[dbcp] Deadlock when evicting dbcp objects (testWhileIdle=true)

The GenericKeyedObjectPool$Evictor thread has probability of deadlock with dbcp
objects.

For example, at a certain time, a normal user thread calls a PoolingConnection
object's synchronized method, which in turn calls GenericKeyedObjectPool
object's synchronzied method.

At the same time, the evictor thread calls the GenericKeyedObjectPool object's
synchronized method 'evict', which in turn calls the PoolingConnection object's
synchronized method. When testWhileIdle=true, the probability of evictor
calling GenericKeyedObjectPool's synchronized method is much greater.

The following is the deadlock information in the thread dump of our program
(testWhileIdle=true):

"Thread-106":
at org.apache.commons.dbcp.AbandonedTrace.removeTrace(AbandonedTrace.java:221)

• waiting to lock <0x6a6b1b80> (a org.apache.commons.dbcp.PoolingConnection)
  at
  org.apache.commons.dbcp.PoolablePreparedStatement.passivate(PoolablePreparedStatement.java:100)
  at
  org.apache.commons.dbcp.PoolingConnection.passivateObject(PoolingConnection.java:239)
  at
  org.apache.commons.pool.impl.GenericKeyedObjectPool.evict(GenericKeyedObjectPool.java:1001)
• locked <0x6a6b1858> (a org.apache.commons.pool.impl.GenericKeyedObjectPool)
  at
  org.apache.commons.pool.impl.GenericKeyedObjectPool$Evictor.run(GenericKeyedObjectPool.java:1156)
  at java.lang.Thread.run(Thread.java:534)
  "Thread-41":
  at
  org.apache.commons.pool.impl.GenericKeyedObjectPool.borrowObject(GenericKeyedObjectPool.java:715)
• waiting to lock <0x6a6b1858> (a
  org.apache.commons.pool.impl.GenericKeyedObjectPool)
  at
  org.apache.commons.dbcp.PoolingConnection.prepareStatement(PoolingConnection.java:87)
• locked <0x6a6b1b80> (a org.apache.commons.dbcp.PoolingConnection)
  at
  org.apache.commons.dbcp.DelegatingConnection.prepareStatement(DelegatingConnection.java:185)
  at
  org.apache.commons.dbcp.PoolingDataSource$PoolGuardConnectionWrapper.prepareStatement(PoolingDataSource.java:278)
  ...

This problem can be worked around by setting testWhileIdle to false, although
there is still very small possibility of deadlock.