Log a warning when retransforming already-loaded classes fails

dd-java-agent/agent-installer/src/main/java/datadog/trace/agent/tooling/AgentInstaller.java

@@ -164,6 +164,7 @@ public static ClassFileTransformer installBytebuddyAgent(
             .with(AgentStrategies.transformerDecorator())
             .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
             .with(AgentStrategies.rediscoveryStrategy())
+            .with(new RedefinitionLoggingListener())
             .with(AgentStrategies.locationStrategy())
             .with(AgentStrategies.poolStrategy())
             .with(AgentBuilder.DescriptionStrategy.Default.POOL_ONLY)
@@ -176,12 +177,7 @@ public static ClassFileTransformer installBytebuddyAgent(
             .ignore(globalIgnoresMatcher(skipAdditionalLibraryMatcher));
 
     if (DEBUG) {
-      agentBuilder =
-          agentBuilder
-              .with(AgentBuilder.RedefinitionStrategy.RETRANSFORMATION)
-              .with(AgentStrategies.rediscoveryStrategy())
-              .with(new RedefinitionLoggingListener())
-              .with(new TransformLoggingListener());
+      agentBuilder = agentBuilder.with(new TransformLoggingListener());
     }
 
     for (final AgentBuilder.Listener listener : listeners) {
@@ -403,6 +399,12 @@ public Iterable<? extends List<Class<?>>> onError(
         final List<Class<?>> types) {
       if (DEBUG) {
         log.debug("Exception while retransforming {} classes: {}", batch.size(), batch, throwable);
+      } else {
+        // a failed batch leaves every class in it uninstrumented, so don't keep this silent
+        log.warn(
+            "Exception while retransforming {} classes, instrumentation may be incomplete: {}",
+            batch.size(),
+            throwable.toString());
       }
       return Collections.emptyList();
     }

dd-java-agent/agent-installer/src/test/java/datadog/trace/agent/tooling/RedefinitionLoggingListenerForkedTest.java

@@ -0,0 +1,46 @@
+package datadog.trace.agent.tooling;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.spi.ILoggingEvent;
+import ch.qos.logback.core.read.ListAppender;
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+import org.slf4j.LoggerFactory;
+
+class RedefinitionLoggingListenerForkedTest {
+
+  @Test
+  void warnsWhenRetransformationBatchFailsWithoutDebugLogging() {
+    // AgentInstaller.DEBUG is fixed when the class initializes, so keep it off in this fork
+    ((Logger) LoggerFactory.getLogger(AgentInstaller.class)).setLevel(Level.INFO);
+    Logger logger =
+        (Logger) LoggerFactory.getLogger(AgentInstaller.RedefinitionLoggingListener.class);
+    logger.setLevel(Level.INFO);
+    ListAppender<ILoggingEvent> appender = new ListAppender<>();
+    appender.start();
+    logger.addAppender(appender);
+    try {
+      List<Class<?>> batch = Arrays.asList(String.class, Integer.class);
+
+      Iterable<? extends List<Class<?>>> retry =
+          new AgentInstaller.RedefinitionLoggingListener()
+              .onError(
+                  0, batch, new UnsupportedOperationException("class redefinition failed"), batch);
+
+      assertFalse(retry.iterator().hasNext());
+      assertEquals(1, appender.list.size());
+      ILoggingEvent event = appender.list.get(0);
+      assertEquals(Level.WARN, event.getLevel());
+      assertTrue(event.getFormattedMessage().contains("retransforming 2 classes"));
+      assertTrue(event.getFormattedMessage().contains("class redefinition failed"));
+    } finally {
+      logger.detachAppender(appender);
+    }
+  }
+}