Fix loop of OffsetForLeaderEpoch requests on quick leader changes

src/rdkafka_topic.c

@@ -681,9 +681,7 @@ static int rd_kafka_toppar_leader_update(rd_kafka_topic_t *rkt,
                 }
         }
 
-        if (rktp->rktp_fetch_state == RD_KAFKA_TOPPAR_FETCH_VALIDATE_EPOCH_WAIT)
-                need_epoch_validation = rd_true;
-        else if (leader_epoch > rktp->rktp_leader_epoch) {
+        if (leader_epoch > rktp->rktp_leader_epoch) {
                 rd_kafka_dbg(rktp->rktp_rkt->rkt_rk, TOPIC, "BROKER",
                              "%s [%" PRId32 "]: leader %" PRId32
                              " epoch %" PRId32 " -> leader %" PRId32
@@ -693,7 +691,9 @@ static int rd_kafka_toppar_leader_update(rd_kafka_topic_t *rkt,
                              rktp->rktp_leader_epoch, leader_id, leader_epoch);
                 rktp->rktp_leader_epoch = leader_epoch;
                 need_epoch_validation   = rd_true;
-        }
+        } else if (rktp->rktp_fetch_state ==
+                   RD_KAFKA_TOPPAR_FETCH_VALIDATE_EPOCH_WAIT)
+                need_epoch_validation = rd_true;
 
         fetching_from_follower =
             leader != NULL && rktp->rktp_broker != NULL &&

tests/0139-offset_validation_mock.c

@@ -212,6 +212,95 @@ static void do_test_ssl_error_retried(void) {
 }
 
 
+/**
+ * @brief If there's an OffsetForLeaderEpoch request which fails, and the leader
+ * changes meanwhile, we end up in an infinite loop of OffsetForLeaderEpoch
+ * requests.
+ * Specifically:
+ * a. Leader Change - causes OffsetForLeaderEpoch
+ *     request 'A'.
+ * b. Request 'A' fails with a retriable error, and we retry it.
+ * c. While waiting for Request 'A', the leader changes again, and we send a
+ *    Request 'B', but the leader epoch is not updated correctly in this
+ *    request, causing a loop.
+ *
+ * See #4425.
+ */
+static void do_test_two_leader_changes(void) {
+        const char *topic      = test_mk_topic_name(__FUNCTION__, 1);
+        const char *c1_groupid = topic;
+        rd_kafka_t *c1;
+        const char *bootstraps;
+        rd_kafka_mock_cluster_t *mcluster;
+        int msg_cnt     = 5;
+        uint64_t testid = test_id_generate();
+        rd_kafka_conf_t *conf;
+
+        SUB_TEST_QUICK();
+
+        mcluster = test_mock_cluster_new(2, &bootstraps);
+        rd_kafka_mock_topic_create(mcluster, topic, 1, 2);
+        rd_kafka_mock_partition_set_leader(mcluster, topic, 0, 1);
+
+        /* Seed the topic with messages */
+        test_produce_msgs_easy_v(topic, testid, 0, 0, msg_cnt, 10,
+                                 "bootstrap.servers", bootstraps,
+                                 "batch.num.messages", "1", NULL);
+
+        test_conf_init(&conf, NULL, 60);
+
+        test_conf_set(conf, "bootstrap.servers", bootstraps);
+        test_conf_set(conf, "auto.offset.reset", "earliest");
+        test_conf_set(conf, "debug", "protocol");
+
+        c1 = test_create_consumer(c1_groupid, NULL, conf, NULL);
+        test_consumer_subscribe(c1, topic);
+
+        /* Consume initial messages and join the group, etc. */
+        test_consumer_poll("MSG_INIT", c1, testid, 0, 0, msg_cnt, NULL);
+
+        /* The leader will change from 1->2, and the OffsetForLeaderEpoch will
+         * be sent to broker 2. We need to first fail it with
+         * an error, and then give enough time to change the leader before
+         * returning a success. */
+        rd_kafka_mock_broker_push_request_error_rtts(
+            mcluster, 2, RD_KAFKAP_OffsetForLeaderEpoch, 1,
+            RD_KAFKA_RESP_ERR_KAFKA_STORAGE_ERROR, 900);
+
+        rd_kafka_mock_broker_push_request_error_rtts(
+            mcluster, 2, RD_KAFKAP_OffsetForLeaderEpoch, 1,
+            RD_KAFKA_RESP_ERR_NO_ERROR, 1000);
+
+        rd_kafka_mock_partition_set_leader(mcluster, topic, 0, 2);
+        rd_kafka_poll(c1, 1000);
+        /* Enough time to make a request, fail with a retriable error, and
+         * retry. */
+        rd_sleep(1);
+
+        /* Reset leader. */
+        rd_kafka_mock_partition_set_leader(mcluster, topic, 0, 1);
+        rd_kafka_poll(c1, 1000);
+        rd_sleep(1);
+
+        /* There should be no infinite loop of OffsetForLeaderEpoch, and
+         * consequently, we should be able to consume these messages as a sign
+         * of success. */
+        test_produce_msgs_easy_v(topic, testid, 0, 0, msg_cnt, 10,
+                                 "bootstrap.servers", bootstraps,
+                                 "batch.num.messages", "1", NULL);
+
+        test_consumer_poll("MSG_INIT", c1, testid, 0, 0, msg_cnt, NULL);
+
+
+        rd_kafka_destroy(c1);
+
+        test_mock_cluster_destroy(mcluster);
+
+        TEST_LATER_CHECK();
+        SUB_TEST_PASS();
+}
+
+
 int main_0139_offset_validation_mock(int argc, char **argv) {
 
         if (test_needs_auth()) {
@@ -223,5 +312,7 @@ int main_0139_offset_validation_mock(int argc, char **argv) {
 
         do_test_ssl_error_retried();
 
+        do_test_two_leader_changes();
+
         return 0;
 }