JIRA | Mike Copley | 6 years ago
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    DBTCPConnector._set() is susceptible to a race condition where two threads call it at the same time, allowing a thread to mistaking think _masterPortPool is set when it is still null, and go on to cause a NullPointerException in DBTCPConnector$MyPort.get(). Further detail: I have two threads invoking DB.getCollection("differentCollectionForEachThread").drop() at approximately the same time. These are the first connections to Mongo. Approximately every 2nd run of this causes a NullPointerException in DBTCPConnector$MyPort.get(). I've tried to make a simple test app to reproduce, but can't - timing issues are tricky to replicate. The problem occurs here: private boolean _set( ServerAddress addr ){ if ( _curMaster == addr) // should check that _masterPortPool != null return false; _curMaster = addr; // _curMaster set before _masterPortPool. At this point _masterPortPool is still null. _masterPortPool = _portHolder.get( addr ); return true; } Then in MyPort.get() the NPE happens: _pool = _masterPortPool; DBPort p = _pool.get(); // NPE In the above code, - Thread 1 enters _set() first, sets _curMaster = addr. - Thread 2 enters _set() next, sees _curMaster already == addr so exits early. - Thread 2 enters get(), assigns _pool to _masterPortPool (null) then calls _pool.get() and NPE - Thread 1 continues in _set(), sets _masterPortPool to addr Stack Trace (based on git head, commit a052b4f35af4069121cbf47adc88d6199563c4d4): java.lang.NullPointerException at com.mongodb.DBTCPConnector$MyPort.get( at at com.mongodb.DBApiLayer$MyCollection.__find( at com.mongodb.DB.command( at com.mongodb.DB.command( at com.mongodb.DBCollection.drop( Suggested fix: private boolean _set( ServerAddress addr ){ if ( _curMaster == addr && _masterPortPool != null) return false; _curMaster = addr; _masterPortPool = _portHolder.get( addr ); return true; }

    JIRA | 6 years ago | Mike Copley

    Root Cause Analysis

    1. java.lang.NullPointerException

      No message provided

      at com.mongodb.DBTCPConnector$MyPort.get()
    2. MongoDB Java Driver
      1. com.mongodb.DBTCPConnector$MyPort.get(
      3. com.mongodb.DBApiLayer$MyCollection.__find(
      4. com.mongodb.DB.command(
      5. com.mongodb.DB.command(
      6. com.mongodb.DBCollection.drop(
      6 frames