chirag-wadhwa5 commented on code in PR #20839:
URL: https://github.com/apache/kafka/pull/20839#discussion_r2511403647


##########
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupCoordinatorService.java:
##########
@@ -1835,27 +1851,139 @@ private 
CompletableFuture<DescribeShareGroupOffsetsResponseData.DescribeShareGro
                     return;
                 }
 
+                // Now compute lag for each partition and build the final 
response.
+                computeShareGroupLagAndBuildResponse(
+                    result,
+                    requestTopicIdToNameMapping,
+                    describeShareGroupOffsetsResponseTopicList,
+                    future,
+                    readSummaryRequestData.groupId()
+                );
+            });
+        return future;
+    }
+
+    private void computeShareGroupLagAndBuildResponse(
+        ReadShareGroupStateSummaryResult readSummaryResult,
+        Map<Uuid, String> requestTopicIdToNameMapping,
+        
List<DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponseTopic>
 describeShareGroupOffsetsResponseTopicList,
+        
CompletableFuture<DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponseGroup>
 responseFuture,
+        String groupId
+    ) {
+
+        // This set keeps track of the partitions for which lag computation is 
needed.
+        Set<TopicPartition> partitionsToComputeLag = new HashSet<>();
+
+        // This map stores the final 
DescribeShareGroupOffsetsResponsePartition, including the lag, for all the 
partitions.
+        Map<TopicIdPartition, 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponsePartition>
 partitionsResponses = new HashMap<>();
+
+        readSummaryResult.topicsData().forEach(topicData -> {
+            topicData.partitions().forEach(partitionData -> {
+                TopicIdPartition tp = new TopicIdPartition(
+                    topicData.topicId(),
+                    new 
TopicPartition(requestTopicIdToNameMapping.get(topicData.topicId()), 
partitionData.partition())
+                );
                 // Return -1 (uninitialized offset) for the situation where 
the persister returned an error.
                 // This is consistent with OffsetFetch for situations in which 
there is no offset information to fetch.
-                // It's treated as absence of data, rather than an error.
-                result.topicsData().forEach(topicData ->
-                    describeShareGroupOffsetsResponseTopicList.add(new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponseTopic()
-                        .setTopicId(topicData.topicId())
-                        
.setTopicName(requestTopicIdToNameMapping.get(topicData.topicId()))
-                        .setPartitions(topicData.partitions().stream().map(
-                            partitionData -> new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponsePartition()
-                                .setPartitionIndex(partitionData.partition())
-                                .setStartOffset(partitionData.errorCode() == 
Errors.NONE.code() ? partitionData.startOffset() : 
PartitionFactory.UNINITIALIZED_START_OFFSET)
-                                .setLeaderEpoch(partitionData.errorCode() == 
Errors.NONE.code() ? partitionData.leaderEpoch() : 
PartitionFactory.DEFAULT_LEADER_EPOCH)
-                        ).toList())
-                    ));
+                // It's treated as absence of data, rather than an error. 
Also, the persister returns startOffset
+                // as -1 (uninitialized offset) for share partitions for which 
consumption hasn't begun yet. Thus,
+                // lag computation is not needed in these situations, and -1 
(uninitialized lag) is returned.
+                if (partitionData.errorCode() != Errors.NONE.code() || 
partitionData.startOffset() == PartitionFactory.UNINITIALIZED_START_OFFSET) {
+                    partitionsResponses.put(
+                        tp,
+                        new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponsePartition()
+                            .setPartitionIndex(partitionData.partition())
+                            
.setStartOffset(PartitionFactory.UNINITIALIZED_START_OFFSET)
+                            
.setLeaderEpoch(PartitionFactory.DEFAULT_LEADER_EPOCH)
+                            .setLag(PartitionFactory.UNINITIALIZED_LAG)
+                    );
+                } else {
+                    // If the readSummaryResult is successful for a partition, 
we need to compute lag.
+                    partitionsToComputeLag.add(new 
TopicPartition(requestTopicIdToNameMapping.get(topicData.topicId()), 
partitionData.partition()));
+                }
+            });
+        });
+
+        // Fetch latest offsets for all partitions that need lag computation.
+        Map<TopicPartition, CompletableFuture<Long>> partitionLatestOffsets = 
partitionsToComputeLag.isEmpty() ? new HashMap<>() :
+                
partitionMetadataClient.listLatestOffsets(partitionsToComputeLag);
 
-                future.complete(
-                    new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponseGroup()
-                        .setGroupId(readSummaryRequestData.groupId())
-                        
.setTopics(describeShareGroupOffsetsResponseTopicList));
+        CompletableFuture.allOf(partitionLatestOffsets.values().toArray(new 
CompletableFuture<?>[0]))
+            .whenComplete((result, error) -> {
+                readSummaryResult.topicsData().forEach(topicData -> {
+                    topicData.partitions().forEach(partitionData -> {
+                        // The partitions that fail this check are already 
handled above, and their corresponding 
DescribeShareGroupOffsetsResponsePartition
+                        // is already present in partitionsResponses.
+                        if (partitionData.errorCode() == Errors.NONE.code() && 
partitionData.startOffset() != PartitionFactory.UNINITIALIZED_START_OFFSET) {
+                            TopicPartition tp = new 
TopicPartition(requestTopicIdToNameMapping.get(topicData.topicId()), 
partitionData.partition());
+                            TopicIdPartition tip = new 
TopicIdPartition(topicData.topicId(), tp);
+                            try {
+                                // This code is reached when allOf above is 
complete, which happens when all the
+                                // individual futures are complete. Thus, the 
call to join() here is safe.
+                                long partitionLatestOffset = 
partitionLatestOffsets.get(tp).join();
+                                // Compute lag as (partition end offset - 
startOffset + 1 - deliveryCompleteCount)
+                                long lag = partitionLatestOffset - 
partitionData.startOffset() + 1 - partitionData.deliveryCompleteCount();
+                                partitionsResponses.put(
+                                    tip,
+                                    new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponsePartition()
+                                        
.setPartitionIndex(partitionData.partition())
+                                        
.setStartOffset(partitionData.startOffset())
+                                        
.setLeaderEpoch(partitionData.leaderEpoch())
+                                        .setLag(lag)
+                                );
+                            } catch (CompletionException e) {
+                                // If fetching latest offset for a partition 
failed, return the error in the response for that partition.
+                                partitionsResponses.put(
+                                    tip,
+                                    new 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponsePartition()
+                                        
.setPartitionIndex(partitionData.partition())
+                                        
.setErrorCode(Errors.forException(e.getCause()).code())
+                                        
.setErrorMessage(e.getCause().getMessage())
+                                );
+                            }
+                        }
+                    });
+                });
+
+                // Build the final response and complete the future.
+                responseFuture.complete(BuildDescribeShareGroupOffsetsResponse(
+                    partitionsResponses,
+                    requestTopicIdToNameMapping,
+                    describeShareGroupOffsetsResponseTopicList,
+                    groupId
+                ));
             });
-        return future;
+    }
+
+    private 
DescribeShareGroupOffsetsResponseData.DescribeShareGroupOffsetsResponseGroup 
BuildDescribeShareGroupOffsetsResponse(

Review Comment:
   My mistake, will replace this in the next commit



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