[jira] [Commented] (HBASE-26688) Threads shared EMPTY_RESULT may lead to unexpected client job down.

["Hudson (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-26688?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17481740#comment-17481740 ] 

Hudson commented on HBASE-26688:
--------------------------------

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> Threads shared EMPTY_RESULT may lead to unexpected client job down.
> -------------------------------------------------------------------
>
>                 Key: HBASE-26688
>                 URL: https://issues.apache.org/jira/browse/HBASE-26688
>             Project: HBase
>          Issue Type: Bug
>          Components: Client
>            Reporter: Yutong Xiao
>            Assignee: Yutong Xiao
>            Priority: Major
>             Fix For: 2.5.0, 3.0.0-alpha-3, 2.4.10
>
>
> Currently, we use a pre-created EMPTY_RESULT in the ProtoBuf.util to reduce the object creation. But these objects could be shared by multi client threads. The Result#cellScannerIndex related methods could throw confusing exception and make the client job down. Could refine the logic of these methods.
> The precreated objects in ProtoBufUtil.java:
> {code:java}
> private final static Cell[] EMPTY_CELL_ARRAY = new Cell[]{};
> private final static Result EMPTY_RESULT = Result.create(EMPTY_CELL_ARRAY);
> final static Result EMPTY_RESULT_EXISTS_TRUE = Result.create(null, true);
> final static Result EMPTY_RESULT_EXISTS_FALSE = Result.create(null, false);
> private final static Result EMPTY_RESULT_STALE = Result.create(EMPTY_CELL_ARRAY, null, true);
> {code}
> Result#advance 
> {code:java}
> public boolean advance() {
>     if (cells == null) return false;
>     cellScannerIndex++;
>     if (cellScannerIndex < this.cells.length) {
>       return true;
>     } else if (cellScannerIndex == this.cells.length) {
>       return false;
>     }
>     // The index of EMPTY_RESULT could be incremented by multi threads and throw this exception.
>     throw new NoSuchElementException("Cannot advance beyond the last cell");
>   }
> {code}
> Result#current
> {code:java}
> public Cell current() {
>     if (cells == null
>             || cellScannerIndex == INITIAL_CELLSCANNER_INDEX
>             || cellScannerIndex >= cells.length)
>       return null;
>    // Although it is almost impossible,
>    // We can arrive here when the client threads share the common reused EMPTY_RESULT.
>     return this.cells[cellScannerIndex];
>   }
> {code}
> In this case, the client can easily got confusing exceptions even if they use different connections, tables in different threads.
> We should change the if condition cells == null to isEmpty() to avoid the client crashed from these exception.



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