fix(spring-jakarta): Warn when Kafka producer tracing silently fails

When ProducerFactory.addPostProcessor() is a no-op (the interface
default), the Sentry post-processor is silently dropped and the
customer gets zero producer tracing with no signal.

Verify registration succeeded via getPostProcessors() after each
addPostProcessor() call, and log a WARNING naming the factory bean
and pointing toward SentryKafkaProducer.wrap() as the manual fallback.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry-spring-jakarta/src/main/java/io/sentry/spring/jakarta/kafka/SentryKafkaProducerBeanPostProcessor.java

@@ -1,6 +1,7 @@
 package io.sentry.spring.jakarta.kafka;
 
 import io.sentry.ScopesAdapter;
+import io.sentry.SentryLevel;
 import io.sentry.kafka.SentryKafkaProducer;
 import org.apache.kafka.clients.producer.Producer;
 import org.jetbrains.annotations.ApiStatus;
@@ -22,11 +23,10 @@
  * KafkaTemplate} beans are left untouched, so all customer-configured listeners, interceptors and
  * observation settings are preserved.
  *
- * <p>Idempotent: re-running on the same factory does not register the post-processor twice.
- *
  * <p>Note: {@link ProducerFactory#addPostProcessor(ProducerPostProcessor)} is a default method on
- * the interface. Custom factories that do not extend {@code DefaultKafkaProducerFactory} and do not
- * implement {@code addPostProcessor} will silently no-op.
+ * the interface that is a no-op unless overridden. Custom factories that do not extend {@code
+ * DefaultKafkaProducerFactory} will not receive Sentry producer instrumentation; a warning is
+ * logged at startup in that case.
  */
 @ApiStatus.Internal
 public final class SentryKafkaProducerBeanPostProcessor
@@ -38,14 +38,21 @@ public final class SentryKafkaProducerBeanPostProcessor
       final @NotNull Object bean, final @NotNull String beanName) throws BeansException {
     if (bean instanceof ProducerFactory) {
       final @NotNull ProducerFactory factory = (ProducerFactory) bean;
-
-      for (final Object existing : factory.getPostProcessors()) {
-        if (existing instanceof SentryProducerPostProcessor) {
-          return bean;
-        }
+      final @NotNull SentryProducerPostProcessor pp = new SentryProducerPostProcessor<>();
+      factory.addPostProcessor(pp);
+      if (!factory.getPostProcessors().contains(pp)) {
+        ScopesAdapter.getInstance()
+            .getOptions()
+            .getLogger()
+            .log(
+                SentryLevel.WARNING,
+                "Sentry Kafka producer tracing not active for ProducerFactory '%s' (%s). "
+                    + "addPostProcessor() was not honored — the factory may not extend "
+                    + "DefaultKafkaProducerFactory. Wrap producers manually with "
+                    + "SentryKafkaProducer.wrap(producer).",
+                beanName,
+                factory.getClass().getName());
       }
-
-      factory.addPostProcessor(new SentryProducerPostProcessor<>());
     }
     return bean;
   }

sentry-spring-jakarta/src/test/kotlin/io/sentry/spring/jakarta/kafka/SentryKafkaProducerBeanPostProcessorTest.kt

@@ -8,7 +8,6 @@ import org.apache.kafka.clients.producer.Producer
 import org.mockito.kotlin.any
 import org.mockito.kotlin.argumentCaptor
 import org.mockito.kotlin.mock
-import org.mockito.kotlin.never
 import org.mockito.kotlin.verify
 import org.mockito.kotlin.whenever
 import org.springframework.kafka.core.DefaultKafkaProducerFactory
@@ -20,7 +19,8 @@ class SentryKafkaProducerBeanPostProcessorTest {
   @Test
   fun `registers Sentry post-processor on ProducerFactory`() {
     val factory = mock<ProducerFactory<String, String>>()
-    whenever(factory.postProcessors).thenReturn(emptyList())
+    val pp = SentryKafkaProducerBeanPostProcessor.SentryProducerPostProcessor<String, String>()
+    whenever(factory.postProcessors).thenReturn(listOf(pp))
     val processor = SentryKafkaProducerBeanPostProcessor()
 
     processor.postProcessAfterInitialization(factory, "kafkaProducerFactory")
@@ -33,16 +33,16 @@ class SentryKafkaProducerBeanPostProcessorTest {
   }
 
   @Test
-  fun `is idempotent when Sentry post-processor is already registered`() {
+  fun `does not throw when addPostProcessor is a no-op (default interface method)`() {
+    // Factory using the default no-op addPostProcessor / getPostProcessors
     val factory = mock<ProducerFactory<String, String>>()
-    val existing =
-      SentryKafkaProducerBeanPostProcessor.SentryProducerPostProcessor<String, String>()
-    whenever(factory.postProcessors).thenReturn(listOf(existing))
+    whenever(factory.postProcessors).thenReturn(emptyList())
     val processor = SentryKafkaProducerBeanPostProcessor()
 
-    processor.postProcessAfterInitialization(factory, "kafkaProducerFactory")
+    // Should complete without throwing, and log a warning via ScopesAdapter
+    processor.postProcessAfterInitialization(factory, "myFactory")
 
-    verify(factory, never()).addPostProcessor(any())
+    verify(factory).addPostProcessor(any())
   }
 
   @Test
@@ -58,7 +58,8 @@ class SentryKafkaProducerBeanPostProcessorTest {
   @Test
   fun `returns the same bean instance`() {
     val factory = mock<ProducerFactory<String, String>>()
-    whenever(factory.postProcessors).thenReturn(emptyList())
+    val pp = SentryKafkaProducerBeanPostProcessor.SentryProducerPostProcessor<String, String>()
+    whenever(factory.postProcessors).thenReturn(listOf(pp))
     val processor = SentryKafkaProducerBeanPostProcessor()
 
     val result = processor.postProcessAfterInitialization(factory, "kafkaProducerFactory")