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### _Why are the changes needed?_
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As `File.listFiles` may return null when spark package not found in folder `${PROJECT_ROOT_DIR}/kyuubi-server/externals/kyuubi-download/target`, the NPE will throw. Improve the error message.
Before thie pr:
```
Caused by: java.lang.RuntimeException: org.apache.kyuubi.KyuubiSQLException:Error opening session SessionHandle [9749221f-5b30-457b-b8e5-25affba25061] for fchen due to null
at org.apache.kyuubi.KyuubiSQLException$.apply(KyuubiSQLException.scala:68)
at org.apache.kyuubi.session.KyuubiSessionManager.openSession(KyuubiSessionManager.scala:74)
at org.apache.kyuubi.service.AbstractBackendService.openSession(AbstractBackendService.scala:45)
at org.apache.kyuubi.service.ThriftFrontendService.getSessionHandle(ThriftFrontendService.scala:190)
at org.apache.kyuubi.service.ThriftFrontendService.OpenSession(ThriftFrontendService.scala:199)
at org.apache.hive.service.rpc.thrift.TCLIService$Processor$OpenSession.getResult(TCLIService.java:1377)
at org.apache.hive.service.rpc.thrift.TCLIService$Processor$OpenSession.getResult(TCLIService.java:1362)
at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:39)
at org.apache.thrift.TBaseProcessor.process(TBaseProcessor.java:39)
at org.apache.kyuubi.service.authentication.TSetIpAddressProcessor.process(TSetIpAddressProcessor.scala:36)
at org.apache.thrift.server.TThreadPoolServer$WorkerProcess.run(TThreadPoolServer.java:286)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException: null
at scala.collection.mutable.ArrayOps$ofRef$.length$extension(ArrayOps.scala:204)
at scala.collection.mutable.ArrayOps$ofRef.length(ArrayOps.scala:204)
at scala.collection.IndexedSeqOptimized.isEmpty(IndexedSeqOptimized.scala:30)
at scala.collection.IndexedSeqOptimized.isEmpty$(IndexedSeqOptimized.scala:30)
at scala.collection.mutable.ArrayOps$ofRef.isEmpty(ArrayOps.scala:198)
at scala.collection.TraversableLike.headOption(TraversableLike.scala:608)
at scala.collection.TraversableLike.headOption$(TraversableLike.scala:608)
at scala.collection.mutable.ArrayOps$ofRef.headOption(ArrayOps.scala:198)
at org.apache.kyuubi.engine.spark.SparkProcessBuilder.$anonfun$executable$1(SparkProcessBuilder.scala:50)
at scala.Option.orElse(Option.scala:447)
at org.apache.kyuubi.engine.spark.SparkProcessBuilder.<init>(SparkProcessBuilder.scala:41)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$1(EngineRef.scala:140)
at org.apache.kyuubi.engine.EngineRef.tryWithLock(EngineRef.scala:116)
at org.apache.kyuubi.engine.EngineRef.create(EngineRef.scala:128)
at org.apache.kyuubi.engine.EngineRef.$anonfun$getOrCreate$1(EngineRef.scala:182)
at scala.Option.getOrElse(Option.scala:189)
at org.apache.kyuubi.engine.EngineRef.getOrCreate(EngineRef.scala:182)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$open$2(KyuubiSessionImpl.scala:63)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$open$2$adapted(KyuubiSessionImpl.scala:62)
at org.apache.kyuubi.ha.client.ZooKeeperClientProvider$.withZkClient(ZooKeeperClientProvider.scala:74)
at org.apache.kyuubi.session.KyuubiSessionImpl.open(KyuubiSessionImpl.scala:62)
at org.apache.kyuubi.session.KyuubiSessionManager.openSession(KyuubiSessionManager.scala:58)
... 12 more
```
After this pr:
```
Caused by: java.lang.RuntimeException: org.apache.kyuubi.KyuubiSQLException:Error opening session SessionHandle [5ea6a8b3-1727-4e0d-b542-992f84afcde8] for fchen due to SPARK_HOME is not set!
at org.apache.kyuubi.KyuubiSQLException$.apply(KyuubiSQLException.scala:68)
at org.apache.kyuubi.session.KyuubiSessionManager.openSession(KyuubiSessionManager.scala:74)
at org.apache.kyuubi.service.AbstractBackendService.openSession(AbstractBackendService.scala:45)
at org.apache.kyuubi.service.ThriftFrontendService.getSessionHandle(ThriftFrontendService.scala:190)
at org.apache.kyuubi.service.ThriftFrontendService.OpenSession(ThriftFrontendService.scala:199)
at org.apache.hive.service.rpc.thrift.TCLIService$Processor$OpenSession.getResult(TCLIService.java:1377)
at org.apache.hive.service.rpc.thrift.TCLIService$Processor$OpenSession.getResult(TCLIService.java:1362)
at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:39)
at org.apache.thrift.TBaseProcessor.process(TBaseProcessor.java:39)
at org.apache.kyuubi.service.authentication.TSetIpAddressProcessor.process(TSetIpAddressProcessor.scala:36)
at org.apache.thrift.server.TThreadPoolServer$WorkerProcess.run(TThreadPoolServer.java:286)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.RuntimeException: org.apache.kyuubi.KyuubiSQLException:SPARK_HOME is not set!
at org.apache.kyuubi.KyuubiSQLException$.apply(KyuubiSQLException.scala:68)
at org.apache.kyuubi.engine.spark.SparkProcessBuilder.$anonfun$executable$5(SparkProcessBuilder.scala:61)
at scala.Option.getOrElse(Option.scala:189)
at org.apache.kyuubi.engine.spark.SparkProcessBuilder.<init>(SparkProcessBuilder.scala:61)
at org.apache.kyuubi.engine.EngineRef.$anonfun$create$1(EngineRef.scala:140)
at org.apache.kyuubi.engine.EngineRef.tryWithLock(EngineRef.scala:116)
at org.apache.kyuubi.engine.EngineRef.create(EngineRef.scala:128)
at org.apache.kyuubi.engine.EngineRef.$anonfun$getOrCreate$1(EngineRef.scala:182)
at scala.Option.getOrElse(Option.scala:189)
at org.apache.kyuubi.engine.EngineRef.getOrCreate(EngineRef.scala:182)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$open$2(KyuubiSessionImpl.scala:63)
at org.apache.kyuubi.session.KyuubiSessionImpl.$anonfun$open$2$adapted(KyuubiSessionImpl.scala:62)
at org.apache.kyuubi.ha.client.ZooKeeperClientProvider$.withZkClient(ZooKeeperClientProvider.scala:74)
at org.apache.kyuubi.session.KyuubiSessionImpl.open(KyuubiSessionImpl.scala:62)
at org.apache.kyuubi.session.KyuubiSessionManager.openSession(KyuubiSessionManager.scala:58)
... 12 more
```
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [x] [Run test](https://kyuubi.readthedocs.io/en/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#991 from cfmcgrady/improve-error-msg.
Closes#991
1e8da683 [Fu Chen] imporove the error message
Authored-by: Fu Chen <cfmcgrady@gmail.com>
Signed-off-by: ulysses-you <ulyssesyou18@gmail.com>
9695f75889