[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.

Mon, 13 Jul 2020 14:14:10 -0700 

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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