feat: Add standalone Java SeekToBeginning test to reproduce the issue

Created:
- SeekToBeginningTest.java: Standalone Java test that reproduces the seekToBeginning() hang
- Dockerfile.seektest: Docker setup for running the test
- pom.xml: Maven build configuration
- Updated docker-compose.yml to include seek-test service

This test simulates what Schema Registry does:
1. Create KafkaConsumer connected to gateway
2. Assign to _schemas topic partition 0
3. Call seekToBeginning()
4. Poll for records

Expected behavior: Should send ListOffsets and then Fetch
Actual behavior: Blocks indefinitely after seekToBeginning()

test/kafka/kafka-client-loadtest/Dockerfile.seektest

@@ -0,0 +1,20 @@
+FROM openjdk:11-jdk-slim
+
+# Install Maven
+RUN apt-get update && apt-get install -y maven && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+
+# Create source directory
+RUN mkdir -p src/main/java
+
+# Copy source and build files
+COPY SeekToBeginningTest.java src/main/java/
+COPY pom.xml .
+
+# Compile and package
+RUN mvn clean package -DskipTests
+
+# Run the test
+ENTRYPOINT ["java", "-cp", "target/seek-test.jar", "SeekToBeginningTest"]
+CMD ["kafka-gateway:9093"]

test/kafka/kafka-client-loadtest/SeekToBeginningTest.java

@@ -0,0 +1,73 @@
+import org.apache.kafka.clients.consumer.*;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.serialization.ByteArrayDeserializer;
+import java.util.*;
+
+/**
+ * Test program to reproduce the seekToBeginning() hang issue
+ * 
+ * This simulates what Schema Registry does:
+ * 1. Create KafkaConsumer
+ * 2. Assign to partition
+ * 3. Call seekToBeginning()
+ * 4. Poll for records
+ * 
+ * Expected behavior: Consumer should send ListOffsets and then Fetch requests
+ * Observed behavior: Consumer sends InitProducerId but not ListOffsets, then
+ * hangs
+ */
+public class SeekToBeginningTest {
+    public static void main(String[] args) throws Exception {
+        String bootstrapServers = "localhost:9093";
+        String topicName = "_schemas";
+
+        if (args.length > 0) {
+            bootstrapServers = args[0];
+        }
+
+        Properties props = new Properties();
+        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, bootstrapServers);
+        props.put(ConsumerConfig.GROUP_ID_CONFIG, "test-seek-group");
+        props.put(ConsumerConfig.CLIENT_ID_CONFIG, "test-seek-client");
+        props.put(ConsumerConfig.AUTO_OFFSET_RESET_CONFIG, "earliest");
+        props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
+        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);
+        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, ByteArrayDeserializer.class);
+        props.put(ConsumerConfig.SESSION_TIMEOUT_MS_CONFIG, "45000");
+        props.put(ConsumerConfig.REQUEST_TIMEOUT_MS_CONFIG, "60000");
+
+        System.out.println("[TEST] Creating KafkaConsumer connecting to " + bootstrapServers);
+        KafkaConsumer<byte[], byte[]> consumer = new KafkaConsumer<>(props);
+
+        TopicPartition tp = new TopicPartition(topicName, 0);
+        List<TopicPartition> partitions = Arrays.asList(tp);
+
+        System.out.println("[TEST] Assigning to partition: " + tp);
+        consumer.assign(partitions);
+
+        System.out.println("[TEST] Calling seekToBeginning()...");
+        long startTime = System.currentTimeMillis();
+        consumer.seekToBeginning(partitions);
+        long seekTime = System.currentTimeMillis() - startTime;
+        System.out.println("[TEST] seekToBeginning() completed in " + seekTime + "ms");
+
+        System.out.println("[TEST] Starting poll loop (30 second timeout per poll)...");
+        for (int i = 0; i < 3; i++) {
+            System.out.println("[POLL " + (i + 1) + "] Polling for records...");
+            long pollStart = System.currentTimeMillis();
+            ConsumerRecords<byte[], byte[]> records = consumer.poll(java.time.Duration.ofSeconds(30));
+            long pollTime = System.currentTimeMillis() - pollStart;
+            System.out.println("[POLL " + (i + 1) + "] Got " + records.count() + " records in " + pollTime + "ms");
+
+            for (ConsumerRecord<byte[], byte[]> record : records) {
+                System.out.println("  [RECORD] offset=" + record.offset() + ", key.len=" +
+                        (record.key() != null ? record.key().length : 0) +
+                        ", value.len=" + (record.value() != null ? record.value().length : 0));
+            }
+        }
+
+        System.out.println("[TEST] Closing consumer...");
+        consumer.close();
+        System.out.println("[TEST] Done!");
+    }
+}

test/kafka/kafka-client-loadtest/docker-compose.yml

@@ -307,6 +307,24 @@ services:
     profiles:
       - debug
 
+  # SeekToBeginning test - reproduces the hang issue
+  seek-test:
+    build:
+      context: .
+      dockerfile: Dockerfile.seektest
+    container_name: loadtest-seek-test
+    depends_on:
+      kafka-gateway:
+        condition: service_healthy
+      schema-registry:
+        condition: service_healthy
+    environment:
+      - KAFKA_BOOTSTRAP_SERVERS=kafka-gateway:9093
+    networks:
+      - kafka-loadtest-net
+    entrypoint: ["java", "-cp", "target/seek-test.jar", "SeekToBeginningTest"]
+    command: ["kafka-gateway:9093"]
+
 volumes:
   prometheus-data:
   grafana-data:

test/kafka/kafka-client-loadtest/pom.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>io.confluent.test</groupId>
+    <artifactId>seek-test</artifactId>
+    <version>1.0</version>
+
+    <properties>
+        <maven.compiler.source>11</maven.compiler.source>
+        <maven.compiler.target>11</maven.compiler.target>
+        <kafka.version>3.6.0</kafka.version>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.apache.kafka</groupId>
+            <artifactId>kafka-clients</artifactId>
+            <version>${kafka.version}</version>
+        </dependency>
+        <dependency>
+            <groupId>org.slf4j</groupId>
+            <artifactId>slf4j-simple</artifactId>
+            <version>2.0.0</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>3.8.1</version>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-shade-plugin</artifactId>
+                <version>3.2.4</version>
+                <executions>
+                    <execution>
+                        <phase>package</phase>
+                        <goals>
+                            <goal>shade</goal>
+                        </goals>
+                        <configuration>
+                            <transformers>
+                                <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
+                                    <mainClass>SeekToBeginningTest</mainClass>
+                                </transformer>
+                            </transformers>
+                            <finalName>seek-test</finalName>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>