Test helper consistency in MuzzleVersionScanPlugin without requiring HelperInjector

This helps decouple internals and provides the same feedback with simpler code

Author: mcculls

dd-java-agent/agent-tooling/src/main/java/datadog/trace/agent/tooling/muzzle/MuzzleVersionScanPlugin.java

@@ -1,7 +1,6 @@
 package datadog.trace.agent.tooling.muzzle;
 
 import datadog.trace.agent.tooling.AdviceShader;
-import datadog.trace.agent.tooling.HelperInjector;
 import datadog.trace.agent.tooling.InstrumenterModule;
 import datadog.trace.agent.tooling.bytebuddy.SharedTypePools;
 import datadog.trace.agent.tooling.bytebuddy.matcher.ClassLoaderMatchers;
@@ -72,16 +71,15 @@ public static void assertInstrumentationMuzzled(
     if (assertPass) {
       for (InstrumenterModule module : toBeTested) {
         try {
-          // verify helper injector works
+          // verify helper consistency
           final String[] helperClassNames = module.helperClassNames();
           if (helperClassNames.length > 0) {
-            new HelperInjector(
-                    module.useAgentCodeSource(),
-                    MuzzleVersionScanPlugin.class.getSimpleName(),
-                    createHelperMap(module))
-                .transform(null, null, testApplicationLoader, null, null);
+            HelperClassLoader helperClassLoader = new HelperClassLoader(testApplicationLoader);
+            for (Map.Entry<String, byte[]> helper : createHelperMap(module).entrySet()) {
+              helperClassLoader.injectClass(helper.getKey(), helper.getValue());
+            }
           }
-        } catch (final Exception e) {
+        } catch (final Throwable e) {
           System.err.println(
               "FAILED HELPER INJECTION. Are Helpers being injected in the correct order?");
           System.err.println(e.getMessage());
@@ -107,6 +105,18 @@ private static synchronized List<InstrumenterModule> toBeTested(
     return toBeTested;
   }
 
+  // Exposes ClassLoader.defineClass() to test helper consistency
+  // without requiring java.lang.instrument.Instrumentation agent
+  static final class HelperClassLoader extends ClassLoader {
+    HelperClassLoader(ClassLoader parent) {
+      super(parent);
+    }
+
+    public void injectClass(String name, byte[] bytecode) {
+      defineClass(name, bytecode, 0, bytecode.length);
+    }
+  }
+
   private static Map<String, byte[]> createHelperMap(final InstrumenterModule module)
       throws IOException {
     String[] helperClasses = module.helperClassNames();