[GitHub] [hadoop] paulward24 opened a new pull request #1030: ArrayList is not thread safe. Field timedOutItems is typically protected by synchronization on itself. However, in one place, the field is inside a synchronized on a different object, which does not ensure protection

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paulward24 opened a new pull request #1030: ArrayList is not thread safe. Field timedOutItems is typically protected by synchronization on itself. However, in one place, the field is inside a synchronized on a different object, which does not ensure protection
URL: https://github.com/apache/hadoop/pull/1030
 
 
   The field ```timedOutItems```  (an ```ArrayList```, i.e., not thread safe):
   
   https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L70
   
   is protected by synchronization on itself (```timedOutItems```):
   
   https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L167-L168
   
   https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L267-L268
   
   https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L178
   
   However, in one place:
   
   https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java#L133-L135
   
   it is (trying to be) protected by synchronized using ```pendingReconstructions``` --- but this cannot protect ```timedOutItems```.
   
   Synchronized on different objects does not ensure mutual exclusion with the other locations.
   
   I.e., 2 code locations, one synchronized by ```pendingReconstructions``` and the other by ```timedOutItems``` can still executed concurrently.
   
   
   This CR adds the synchronized on ```timedOutItems```.
   
   Note that this CR keeps the synchronized on ```pendingReconstructions```, which is needed for a different purpose (protect ```pendingReconstructions```)
   

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