fix(virtual-thread): Preload scope classes to prevent virtual thread deadlock

PerfectSlayer · PerfectSlayer · commit c521b50e1c4e · 2026-04-16T10:49:54.000+02:00
Preload ScopeContext and ScopeStack classes in ContinuableScopeManager constructor to avoid class loading on virtual thread mount path. DatadogClassLoader's synchronized I/O from JarFile can pin carrier threads and deadlock the application.
diff --git a/dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/main/java/datadog/trace/instrumentation/java/lang/jdk21/VirtualThreadInstrumentation.java b/dd-java-agent/instrumentation/java/java-lang/java-lang-21.0/src/main/java/datadog/trace/instrumentation/java/lang/jdk21/VirtualThreadInstrumentation.java
@@ -66,10 +66,22 @@ 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.
+  private static final String[] PRELOAD_CLASS_NAMES = {
+    "datadog.trace.core.scopemanager.ScopeContext", "datadog.trace.core.scopemanager.ScopeStack"
+  };
+
   public VirtualThreadInstrumentation() {
     super("java-lang", "java-lang-21", "virtual-thread");
   }
 
+  @Override
+  public String[] preloadClassNames() {
+    return PRELOAD_CLASS_NAMES;
+  }
+
   @Override
   public String instrumentedType() {
     return VIRTUAL_THREAD_CLASS_NAME;
