chore: fix unit test and add some comments

src/common/meta/src/wal_options_allocator.rs

@@ -192,7 +192,8 @@ mod tests {
                     ..Default::default()
                 };
                 let kv_backend = Arc::new(MemoryKvBackend::new()) as KvBackendRef;
-                let topic_pool = KafkaTopicPool::new(config.clone(), kv_backend);
+                let mut topic_pool = KafkaTopicPool::new(config.clone(), kv_backend);
+                topic_pool.topics.clone_from(&topics);
 
                 // Creates an options allocator.
                 let allocator = WalOptionsAllocator::Kafka(topic_pool);

src/common/meta/src/wal_options_allocator/kafka_topic_pool.rs

@@ -32,7 +32,7 @@ use crate::wal_options_allocator::selector::{RoundRobinTopicSelector, TopicSelec
 /// 2. Creates topics in kafka.
 /// 3. Selects topic
 pub struct KafkaTopicPool {
-    topics: Vec<String>,
+    pub(crate) topics: Vec<String>,
     topic_kvbackend_manager: TopicKvBackendManager,
     topic_kafka_manager: TopicKafkaManager,
     selector: TopicSelectorRef,

src/common/meta/src/wal_options_allocator/kafka_topic_pool/kafka_topic_manager.rs

@@ -33,7 +33,7 @@ use crate::error::{
 // The `DEFAULT_PARTITION` refers to the index of the partition.
 const DEFAULT_PARTITION: i32 = 0;
 
-/// Manages topics in kafka.
+/// Creates topics in kafka.
 pub struct TopicKafkaManager {
     pub(super) config: MetasrvKafkaConfig,
 }

src/common/meta/src/wal_options_allocator/kvbackend.rs

@@ -18,6 +18,10 @@ use crate::error::Result;
 use crate::key::topic_name::{TopicNameKey, TopicNameKeyManager};
 use crate::kv_backend::KvBackendRef;
 
+/// Manages topics in kvbackend.
+/// Responsible for:
+/// 1. Restors and persisting topics in kvbackend.
+/// 2. Clears topics in legacy format and restores them in the new format.
 pub struct TopicKvBackendManager {
     manager: TopicNameKeyManager,
 }