Attribute non-test thread output to most recent test thread (#4200)

When using frameworks or running external processes, test output is
often written on other threads than the test. When tests are executed
sequentially that output can be attributed unambiguously to the current
test. If tests are run in parallel, picking the right test to attribute
output to becomes much harder, though.

Author: marcphilipp

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());
+	}
 }