Better classLoaderMatcher testing

Author: trask

conventions/src/main/kotlin/io.opentelemetry.instrumentation.javaagent-testing.gradle.kts

@@ -10,6 +10,10 @@ val failOnContextLeakProperty = providers.gradleProperty("failOnContextLeak")
   .map { it != "false" }
   .orElse(true)
 
+val failOnMuzzleMismatchProperty = providers.gradleProperty("failOnMuzzleMismatch")
+  .map { it == "true" }
+  .orElse(false)
+
 val testIndyProperty = providers.gradleProperty("testIndy")
   .map { it == "true" }
   .orElse(false)
@@ -85,6 +89,9 @@ class JavaagentTestArgumentsProvider(
   @get:Input
   val failOnContextLeak: Boolean,
 
+  @get:Input
+  val failOnMuzzleMismatch: Boolean,
+
   @get:Input
   val testIndy: Boolean,
 
@@ -100,6 +107,7 @@ class JavaagentTestArgumentsProvider(
       "-Dotel.javaagent.experimental.initializer.jar=${shadowJar.absolutePath}",
       "-Dotel.javaagent.testing.additional-library-ignores.enabled=false",
       "-Dotel.javaagent.testing.fail-on-context-leak=$failOnContextLeak",
+      "-Dotel.javaagent.testing.fail-on-muzzle-mismatch=$failOnMuzzleMismatch",
       // prevent sporadic gradle deadlocks, see SafeLogger for more details
       "-Dotel.javaagent.testing.transform-safe-logging.enabled=true",
       // Reduce noise in assertion messages since we don't need to verify this in most tests. We check
@@ -140,13 +148,15 @@ afterEvaluate {
     dependsOn(agentForTesting.buildDependencies)
 
     val failOnContextLeakOverride = failOnContextLeakProperty.get()
+    val failOnMuzzleMismatchOverride = failOnMuzzleMismatchProperty.get()
     val testIndyEnabled = testIndyProperty.get()
 
     jvmArgumentProviders.add(
       JavaagentTestArgumentsProvider(
         agentShadowJar,
         shadowJar.archiveFile.get().asFile,
         failOnContextLeakOverride,
+        failOnMuzzleMismatchOverride,
         testIndyEnabled,
         denyUnsafe
       )

javaagent-tooling/src/main/java/io/opentelemetry/javaagent/tooling/instrumentation/MuzzleMatcher.java

@@ -34,6 +34,9 @@
  */
 class MuzzleMatcher implements AgentBuilder.RawMatcher {
 
+  private static final boolean FAIL_ON_MUZZLE_MISMATCH =
+      Boolean.getBoolean("otel.javaagent.testing.fail-on-muzzle-mismatch");
+
   private static final Logger muzzleLogger = Logger.getLogger(MuzzleMatcher.class.getName());
 
   private final TransformSafeLogger instrumentationLogger;
@@ -94,6 +97,11 @@ private boolean doesMatch(ClassLoader classLoader) {
           muzzleLogger.log(muzzleLogLevel, "-- {0}", mismatch);
         }
       }
+      if (FAIL_ON_MUZZLE_MISMATCH) {
+        throw new IllegalStateException(
+            "Muzzle mismatch detected for instrumentation "
+                + instrumentationModule.instrumentationName());
+      }
     } else {
       if (instrumentationLogger.isLoggable(FINE)) {
         instrumentationLogger.log(