Fix startup deadlock with virtual threads

dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/main/java/datadog/trace/instrumentation/java/lang/jdk21/VirtualThreadInstrumentation.java

@@ -66,11 +66,14 @@ public final class VirtualThreadInstrumentation extends InstrumenterModule.Conte
         Instrumenter.HasMethodAdvice,
         ExcludeFilterProvider {
 
-  // Preload classes used by Context.swap() to avoid class loading on the virtual thread mount path.
-  // DatadogClassLoader loads these from a JarFile using synchronized I/O, which pins
-  // virtual thread carrier threads and can deadlock the application.
+  /**
+   * Preloads mount/unmount helpers to avoid class loading during a JVMTI mount transition. Class
+   * loading can wait for that same transition to finish, or pin carriers in synchronized jar I/O.
+   */
   private static final String[] PRELOAD_CLASS_NAMES = {
-    "datadog.trace.core.scopemanager.ScopeContext", "datadog.trace.core.scopemanager.ScopeStack"
+    VIRTUAL_THREAD_STATE_CLASS_NAME,
+    "datadog.trace.core.scopemanager.ScopeContext",
+    "datadog.trace.core.scopemanager.ScopeStack"
   };
 
   public VirtualThreadInstrumentation() {

dd-smoke-tests/concurrent/java-21/src/main/java/datadog/smoketest/concurrent/VirtualThreadStartup.java

@@ -0,0 +1,12 @@
+package datadog.smoketest.concurrent;
+
+public class VirtualThreadStartup {
+  public static void main(String[] args) throws InterruptedException {
+    boolean[] ran = {false};
+    Thread thread = Thread.startVirtualThread(() -> ran[0] = true);
+    thread.join();
+    if (!ran[0]) {
+      throw new AssertionError("Virtual thread did not execute");
+    }
+  }
+}

dd-smoke-tests/concurrent/java-21/src/test/java/datadog/smoketest/concurrent/VirtualThreadStartupSmokeTest.java

@@ -0,0 +1,28 @@
+package datadog.smoketest.concurrent;
+
+import static datadog.smoketest.backend.AgentBackend.testAgent;
+import static java.util.concurrent.TimeUnit.SECONDS;
+
+import datadog.smoketest.SmokeCliApp;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.RegisterExtension;
+
+class VirtualThreadStartupSmokeTest {
+  @RegisterExtension
+  static final SmokeCliApp app =
+      SmokeCliApp.named("virtual-thread-startup")
+          .mainClass(
+              "datadog.smoketest.concurrent.VirtualThreadStartup",
+              System.getProperty("datadog.smoketest.shadowJar.path"))
+          .jvmArgs(
+              "-Djdk.virtualThreadScheduler.parallelism=1",
+              "-Djdk.virtualThreadScheduler.maxPoolSize=1")
+          .backend(testAgent())
+          .skipTelemetryCheck()
+          .build();
+
+  @Test
+  void startsVirtualThreadWithoutActiveContext() {
+    app.assertCompletesWithValue(30, SECONDS, 0);
+  }
+}