Attribute non-test thread output to most recent test thread

documentation/src/docs/asciidoc/release-notes/release-notes-5.12.0-M1.adoc

@@ -52,6 +52,8 @@ JUnit repository on GitHub.
   - If <<../user-guide/index.adoc#running-tests-capturing-output, output capturing>> is
     enabled, the captured output written to `System.out` and `System.err` is now included
     in the XML report.
+* Output written to `System.out` and `System.err` from non-test threads is now attributed
+  to the most recent test or container that was started or has written output.
 * Introduced contracts for Kotlin-specific assertion methods.
 
 

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/StreamInterceptor.java

@@ -15,13 +15,16 @@
 import java.util.ArrayDeque;
 import java.util.Deque;
 import java.util.Optional;
+import java.util.concurrent.ConcurrentLinkedDeque;
 import java.util.function.Consumer;
 
 /**
  * @since 1.3
  */
 class StreamInterceptor extends PrintStream {
 
+	private final Deque<RewindableByteArrayOutputStream> mostRecentOutputs = new ConcurrentLinkedDeque<>();
+
 	private final PrintStream originalStream;
 	private final Consumer<PrintStream> unregisterAction;
 	private final int maxNumberOfBytesPerThread;
@@ -56,11 +59,18 @@ private StreamInterceptor(PrintStream originalStream, Consumer<PrintStream> unre
 	}
 
 	void capture() {
-		output.get().mark();
+		RewindableByteArrayOutputStream out = output.get();
+		out.mark();
+		pushToTop(out);
 	}
 
 	String consume() {
-		return output.get().rewind();
+		RewindableByteArrayOutputStream out = output.get();
+		String result = out.rewind();
+		if (!out.isMarked()) {
+			mostRecentOutputs.remove(out);
+		}
+		return result;
 	}
 
 	void unregister() {
@@ -69,8 +79,9 @@ void unregister() {
 
 	@Override
 	public void write(int b) {
-		RewindableByteArrayOutputStream out = output.get();
-		if (out.isMarked() && out.size() < maxNumberOfBytesPerThread) {
+		RewindableByteArrayOutputStream out = getOutput();
+		if (out != null && out.size() < maxNumberOfBytesPerThread) {
+			pushToTop(out);
 			out.write(b);
 		}
 		super.write(b);
@@ -83,16 +94,29 @@ public void write(byte[] b) {
 
 	@Override
 	public void write(byte[] buf, int off, int len) {
-		RewindableByteArrayOutputStream out = output.get();
-		if (out.isMarked()) {
+		RewindableByteArrayOutputStream out = getOutput();
+		if (out != null) {
 			int actualLength = Math.max(0, Math.min(len, maxNumberOfBytesPerThread - out.size()));
 			if (actualLength > 0) {
+				pushToTop(out);
 				out.write(buf, off, actualLength);
 			}
 		}
 		super.write(buf, off, len);
 	}
 
+	private void pushToTop(RewindableByteArrayOutputStream out) {
+		if (!out.equals(mostRecentOutputs.peek())) {
+			mostRecentOutputs.remove(out);
+			mostRecentOutputs.push(out);
+		}
+	}
+
+	private RewindableByteArrayOutputStream getOutput() {
+		RewindableByteArrayOutputStream out = output.get();
+		return out.isMarked() ? out : mostRecentOutputs.peek();
+	}
+
 	static class RewindableByteArrayOutputStream extends ByteArrayOutputStream {
 
 		private final Deque<Integer> markedPositions = new ArrayDeque<>();

platform-tests/src/test/java/org/junit/platform/launcher/core/StreamInterceptorTests.java

@@ -20,24 +20,27 @@
 import java.io.PrintStream;
 import java.util.stream.IntStream;
 
+import org.junit.jupiter.api.AutoClose;
 import org.junit.jupiter.api.Test;
 
 /**
  * @since 1.3
  */
 class StreamInterceptorTests {
 
-	private ByteArrayOutputStream originalOut = new ByteArrayOutputStream();
-	private PrintStream targetStream = new PrintStream(originalOut);
+	final ByteArrayOutputStream originalOut = new ByteArrayOutputStream();
+	PrintStream targetStream = new PrintStream(originalOut);
+
+	@AutoClose
+	StreamInterceptor streamInterceptor;
 
 	@Test
 	void interceptsWriteOperationsToStreamPerThread() {
-		var streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
+		streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
 			3).orElseThrow(RuntimeException::new);
 		// @formatter:off
 		IntStream.range(0, 1000)
 				.parallel()
-				.peek(i -> targetStream.println(i))
 				.mapToObj(String::valueOf)
 				.peek(i -> streamInterceptor.capture())
 				.peek(i -> targetStream.println(i))
@@ -49,7 +52,7 @@ void interceptsWriteOperationsToStreamPerThread() {
 	void unregisterRestoresOriginalStream() {
 		var originalStream = targetStream;
 
-		var streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
+		streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
 			3).orElseThrow(RuntimeException::new);
 		assertSame(streamInterceptor, targetStream);
 
@@ -61,8 +64,8 @@ void unregisterRestoresOriginalStream() {
 	void writeForwardsOperationsToOriginalStream() throws IOException {
 		var originalStream = targetStream;
 
-		StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream, 2).orElseThrow(
-			RuntimeException::new);
+		streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
+			2).orElseThrow(RuntimeException::new);
 		assertNotSame(originalStream, targetStream);
 
 		targetStream.write('a');
@@ -73,7 +76,7 @@ void writeForwardsOperationsToOriginalStream() throws IOException {
 
 	@Test
 	void handlesNestedCaptures() {
-		var streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
+		streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
 			100).orElseThrow(RuntimeException::new);
 
 		String outermost, inner, innermost;
@@ -100,4 +103,19 @@ void handlesNestedCaptures() {
 			() -> assertEquals("innermost", innermost) //
 		);
 	}
+
+	@Test
+	void capturesOutputFromNonTestThreads() throws Exception {
+		streamInterceptor = StreamInterceptor.register(targetStream, newStream -> this.targetStream = newStream,
+			100).orElseThrow(RuntimeException::new);
+
+		streamInterceptor.capture();
+		var thread = new Thread(() -> {
+			targetStream.println("from non-test thread");
+		});
+		thread.start();
+		thread.join();
+
+		assertEquals("from non-test thread", streamInterceptor.consume().trim());
+	}
 }