Fix race condition in DrillComplexWriterFuncHolder

select flatten(sub.fk.`value`) from (select flatten(kvgen(map)) fk from `json_kvgenflatten/nested3.json`) sub
Failed with exception
java.sql.SQLException: UNSUPPORTED_OPERATION ERROR: Flatten does not support inputs of non-list values.

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[Error Id: 90026283-0b95-4bda-948e-54ed57a62edf on qa-node183.qa.lab:31010]


	at org.apache.drill.jdbc.impl.DrillCursor.nextRowInternally(DrillCursor.java:489)
	at org.apache.drill.jdbc.impl.DrillCursor.loadInitialSchema(DrillCursor.java:561)
	at org.apache.drill.jdbc.impl.DrillResultSetImpl.execute(DrillResultSetImpl.java:1895)
	at org.apache.drill.jdbc.impl.DrillResultSetImpl.execute(DrillResultSetImpl.java:61)
	at oadd.org.apache.calcite.avatica.AvaticaConnection$1.execute(AvaticaConnection.java:473)
	at org.apache.drill.jdbc.impl.DrillMetaImpl.prepareAndExecute(DrillMetaImpl.java:1100)
	at oadd.org.apache.calcite.avatica.AvaticaConnection.prepareAndExecuteInternal(AvaticaConnection.java:477)
	at org.apache.drill.jdbc.impl.DrillConnectionImpl.prepareAndExecuteInternal(DrillConnectionImpl.java:180)
	at oadd.org.apache.calcite.avatica.AvaticaStatement.executeInternal(AvaticaStatement.java:109)
	at oadd.org.apache.calcite.avatica.AvaticaStatement.executeQuery(AvaticaStatement.java:130)
	at org.apache.drill.jdbc.impl.DrillStatementImpl.executeQuery(DrillStatementImpl.java:112)
	at org.apache.drill.test.framework.DrillTestJdbc.executeQuery(DrillTestJdbc.java:177)
	at org.apache.drill.test.framework.DrillTestJdbc.run(DrillTestJdbc.java:101)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)
Caused by: oadd.org.apache.drill.common.exceptions.UserRemoteException: UNSUPPORTED_OPERATION ERROR: Flatten does not support inputs of non-list values.

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[Error Id: 90026283-0b95-4bda-948e-54ed57a62edf on qa-node183.qa.lab:31010]


	at oadd.org.apache.drill.exec.rpc.user.QueryResultHandler.resultArrived(QueryResultHandler.java:123)
	at oadd.org.apache.drill.exec.rpc.user.UserClient.handle(UserClient.java:343)
	at oadd.org.apache.drill.exec.rpc.user.UserClient.handle(UserClient.java:88)
	at oadd.org.apache.drill.exec.rpc.RpcBus$InboundHandler.decode(RpcBus.java:274)
	at oadd.org.apache.drill.exec.rpc.RpcBus$InboundHandler.decode(RpcBus.java:244)
	at oadd.io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:89)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
	at oadd.io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:254)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
	at oadd.io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
	at oadd.io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:242)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
	at oadd.io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:86)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:339)
	at oadd.io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:324)
	at oadd.io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:847)
	at oadd.io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:131)
	at oadd.io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:511)
	at oadd.io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:468)
	at oadd.io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:382)
	at oadd.io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
	at oadd.io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:111)
	... 1 more