50,15 @@ class OutputCapture implements CapturedOutput {
 
 	private AnsiOutputState ansiOutputState;
 
-	private final AtomicReference<String> out = new AtomicReference<>(null);
+	private final AtomicReference<Object> out = new AtomicReference<>();
 
-	private final AtomicReference<String> err = new AtomicReference<>(null);
+	private final AtomicReference<Object> err = new AtomicReference<>();
 
-	private final AtomicReference<String> all = new AtomicReference<>(null);
+	private final AtomicReference<Object> all = new AtomicReference<>();
+
+	OutputCapture() {
+		clearExisting();
+	}
 
 	/**
 	 * Push a new system capture session onto the stack.
@@ -136,20 +141,21 @@ void reset() {
 	}
 
 	void clearExisting() {
-		this.out.set(null);
-		this.err.set(null);
-		this.all.set(null);
+		this.out.set(new NoOutput());
+		this.err.set(new NoOutput());
+		this.all.set(new NoOutput());
 	}
 
-	private String get(AtomicReference<String> existing, Predicate<Type> filter) {
+	private String get(AtomicReference<Object> existing, Predicate<Type> filter) {
 		Assert.state(!this.systemCaptures.isEmpty(),
 				"No system captures found. Please check your output capture registration.");
-		String result = existing.get();
-		if (result == null) {
-			result = build(filter);
-			existing.compareAndSet(null, result);
+		Object existingOutput = existing.get();
+		if (existingOutput instanceof String) {
+			return (String) existingOutput;
 		}
-		return result;
+		String builtOutput = build(filter);
+		existing.compareAndSet(existingOutput, builtOutput);
+		return builtOutput;
 	}
 
 	String build(Predicate<Type> filter) {
@@ -339,4 +345,8 @@ static AnsiOutputState saveAndDisable() {
 
 	}
 
+	static class NoOutput {
+
+	}
+
 }

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/system/OutputCaptureTests.java

@@ -19,6 +19,11 @@
 import java.io.ByteArrayOutputStream;
 import java.io.PrintStream;
 import java.util.NoSuchElementException;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
+import java.util.concurrent.TimeUnit;
 import java.util.function.Predicate;
 
 import org.junit.jupiter.api.AfterEach;
@@ -32,6 +37,7 @@
  * Tests for {@link OutputCapture}.
  *
  * @author Phillip Webb
+ * @author Daniel Schmidt
  */
 class OutputCaptureTests {
 
@@ -188,6 +194,25 @@ void getErrUsesCache() {
 		assertThat(this.output.buildCount).isEqualTo(2);
 	}
 
+	@Test
+	void getOutCacheShouldNotReturnStaleDataWhenDataIsLoggedWhileReading() throws Exception {
+		TestLatchedOutputCapture output = new TestLatchedOutputCapture();
+		output.push();
+		System.out.print("A");
+		ExecutorService executor = Executors.newFixedThreadPool(2);
+		try {
+			Future<?> reader = executor.submit(output::releaseAfterBuildAndAssertResultIsA);
+			Future<?> writer = executor.submit(output::awaitReleaseAfterBuildThenWriteBAndRelease);
+			reader.get();
+			writer.get();
+		}
+		finally {
+			executor.shutdown();
+			executor.awaitTermination(10, TimeUnit.SECONDS);
+		}
+		assertThat(output.getOut()).isEqualTo("AB");
+	}
+
 	private void pushAndPrint() {
 		this.output.push();
 		System.out.print("A");
@@ -220,4 +245,40 @@ String build(Predicate<Type> filter) {
 
 	}
 
+	static class TestLatchedOutputCapture extends OutputCapture {
+
+		private final CountDownLatch waitAfterBuild = new CountDownLatch(1);
+
+		private final CountDownLatch releaseAfterBuild = new CountDownLatch(1);
+
+		@Override
+		String build(Predicate<Type> filter) {
+			var result = super.build(filter);
+			this.releaseAfterBuild.countDown();
+			await(this.waitAfterBuild);
+			return result;
+		}
+
+		void releaseAfterBuildAndAssertResultIsA() {
+			assertThat(getOut()).isEqualTo("A");
+		}
+
+		void awaitReleaseAfterBuildThenWriteBAndRelease() {
+			await(this.releaseAfterBuild);
+			System.out.print("B");
+			this.waitAfterBuild.countDown();
+		}
+
+		private void await(CountDownLatch latch) {
+			try {
+				latch.await();
+			}
+			catch (InterruptedException ex) {
+				Thread.currentThread().interrupt();
+				throw new RuntimeException(ex);
+			}
+		}
+
+	}
+
 }