[jira] [Created] (HIVE-23841) Field writers is an HashSet, i.e., not thread-safe. Field writers is typically protected by synchronization on lock, but not in 1 location.

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

Adrian Nistor created HIVE-23841:
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             Summary: Field writers is an HashSet, i.e., not thread-safe.  Field writers is typically protected by synchronization on lock, but not in 1 location.
                 Key: HIVE-23841
                 URL: https://issues.apache.org/jira/browse/HIVE-23841
             Project: Hive
          Issue Type: Bug
         Environment: Any environment
            Reporter: Adrian Nistor


Field {{writers}} is a {{HashSet}} ([line 70|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L70]), i.e., not thread-safe.

Accesses to field {{writers}}  are protected by synchronization on {{lock}}, e.g., at lines: [141-144|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L141-L144], [212-213|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L212-L213], and [212-215|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L212-L215].

However, the {{writers.remove()}} at  [line 249|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L249] is protected by synchronization on {{INSTANCE}}, *not* on {{lock}}.

Synchronizing on 2 different objects does not ensure mutual exclusion.  This is because 2 threads synchronizing on different objects can still execute in parallel at the same time.

Note that lines [215|https://github.com/apache/hive/blob/c93d7797329103d6c509bada68b6da7f907b3dee/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L215] and [249|https://github.com/apache/hive/blob/master/ql/src/java/org/apache/hadoop/hive/llap/LlapOutputFormatService.java#L249] are modifying {{writers}} with {{put()}} and {{remove()}}, respectively. 


h1. The Code for This Fix

This fix is very simple: just change {{synchronized (INSTANCE)}} to {{synchronized (lock)}}, just like the methods containing the other lines listed above.




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