Prevent smoke-test teardown race

dd-smoke-tests/src/main/java/datadog/smoketest/OutputThreads.java

@@ -1,5 +1,6 @@
 package datadog.smoketest;
 
+import datadog.trace.api.internal.VisibleForTesting;
 import java.io.Closeable;
 import java.io.File;
 import java.io.FileNotFoundException;
@@ -23,17 +24,26 @@ public class OutputThreads implements Closeable {
   private static final int MAX_LINE_SIZE = 1024 * 1024;
   private static final int DEFAULT_TIMEOUT_MILLIS = 10_000;
 
-  final ThreadGroup tg = new ThreadGroup("smoke-output");
+  final ThreadGroup tg;
   final List<String> testLogMessages = new ArrayList<>();
 
+  public OutputThreads() {
+    this(new ThreadGroup("smoke-output"));
+  }
+
+  @VisibleForTesting
+  OutputThreads(ThreadGroup tg) {
+    this.tg = tg;
+  }
+
   public void close() {
     tg.interrupt();
     Thread[] threads = new Thread[tg.activeCount()];
-    tg.enumerate(threads);
+    int threadCount = tg.enumerate(threads);
 
-    for (Thread thread : threads) {
+    for (int i = 0; i < threadCount; i++) {
       try {
-        thread.join(THREAD_JOIN_TIMEOUT_MILLIS);
+        threads[i].join(THREAD_JOIN_TIMEOUT_MILLIS);
       } catch (InterruptedException e) {
         // ignore
       }

dd-smoke-tests/src/test/java/datadog/smoketest/OutputThreadsTest.java

@@ -0,0 +1,26 @@
+package datadog.smoketest;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import org.junit.jupiter.api.Test;
+
+class OutputThreadsTest {
+
+  @Test
+  void closeOnlyJoinsEnumeratedThreads() {
+    ThreadGroup shrinkingThreadGroup =
+        new ThreadGroup("shrinking-smoke-output") {
+          @Override
+          public int activeCount() {
+            return 1;
+          }
+
+          @Override
+          public int enumerate(Thread[] threads) {
+            return 0;
+          }
+        };
+
+    assertDoesNotThrow(() -> new OutputThreads(shrinkingThreadGroup).close());
+  }
+}