Fix race condition in OutputCapture

This race could occur because of the existing cache in OutputCapture: When data was written, while it was read, any following reading of data might miss the last output. The cache was not invalidated correctly in this case.

We are now versioning the cache entries to make sure this can't happen anymore.

Signed-off-by: Daniel Schmidt <daniel-github@ad-schmidt.de>

Author: schmidti159

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

@@ -23,6 +23,7 @@
 import java.util.ArrayList;
 import java.util.Deque;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 import java.util.function.Predicate;
@@ -42,6 +43,7 @@
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @author Sam Brannen
+ * @author Daniel Schmidt
  * @see OutputCaptureExtension
  * @see OutputCaptureRule
  */
@@ -51,11 +53,14 @@ class OutputCapture implements CapturedOutput {
 
 	private @Nullable AnsiOutputState ansiOutputState;
 
-	private final AtomicReference<String> out = new AtomicReference<>(null);
+	private final AtomicLong outVersion = new AtomicLong();
+	private final AtomicReference<VersionedCacheResult> out = new AtomicReference<>(null);
 
-	private final AtomicReference<String> err = new AtomicReference<>(null);
+	private final AtomicLong errVersion = new AtomicLong();
+	private final AtomicReference<VersionedCacheResult> err = new AtomicReference<>(null);
 
-	private final AtomicReference<String> all = new AtomicReference<>(null);
+	private final AtomicLong allVersion = new AtomicLong();
+	private final AtomicReference<VersionedCacheResult> all = new AtomicReference<>(null);
 
 	/**
 	 * Push a new system capture session onto the stack.
@@ -108,7 +113,7 @@ public String toString() {
 	 */
 	@Override
 	public String getAll() {
-		return get(this.all, (type) -> true);
+		return get(this.all, this.allVersion, (type) -> true);
 	}
 
 	/**
@@ -117,7 +122,7 @@ public String getAll() {
 	 */
 	@Override
 	public String getOut() {
-		return get(this.out, Type.OUT::equals);
+		return get(this.out, this.outVersion, Type.OUT::equals);
 	}
 
 	/**
@@ -126,7 +131,7 @@ public String getOut() {
 	 */
 	@Override
 	public String getErr() {
-		return get(this.err, Type.ERR::equals);
+		return get(this.err, this.errVersion, Type.ERR::equals);
 	}
 
 	/**
@@ -138,19 +143,24 @@ void reset() {
 	}
 
 	void clearExisting() {
+		this.outVersion.incrementAndGet();
 		this.out.set(null);
+		this.errVersion.incrementAndGet();
 		this.err.set(null);
+		this.allVersion.incrementAndGet();
 		this.all.set(null);
 	}
 
-	private String get(AtomicReference<String> existing, Predicate<Type> filter) {
+	private String get(AtomicReference<VersionedCacheResult> resultCache, AtomicLong version, 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);
+		long currentVersion = version.get();
+		VersionedCacheResult cached = resultCache.get();
+		if (cached != null && cached.version == currentVersion) {
+			return cached.result;
 		}
+		String result = build(filter);
+		resultCache.compareAndSet(null, new VersionedCacheResult(result, currentVersion));
 		return result;
 	}
 
@@ -162,6 +172,10 @@ String build(Predicate<Type> filter) {
 		return builder.toString();
 	}
 
+	private record VersionedCacheResult(String result, long version) {
+
+	}
+
 	/**
 	 * A capture session that captures {@link System#out System.out} and {@link System#out
 	 * System.err}.

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

@@ -19,8 +19,13 @@
 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.TimeUnit;
 import java.util.function.Predicate;
 
+import org.jspecify.annotations.Nullable;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -32,6 +37,7 @@
  * Tests for {@link OutputCapture}.
  *
  * @author Phillip Webb
+ * @author Daniel Schmidt
  */
 class OutputCaptureTests {
 
@@ -188,6 +194,45 @@ void getErrUsesCache() {
 		assertThat(this.output.buildCount).isEqualTo(2);
 	}
 
+	@Test
+	void getOutCacheShouldNotReturnStaleDataWhenDataIsLoggedWhileReading() throws Exception {
+		this.output.push();
+		System.out.print("A");
+		this.output.waitAfterBuildLatch = new CountDownLatch(1);
+
+		ExecutorService executorService = null;
+		try {
+			executorService = Executors.newFixedThreadPool(2);
+			var readingThreadFuture = executorService.submit(() -> {
+				// this will release the releaseAfterBuildLatch and block on the waitAfterBuildLatch
+				assertThat(this.output.getOut()).isEqualTo("A");
+			});
+			var writingThreadFuture = executorService.submit(() -> {
+				// wait until we finished building the first result (but did not yet update the cache)
+				try {
+					this.output.releaseAfterBuildLatch.await();
+				}
+				catch (InterruptedException e) {
+					throw new RuntimeException(e);
+				}
+				// print something else and then release the latch, for the other thread to continue
+				System.out.print("B");
+				this.output.waitAfterBuildLatch.countDown();
+			});
+			readingThreadFuture.get();
+			writingThreadFuture.get();
+		}
+		finally {
+			if (executorService != null) {
+				executorService.shutdown();
+				executorService.awaitTermination(10, TimeUnit.SECONDS);
+			}
+		}
+
+		// If not synchronized correctly this will fail, because the second print did not clear the cache and the cache will return stale data.
+		assertThat(this.output.getOut()).isEqualTo("AB");
+	}
+
 	private void pushAndPrint() {
 		this.output.push();
 		System.out.print("A");
@@ -212,10 +257,26 @@ static class TestOutputCapture extends OutputCapture {
 
 		int buildCount;
 
+		@Nullable
+		CountDownLatch waitAfterBuildLatch = null;
+
+		CountDownLatch releaseAfterBuildLatch = new CountDownLatch(1);
+
 		@Override
 		String build(Predicate<Type> filter) {
 			this.buildCount++;
-			return super.build(filter);
+			var result = super.build(filter);
+			this.releaseAfterBuildLatch.countDown();
+			if (this.waitAfterBuildLatch != null) {
+				try {
+					this.waitAfterBuildLatch.await();
+				}
+				catch (InterruptedException e) {
+					Thread.currentThread().interrupt();
+					throw new RuntimeException(e);
+				}
+			}
+			return result;
 		}
 
 	}