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KAFKA-18646: Null records in fetch response breaks librdkafka #18726

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Jan 29, 2025
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KAFKA-18646: Null records in fetch response breaks librdkafka #18726

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b679c00
KAFKA-18646: Null records in fetch response breaks librdkafka
ijuma Jan 28, 2025
d3ccbd9
Make records non-nullable
ijuma Jan 28, 2025
072533d
Fix tests
ijuma Jan 28, 2025
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5 changes: 3 additions & 2 deletions clients/src/main/java/org/apache/kafka/common/requests/FetchResponse.java
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Expand Up @@ -196,7 +196,8 @@ public static FetchResponseData.PartitionData partitionResponse(int partition, E
return new FetchResponseData.PartitionData()
.setPartitionIndex(partition)
.setErrorCode(error.code())
.setHighWatermark(FetchResponse.INVALID_HIGH_WATERMARK);
.setHighWatermark(FetchResponse.INVALID_HIGH_WATERMARK)
.setRecords(MemoryRecords.EMPTY);
}

/**
Expand Down Expand Up @@ -285,4 +286,4 @@ private static FetchResponseData toMessage(Errors error,
.setSessionId(sessionId)
.setResponses(topicResponseList);
}
}
}
2 changes: 1 addition & 1 deletion clients/src/main/resources/common/message/FetchResponse.json
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Expand Up @@ -106,7 +106,7 @@
]},
{ "name": "PreferredReadReplica", "type": "int32", "versions": "11+", "default": "-1", "ignorable": false, "entityType": "brokerId",
"about": "The preferred read replica for the consumer to use on its next fetch request."},
{ "name": "Records", "type": "records", "versions": "0+", "nullableVersions": "0+", "about": "The record data."}
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Removing nullableVersions would prevent Kafka from "reading" null records. This could potentially pose a risk. Therefore, we might consider a strategy where Kafka strictly enforces that null records are never "written" while still maintaining the ability to "read" them.

In summary, I favor @mumrah's suggestion of adding a null check. Furthermore, we should include a comment explaining the rationale behind preventing Kafka from writing null records.

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Please elaborate on the risk - what is the exact use case where that would happen? I couldn't come up with one.

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@ijuma ijuma Jan 29, 2025 •
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Keep in mind that released versions of Kafka never return null records - if that ever happened, it would have broken librdkafka. Same for any other implementation of the Kafka protocol.

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Please elaborate on the risk - what is the exact use case where that would happen? I couldn't come up with one.

I believe FetchResponse.json should be considered part of the public interface. Consequently, modifying the "released spec" carries inherent risks. We cannot guarantee that no external implementations adhere to our specification. For instance, other server implementations might return null records, and after this PR, our 4.0 client would no longer be able to read them.

Keep in mind that released versions of Kafka never return null records

that is true and it does not violate the spec, right? I mean "apache kafka can never return null even though the spec says it is valid to return null"

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@ijuma ijuma Jan 29, 2025 •
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I disagree, the actual spec here is that this field can never be null. As I said, there is no Kafka implementation that doesn't work with librdkafka and related clients - it constitutes 30-50% of Kafka clients in the wild.

What happened here is that the spec definition was actually incorrect - it specified the field as nullable even though it couldn't be. And that bug meant we accidentally broke a large part of the ecosystem. I don't see the benefit in not fixing the spec.

Again, let's discuss a concrete use case - you're worried that the Java consumer might break because there is an unknown Kafka broker that implements this differently?

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Again, let's discuss a concrete use case - you're worried that the Java consumer might break because there is an unknown Kafka broker that implements this differently?

I have no evidence that there is a broker implementation that returns null records. If librdkafka and related clients constitute 30-50% of clients, it is acceptable to fix the specification.

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Since this behavior in librdkafka has existed for a long time, and we have not seen an issue with null records prior to the recent changes, I think it is safe to say we have never returned null records.

I think fixing the protocol spec makes sense.

{ "name": "Records", "type": "records", "versions": "0+", "about": "The record data."}
]}
]},
{ "name": "NodeEndpoints", "type": "[]NodeEndpoint", "versions": "16+", "taggedVersions": "16+", "tag": 0,
Expand Down
6 changes: 4 additions & 2 deletions clients/src/test/java/org/apache/kafka/common/requests/RequestResponseTest.java
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Expand Up @@ -2024,7 +2024,8 @@ private FetchResponse createFetchResponse(int sessionId) {
.setPartitionIndex(1)
.setHighWatermark(1000000)
.setLogStartOffset(0)
.setAbortedTransactions(abortedTransactions));
.setAbortedTransactions(abortedTransactions)
.setRecords(MemoryRecords.EMPTY));
return FetchResponse.parse(FetchResponse.of(Errors.NONE, 25, sessionId,
responseData).serialize(FETCH.latestVersion()), FETCH.latestVersion());
}
Expand All @@ -2048,7 +2049,8 @@ private FetchResponse createFetchResponse(boolean includeAborted) {
.setPartitionIndex(1)
.setHighWatermark(1000000)
.setLogStartOffset(0)
.setAbortedTransactions(abortedTransactions));
.setAbortedTransactions(abortedTransactions)
.setRecords(MemoryRecords.EMPTY));
return FetchResponse.parse(FetchResponse.of(Errors.NONE, 25, INVALID_SESSION_ID,
responseData).serialize(FETCH.latestVersion()), FETCH.latestVersion());
}
Expand Down
31 changes: 30 additions & 1 deletion core/src/test/scala/integration/kafka/api/AuthorizerIntegrationTest.scala
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Expand Up @@ -37,7 +37,7 @@ import org.apache.kafka.common.message.JoinGroupRequestData.JoinGroupRequestProt
import org.apache.kafka.common.message.LeaveGroupRequestData.MemberIdentity
import org.apache.kafka.common.message.ListOffsetsRequestData.{ListOffsetsPartition, ListOffsetsTopic}
import org.apache.kafka.common.message.OffsetForLeaderEpochRequestData.{OffsetForLeaderPartition, OffsetForLeaderTopic, OffsetForLeaderTopicCollection}
import org.apache.kafka.common.message.{AddOffsetsToTxnRequestData, AlterPartitionReassignmentsRequestData, AlterReplicaLogDirsRequestData, ConsumerGroupDescribeRequestData, ConsumerGroupHeartbeatRequestData, CreateAclsRequestData, CreatePartitionsRequestData, CreateTopicsRequestData, DeleteAclsRequestData, DeleteGroupsRequestData, DeleteRecordsRequestData, DeleteTopicsRequestData, DescribeClusterRequestData, DescribeConfigsRequestData, DescribeGroupsRequestData, DescribeLogDirsRequestData, DescribeProducersRequestData, DescribeTransactionsRequestData, FindCoordinatorRequestData, HeartbeatRequestData, IncrementalAlterConfigsRequestData, JoinGroupRequestData, ListPartitionReassignmentsRequestData, ListTransactionsRequestData, MetadataRequestData, OffsetCommitRequestData, ProduceRequestData, SyncGroupRequestData, WriteTxnMarkersRequestData}
import org.apache.kafka.common.message.{AddOffsetsToTxnRequestData, AlterPartitionReassignmentsRequestData, AlterReplicaLogDirsRequestData, ConsumerGroupDescribeRequestData, ConsumerGroupHeartbeatRequestData, CreateAclsRequestData, CreatePartitionsRequestData, CreateTopicsRequestData, DeleteAclsRequestData, DeleteGroupsRequestData, DeleteRecordsRequestData, DeleteTopicsRequestData, DescribeClusterRequestData, DescribeConfigsRequestData, DescribeGroupsRequestData, DescribeLogDirsRequestData, DescribeProducersRequestData, DescribeTransactionsRequestData, FetchResponseData, FindCoordinatorRequestData, HeartbeatRequestData, IncrementalAlterConfigsRequestData, JoinGroupRequestData, ListPartitionReassignmentsRequestData, ListTransactionsRequestData, MetadataRequestData, OffsetCommitRequestData, ProduceRequestData, SyncGroupRequestData, WriteTxnMarkersRequestData}
import org.apache.kafka.common.protocol.{ApiKeys, Errors}
import org.apache.kafka.common.record.{MemoryRecords, RecordBatch, SimpleRecord}
import org.apache.kafka.common.requests.OffsetFetchResponse.PartitionData
Expand All @@ -59,6 +59,7 @@ import java.util.Collections.singletonList
import org.apache.kafka.common.message.MetadataRequestData.MetadataRequestTopic
import org.apache.kafka.common.message.WriteTxnMarkersRequestData.{WritableTxnMarker, WritableTxnMarkerTopic}
import org.apache.kafka.coordinator.group.GroupConfig
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.function.Executable

import scala.collection.mutable
Expand Down Expand Up @@ -808,6 +809,34 @@ class AuthorizerIntegrationTest extends AbstractAuthorizerIntegrationTest {
sendRequestAndVerifyResponseError(request, resources, isAuthorized = true)
}

@Test
def testFetchConsumerRequest(): Unit = {
createTopicWithBrokerPrincipal(topic)

val request = createFetchRequest
val topicNames = getTopicNames().asJava

def partitionDatas(response: AbstractResponse): Iterable[FetchResponseData.PartitionData] = {
assertTrue(response.isInstanceOf[FetchResponse])
response.asInstanceOf[FetchResponse].responseData(topicNames, ApiKeys.FETCH.latestVersion).values().asScala
}

removeAllClientAcls()
val resources = Set(topicResource.resourceType, clusterResource.resourceType)
val failedResponse = sendRequestAndVerifyResponseError(request, resources, isAuthorized = false)
val failedPartitionDatas = partitionDatas(failedResponse)
assertEquals(1, failedPartitionDatas.size)
// Some clients (like librdkafka) always expect non-null records - even for the cases where an error is returned
failedPartitionDatas.foreach(partitionData => assertEquals(MemoryRecords.EMPTY, partitionData.records))

val readAcls = topicReadAcl(topicResource)
addAndVerifyAcls(readAcls, topicResource)
val succeededResponse = sendRequestAndVerifyResponseError(request, resources, isAuthorized = true)
val succeededPartitionDatas = partitionDatas(succeededResponse)
assertEquals(1, succeededPartitionDatas.size)
succeededPartitionDatas.foreach(partitionData => assertEquals(MemoryRecords.EMPTY, partitionData.records))
}

@ParameterizedTest
@ValueSource(strings = Array("kraft"))
def testIncrementalAlterConfigsRequestRequiresClusterPermissionForBrokerLogger(quorum: String): Unit = {
Expand Down
4 changes: 4 additions & 0 deletions raft/src/test/java/org/apache/kafka/raft/RaftClientTestContext.java
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Expand Up @@ -1802,6 +1802,8 @@ FetchResponseData divergingFetchResponse(
partitionData.divergingEpoch()
.setEpoch(divergingEpoch)
.setEndOffset(divergingEpochEndOffset);

partitionData.setRecords(MemoryRecords.EMPTY);
}
);
}
Expand Down Expand Up @@ -1830,6 +1832,8 @@ FetchResponseData snapshotFetchResponse(
partitionData.snapshotId()
.setEpoch(snapshotId.epoch())
.setEndOffset(snapshotId.offset());

partitionData.setRecords(MemoryRecords.EMPTY);
}
);
}
Expand Down
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