KAFKA-19955: Fix performance regression in server-side assignors

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/MemberAssignmentImpl.java

@@ -19,7 +19,6 @@
 import org.apache.kafka.common.Uuid;
 import org.apache.kafka.coordinator.group.api.assignor.MemberAssignment;
 
-import java.util.Collections;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Set;
@@ -28,10 +27,12 @@
  * The partition assignment for a modern group member.
  *
  * @param partitions The partitions assigned to this member keyed by topicId.
+ *                   The map will not be made immutable, since the server-side assignors rely on
+ *                   being able to mutate the map while building new assignments.
  */
 public record MemberAssignmentImpl(Map<Uuid, Set<Integer>> partitions) implements MemberAssignment {
     public MemberAssignmentImpl {
-        partitions = Collections.unmodifiableMap(Objects.requireNonNull(partitions));
+        Objects.requireNonNull(partitions);
     }
 
     /**

group-coordinator/src/test/java/org/apache/kafka/coordinator/group/modern/MemberAssignmentImplTest.java

@@ -0,0 +1,49 @@
+/*
+ * 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 org.apache.kafka.coordinator.group.modern;
+
+import org.apache.kafka.common.Uuid;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+public class MemberAssignmentImplTest {
+
+    @Test
+    public void testPartitionsMutable() {
+        // We depend on the the map inside MemberAssignmentImpl remaining mutable in the server-side
+        // assignors, otherwise we end up deep copying the map unnecessarily.
+        Uuid topicId1 = Uuid.randomUuid();
+        Uuid topicId2 = Uuid.randomUuid();
+
+        HashMap<Uuid, Set<Integer>> partitions = new HashMap<>();
+        partitions.put(topicId1, new HashSet<>());
+
+        MemberAssignmentImpl memberAssignment = new MemberAssignmentImpl(partitions);
+
+        // The map should remain mutable.
+        assertDoesNotThrow(() -> memberAssignment.partitions().put(topicId2, Set.of(3, 4, 5)));
+
+        // The sets inside the map should remain mutable.
+        assertDoesNotThrow(() -> memberAssignment.partitions().get(topicId1).add(3));
+    }
+}