AndrewJSchofield commented on code in PR #16969:
URL: https://github.com/apache/kafka/pull/16969#discussion_r1729216536


##########
core/src/main/java/kafka/server/share/SharePartitionManager.java:
##########
@@ -533,178 +538,52 @@ void maybeProcessFetchQueue() {
                     shareFetchPartitionData.groupId,
                     topicIdPartition
                 );
-                SharePartition sharePartition = 
partitionCacheMap.computeIfAbsent(sharePartitionKey,
-                    k -> {
-                        long start = time.hiResClockMs();
-                        SharePartition partition = new 
SharePartition(shareFetchPartitionData.groupId, topicIdPartition, 
maxInFlightMessages, maxDeliveryCount,
-                            recordLockDurationMs, timer, time, persister);
-                        this.shareGroupMetrics.partitionLoadTime(start);
-                        return partition;
-                    });
-                int partitionMaxBytes = 
shareFetchPartitionData.partitionMaxBytes.getOrDefault(topicIdPartition, 0);
-                // Add the share partition to the list of partitions to be 
fetched only if we can
-                // acquire the fetch lock on it.
-                if (sharePartition.maybeAcquireFetchLock()) {
-                    // If the share partition is already at capacity, we 
should not attempt to fetch.
-                    if (sharePartition.canAcquireRecords()) {
-                        topicPartitionData.put(
-                            topicIdPartition,
-                            new FetchRequest.PartitionData(
-                                topicIdPartition.topicId(),
-                                sharePartition.nextFetchOffset(),
-                                0,
-                                partitionMaxBytes,
-                                Optional.empty()
-                            )
-                        );
-                    } else {
-                        sharePartition.releaseFetchLock();
-                        log.info("Record lock partition limit exceeded for 
SharePartition with key {}, " +
-                            "cannot acquire more records", sharePartitionKey);
-                    }
-                }
+                partitionCacheMap.computeIfAbsent(sharePartitionKey, k -> {
+                    long start = time.hiResClockMs();
+                    SharePartition partition = new 
SharePartition(shareFetchPartitionData.groupId, topicIdPartition, 
maxInFlightMessages, maxDeliveryCount,
+                        recordLockDurationMs, timer, time, persister);
+                    this.shareGroupMetrics.partitionLoadTime(start);
+                    return partition;
+                });
             });
 
-            if (topicPartitionData.isEmpty()) {
-                // No locks for share partitions could be acquired, so we 
complete the request and
-                // will re-fetch for the client in next poll.
-                
shareFetchPartitionData.future.complete(Collections.emptyMap());
-                // Though if no partitions can be locked then there must be 
some other request which
-                // is in-flight and should release the lock. But it's safe to 
release the lock as
-                // the lock on share partition already exists which 
facilitates correct behaviour
-                // with multiple requests from queue being processed.
-                releaseProcessFetchQueueLock();
-                if (!fetchQueue.isEmpty())
-                    maybeProcessFetchQueue();
-                return;
-            }
+            Set<Object> delayedShareFetchWatchKeys = new HashSet<>();
+            delayedShareFetchWatchKeys.add(new DelayedShareFetchKey(
+                shareFetchPartitionData.partitionMaxBytes.keySet(),
+                shareFetchPartitionData.groupId,
+                shareFetchPartitionData.memberId));
 
-            log.trace("Fetchable share partitions data: {} with groupId: {} 
fetch params: {}",
-                topicPartitionData, shareFetchPartitionData.groupId, 
shareFetchPartitionData.fetchParams);
-
-            replicaManager.fetchMessages(
-                shareFetchPartitionData.fetchParams,
-                CollectionConverters.asScala(
-                    topicPartitionData.entrySet().stream().map(entry ->
-                        new Tuple2<>(entry.getKey(), 
entry.getValue())).collect(Collectors.toList())
-                ),
-                QuotaFactory.UnboundedQuota$.MODULE$,
-                responsePartitionData -> {
-                    log.trace("Data successfully retrieved by replica manager: 
{}", responsePartitionData);
-                    List<Tuple2<TopicIdPartition, FetchPartitionData>> 
responseData = CollectionConverters.asJava(
-                        responsePartitionData);
-                    processFetchResponse(shareFetchPartitionData, 
responseData).whenComplete(
-                        (result, throwable) -> {
-                            if (throwable != null) {
-                                log.error("Error processing fetch response for 
share partitions", throwable);
-                                
shareFetchPartitionData.future.completeExceptionally(throwable);
-                            } else {
-                                
shareFetchPartitionData.future.complete(result);
-                            }
-                            // Releasing the lock to move ahead with the next 
request in queue.
-                            
releaseFetchQueueAndPartitionsLock(shareFetchPartitionData.groupId, 
topicPartitionData.keySet());
-                        });
-                    return BoxedUnit.UNIT;
-                });
+            // Add the share fetch to the delayed share fetch purgatory to 
process the fetch request.
+            addDelayedShareFetch(new 
DelayedShareFetch(shareFetchPartitionData, replicaManager, partitionCacheMap),
+                delayedShareFetchWatchKeys);
 
+            // Release the lock so that other threads can process the queue.
+            releaseProcessFetchQueueLock();

Review Comment:
   If `maybeProcessFetchQueue` throws an exception, you might find the catch 
block releases the fetch queue lock which has been obtained by another thread.



##########
core/src/main/java/kafka/server/share/DelayedShareFetch.java:
##########
@@ -0,0 +1,271 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server.share;
+
+import kafka.server.DelayedOperation;
+import kafka.server.QuotaFactory;
+import kafka.server.ReplicaManager;
+
+import org.apache.kafka.common.TopicIdPartition;
+import org.apache.kafka.common.message.ShareFetchResponseData;
+import org.apache.kafka.common.protocol.Errors;
+import org.apache.kafka.common.record.FileRecords;
+import org.apache.kafka.common.requests.FetchRequest;
+import org.apache.kafka.common.requests.ListOffsetsRequest;
+import org.apache.kafka.storage.internals.log.FetchPartitionData;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.concurrent.CompletableFuture;
+import java.util.stream.Collectors;
+
+import scala.Option;
+import scala.Tuple2;
+import scala.jdk.javaapi.CollectionConverters;
+import scala.runtime.BoxedUnit;
+
+/**
+ * A delayed share fetch operation has been introduced in case there is no 
share partition for which we can acquire records. We will try to wait
+ * for MaxWaitMs for records to be released else complete the share fetch 
request.
+ */
+public class DelayedShareFetch extends DelayedOperation {
+    private final SharePartitionManager.ShareFetchPartitionData 
shareFetchPartitionData;
+    private final ReplicaManager replicaManager;
+    private final Map<SharePartitionManager.SharePartitionKey, SharePartition> 
partitionCacheMap;
+
+    private static final Logger log = 
LoggerFactory.getLogger(DelayedShareFetch.class);
+
+    DelayedShareFetch(
+            SharePartitionManager.ShareFetchPartitionData 
shareFetchPartitionData,
+            ReplicaManager replicaManager,
+            Map<SharePartitionManager.SharePartitionKey, SharePartition> 
partitionCacheMap) {
+        super(shareFetchPartitionData.fetchParams().maxWaitMs, Option.empty());
+        this.shareFetchPartitionData = shareFetchPartitionData;
+        this.replicaManager = replicaManager;
+        this.partitionCacheMap = partitionCacheMap;
+    }
+
+    @Override
+    public void onExpiration() {
+    }
+
+    /**
+     * Complete the share fetch operation by fetching records for all 
partitions in the share fetch request irrespective
+     * of whether they have any acquired records. This is called when the 
fetch operation is forced to complete either
+     * because records can be acquired for some partitions or due to MaxWaitMs 
timeout.
+     */
+    @Override
+    public void onComplete() {
+        log.trace("onCompletion of delayed share fetch request for group {}, 
member {}, " +
+                        "topic partitions {}", 
shareFetchPartitionData.groupId(),
+                shareFetchPartitionData.memberId(), 
shareFetchPartitionData.partitionMaxBytes().keySet());
+
+        Map<TopicIdPartition, FetchRequest.PartitionData> topicPartitionData = 
topicPartitionDataForAcquirablePartitions();
+        try {
+            if (topicPartitionData.isEmpty()) {
+                // No locks for share partitions could be acquired, so we 
complete the request with an empty response.
+                
shareFetchPartitionData.future().complete(Collections.emptyMap());
+                return;
+            }
+            log.trace("Fetchable share partitions data: {} with groupId: {} 
fetch params: {}",
+                    topicPartitionData, shareFetchPartitionData.groupId(), 
shareFetchPartitionData.fetchParams());
+
+            replicaManager.fetchMessages(
+                    shareFetchPartitionData.fetchParams(),
+                    CollectionConverters.asScala(
+                            topicPartitionData.entrySet().stream().map(entry ->
+                                    new Tuple2<>(entry.getKey(), 
entry.getValue())).collect(Collectors.toList())
+                    ),
+                    QuotaFactory.UnboundedQuota$.MODULE$,
+                    responsePartitionData -> {
+                        log.trace("Data successfully retrieved by replica 
manager: {}", responsePartitionData);
+                        List<Tuple2<TopicIdPartition, FetchPartitionData>> 
responseData = CollectionConverters.asJava(
+                                responsePartitionData);
+                        processFetchResponse(shareFetchPartitionData, 
responseData).whenComplete(
+                                (result, throwable) -> {
+                                    if (throwable != null) {
+                                        log.error("Error processing fetch 
response for share partitions", throwable);
+                                        
shareFetchPartitionData.future().completeExceptionally(throwable);
+                                    } else {
+                                        
shareFetchPartitionData.future().complete(result);
+                                    }
+                                    // Releasing the lock to move ahead with 
the next request in queue.
+                                    
releasePartitionsLock(shareFetchPartitionData.groupId(), 
topicPartitionData.keySet());
+                                });
+                        return BoxedUnit.UNIT;
+                    });
+        } catch (Exception e) {
+            // Release the locks acquired for the partitions in the share 
fetch request in case there is an exception
+            log.error("Error processing delayed share fetch request", e);
+            shareFetchPartitionData.future().completeExceptionally(e);
+            releasePartitionsLock(shareFetchPartitionData.groupId(), 
topicPartitionData.keySet());
+        }
+    }
+
+    /**
+     * Try to complete the fetch operation if we can acquire records for any 
partition in the share fetch request.
+     */
+    @Override
+    public boolean tryComplete() {
+        log.trace("onTry of delayed share fetch request for group {}, member 
{}, topic partitions {}",

Review Comment:
   onTry seems like a typo.



##########
core/src/test/java/kafka/server/share/DelayedShareFetchKeyTest.java:
##########
@@ -0,0 +1,60 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package kafka.server.share;
+
+import org.apache.kafka.common.TopicIdPartition;
+import org.apache.kafka.common.TopicPartition;
+import org.apache.kafka.common.Uuid;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+public class DelayedShareFetchKeyTest {
+
+    @Test
+    public void testDelayedShareFetchEqualsAndHashcode() {
+        TopicIdPartition tp0 = new TopicIdPartition(Uuid.randomUuid(), new 
TopicPartition("topic", 0));

Review Comment:
   You really ought to use the same UUID for the partitions of the topic 
"topic".



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