The session variable in export job should be copied from session variable in connection context.
Because both session variable in connection context and in export job may be modified at same time,
cause ConcurrentModificationException like:
2023-10-07 22:56:12,818 WARN (mysql-nio-pool-2|249) [ConnectProcessor.handleQueryException():396] Process one query failed because unknown reason:
java.util.ConcurrentModificationException: null
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437) ~[?:1.8.0_131]
at java.util.HashMap$KeyIterator.next(HashMap.java:1461) ~[?:1.8.0_131]
at org.apache.doris.qe.VariableMgr.revertSessionValue(VariableMgr.java:238) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.qe.StmtExecutor.execute(StmtExecutor.java:474) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.qe.StmtExecutor.execute(StmtExecutor.java:438) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.qe.ConnectProcessor.handleQuery(ConnectProcessor.java:353) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.qe.ConnectProcessor.dispatch(ConnectProcessor.java:501) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.qe.ConnectProcessor.processOnce(ConnectProcessor.java:752) ~[doris-fe.jar:1.2-SNAPSHOT]
at org.apache.doris.mysql.ReadListener.lambda$handleEvent$0(ReadListener.java:52) ~[doris-fe.jar:1.2-SNAPSHOT]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) ~[?:1.8.0_131]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) ~[?:1.8.0_131]
at java.lang.Thread.run(Thread.java:748) ~[?:1.8.0_131]
This error is reported by external_table_p0/export/test_export_external_table.groovy
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