Fix kafka read session partitions releases (#3528)

Author: niksaveliev

ydb/core/kafka_proxy/actors/kafka_read_session_actor.cpp

@@ -599,18 +599,19 @@ void TKafkaReadSessionActor::HandleReleasePartition(TEvPersQueue::TEvReleasePart
         }
 
         NeedRebalance = true;
-        size_t partitionToReleaseIndex = 0;
-        size_t i = 0;
+        ui32 partitionToRelease = 0;
+        ui32 i = 0;
 
-        for (size_t partIndex = 0; partIndex < topicPartitionsIt->second.ReadingNow.size(); partIndex++) {
-            if (!topicPartitionsIt->second.ToRelease.contains(partIndex) && (group == 0 || partIndex + 1 == group)) {
+        for (auto curPartition : topicPartitionsIt->second.ReadingNow) {
+            if (!topicPartitionsIt->second.ToRelease.contains(curPartition) && (group == 0 || curPartition + 1 == group)) {
                 ++i;
-                if (rand() % i == 0) { // will lead to 1/n probability for each of n partitions
-                    partitionToReleaseIndex = partIndex;
+                if (rand() % i == 0) {
+                    partitionToRelease = curPartition;
                 }
             }
         }
-        topicPartitionsIt->second.ToRelease.emplace(partitionToReleaseIndex);
+
+        topicPartitionsIt->second.ToRelease.emplace(partitionToRelease);
     }
 }