Fix GraalVM smoke test on Java 21

The root issue was that the reference to MemFDUnixWriter in the TracerInstaller
bytecode was being picked up by HelperScanner, which is a utility that we use to
avoid having to maintain an exact list of classes to inject into GraalVM in order
to activate the tracer in the final native binary via VMRuntimeInstrumentation
and TracerActivation.

The simplest solution was to use reflection to load MemFDUnixWriter.

The alternative would have been to enhance HelperScanner to be able to exclude
types from the list of classes it finds. However this would be more complicated.

Author: mcculls

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/TracerInstaller.java

@@ -2,13 +2,14 @@
 
 import datadog.communication.ddagent.SharedCommunicationObjects;
 import datadog.environment.OperatingSystem;
-import datadog.trace.agent.tooling.servicediscovery.MemFDUnixWriter;
 import datadog.trace.api.Config;
 import datadog.trace.api.GlobalTracer;
 import datadog.trace.api.Platform;
 import datadog.trace.bootstrap.instrumentation.api.AgentTracer;
 import datadog.trace.bootstrap.instrumentation.api.ProfilingContextIntegration;
 import datadog.trace.core.CoreTracer;
+import datadog.trace.core.CoreTracer.CoreTracerBuilder;
+import datadog.trace.core.servicediscovery.ForeignMemoryWriter;
 import datadog.trace.core.servicediscovery.ServiceDiscovery;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -21,15 +22,16 @@ public static synchronized void installGlobalTracer(
       ProfilingContextIntegration profilingContextIntegration) {
     if (Config.get().isTraceEnabled() || Config.get().isCiVisibilityEnabled()) {
       if (!(GlobalTracer.get() instanceof CoreTracer)) {
-        CoreTracer tracer =
+        CoreTracerBuilder tracerBuilder =
             CoreTracer.builder()
                 .sharedCommunicationObjects(sharedCommunicationObjects)
                 .profilingContextIntegration(profilingContextIntegration)
                 .reportInTracerFlare()
-                .pollForTracingConfiguration()
-                .serviceDiscovery(getServiceDiscovery())
-                .build();
-        installGlobalTracer(tracer);
+                .pollForTracingConfiguration();
+
+        maybeEnableServiceDiscovery(tracerBuilder);
+
+        installGlobalTracer(tracerBuilder.build());
       } else {
         log.debug("GlobalTracer already registered.");
       }
@@ -38,17 +40,26 @@ public static synchronized void installGlobalTracer(
     }
   }
 
-  private static ServiceDiscovery getServiceDiscovery() {
+  private static void maybeEnableServiceDiscovery(CoreTracerBuilder tracerBuilder) {
     if (!OperatingSystem.isLinux()) {
       log.debug("service discovery not supported outside linux");
-      return null;
+      return;
     }
     // make sure this branch is not considered possible for graalvm artifact
-    if (!Platform.isNativeImageBuilder() && !Platform.isNativeImage()) {
-      return new ServiceDiscovery(new MemFDUnixWriter());
+    if (Platform.isNativeImageBuilder()) {
+      log.debug("service discovery not supported on native images");
+      return;
+    }
+    try {
+      // use reflection to load MemFDUnixWriter so it doesn't get picked up when we transitively
+      // look for all tracer class dependencies to install in GraalVM via VMRuntimeInstrumentation
+      Class<?> memFdClass =
+          Class.forName("datadog.trace.agent.tooling.servicediscovery.MemFDUnixWriter");
+      ForeignMemoryWriter memFd = (ForeignMemoryWriter) memFdClass.getConstructor().newInstance();
+      tracerBuilder.serviceDiscovery(new ServiceDiscovery(memFd));
+    } catch (Throwable e) {
+      log.debug("service discovery not supported", e);
     }
-    log.debug("service discovery not supported on native images");
-    return null;
   }
 
   public static void installGlobalTracer(final CoreTracer tracer) {