[I] [Bug] unique-key index concurrent issue [incubator-hugegraph]

[via GitHub]

qwtsc opened a new issue, #2334:
URL: https://github.com/apache/incubator-hugegraph/issues/2334

   ### Bug Type (问题类型)
   
   other exception / error (其他异常报错)
   
   ### Before submit
   
   - [X] 我已经确认现有的 [Issues](https://github.com/apache/hugegraph/issues) 与 
[FAQ](https://hugegraph.apache.org/docs/guides/faq/) 中没有相同 / 重复问题 (I have 
confirmed and searched that there are no similar problems in the historical 
issue and documents)
   
   ### Environment (环境信息)
   
   unique key index is not valid when insert two vertex concurrently using 
mysql as backend store.
   
   ### Expected & Actual behavior (期望与实际表现)
   
   
![image](https://github.com/apache/incubator-hugegraph/assets/34210641/e54240ec-a5c9-4305-8093-f78305f19de9)
   
   More than one element id can be found with the same index label and the same 
field value, which is unacceptable for production env. When I read the code, I 
found the following comment that made in 2019.
   ```java
                   Id id = element.id();
                   // TODO: add lock for updating unique index
                   if (!removed && this.existUniqueValue(indexLabel, value, 
id)) {
                       throw new IllegalArgumentException(String.format(
                                 "Unique constraint %s conflict is found for 
%s",
                                 indexLabel, element));
                   }
                   this.updateIndex(indexLabel, value, element.id(),
                                    expiredTime, removed);
   ```
   and if unique index update process is not lock-protected by now, the schema 
of mysql unique index table should not reuse the definition of secondary index 
and maybe change to use FIELD_VALUE and INDEX_LABEL_ID as primary keys. 
However, this is a break change and will impact lots of current users.
   ```java
       public static class UniqueIndex extends SecondaryIndex {
   
           public static final String TABLE = HugeType.UNIQUE_INDEX.name();
   
           public UniqueIndex(String store) {
               // should overwrite the constructor to not use the same keys 
with SecondaryIndex.
               super(store, TABLE, TYPES_MAPPING);
           }
       }
   
       public static class SecondaryIndex extends Index {
   
           public static final String TABLE = HugeType.SECONDARY_INDEX.name();
   
           public SecondaryIndex(String store) {
               this(store, TABLE, TYPES_MAPPING);
           }
   
           public SecondaryIndex(String store, String table,
                                 Map<String, String> typesMapping) {
               super(joinTableName(store, table));
   
               this.define = new TableDefine(typesMapping);
               this.define.column(HugeKeys.FIELD_VALUES, SMALL_TEXT);
               this.define.column(HugeKeys.INDEX_LABEL_ID, DATATYPE_IL);
               this.define.column(HugeKeys.ELEMENT_IDS, SMALL_TEXT);
               this.define.column(HugeKeys.EXPIRED_TIME, BIGINT);
               this.define.keys(HugeKeys.FIELD_VALUES,
                                HugeKeys.INDEX_LABEL_ID,
                                HugeKeys.ELEMENT_IDS);
           }
   
           @Override
           public final String entryId(MysqlBackendEntry entry) {
               String fieldValues = entry.column(HugeKeys.FIELD_VALUES);
               Integer labelId = entry.column(HugeKeys.INDEX_LABEL_ID);
               return SplicingIdGenerator.concat(fieldValues, 
labelId.toString());
           }
       }
   ```
   To be honest, it's a little bit frustrating.
   
   ### Vertex/Edge example (问题点 / 边数据举例)
   
   _No response_
   
   ### Schema [VertexLabel, EdgeLabel, IndexLabel] (元数据结构)
   
   _No response_


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