Sentry starts but dysfunctional when Web Port is unavailable

Sentry clients (e.g., HMS) show errors like:

2019-02-26 14:04:13,451 ERROR org.apache.sentry.core.common.transport.SentryTransportPool: [pool-9-thread-131]: Failed to obtain transport for XXXX.net:8038: null
2019-02-26 14:04:40,815 ERROR org.apache.sentry.core.common.transport.SentryTransportPool: [pool-9-thread-130]: Failed to obtain transport for XXXX.net:8038: null
2019-02-26 14:04:43,816 ERROR org.apache.sentry.binding.metastore.SentrySyncHMSNotificationsPostEventListener: [pool-9-thread-130]: Encountered failure while notifying notification for CREATE_DATABASE (id: 19)
org.apache.sentry.core.common.exception.SentryUserException: java.net.SocketException: Connection reset
at org.apache.sentry.core.common.transport.RetryClientInvocationHandler.connect(RetryClientInvocationHandler.java:166) ~[sentry-core-common-2.1.0-cdh6.1.1.jar:?]
at org.apache.sentry.core.common.transport.RetryClientInvocationHandler.invokeImpl(RetryClientInvocationHandler.java:90) ~[sentry-core-common-2.1.0-cdh6.1.1.jar:?]
at org.apache.sentry.core.common.transport.SentryClientInvocationHandler.invoke(SentryClientInvocationHandler.java:41) ~[sentry-core-common-2.1.0-cdh6.1.1.jar:?]
at com.sun.proxy.$Proxy34.notifyHmsEvent(Unknown Source) ~[?:?]
::::::

Sentry stderr log shows:

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.Exception: Error starting server
at java.util.concurrent.FutureTask.report(FutureTask.java:122)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at org.apache.sentry.service.thrift.SentryService$CommandImpl.run(SentryService.java:596)
at org.apache.sentry.SentryMain.main(SentryMain.java:120)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.hadoop.util.RunJar.run(RunJar.java:313)
at org.apache.hadoop.util.RunJar.main(RunJar.java:227)
Caused by: java.lang.Exception: Error starting server
at org.apache.sentry.service.thrift.SentryService.call(SentryService.java:207)
at org.apache.sentry.service.thrift.SentryService.call(SentryService.java:81)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.security.PrivilegedActionException: java.net.BindException: Address already in use
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.sentry.service.thrift.SentryService.call(SentryService.java:195)
... 5 more
Caused by: java.net.BindException: Address already in use
at sun.nio.ch.Net.bind0(Native Method)
at sun.nio.ch.Net.bind(Net.java:433)
at sun.nio.ch.Net.bind(Net.java:425)
at sun.nio.ch.ServerSocketChannelImpl.bind(ServerSocketChannelImpl.java:223)
at sun.nio.ch.ServerSocketAdaptor.bind(ServerSocketAdaptor.java:74)
at org.eclipse.jetty.server.ServerConnector.open(ServerConnector.java:317)
at org.eclipse.jetty.server.AbstractNetworkConnector.doStart(AbstractNetworkConnector.java:80)
at org.eclipse.jetty.server.ServerConnector.doStart(ServerConnector.java:235)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.eclipse.jetty.server.Server.doStart(Server.java:401)
at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
at org.apache.sentry.api.service.thrift.SentryWebServer.start(SentryWebServer.java:190)
at org.apache.sentry.service.thrift.SentryService.startSentryWebServer(SentryService.java:428)
at org.apache.sentry.service.thrift.SentryService.runServer(SentryService.java:273)
at org.apache.sentry.service.thrift.SentryService.access$000(SentryService.java:81)
at org.apache.sentry.service.thrift.SentryService$1.run(SentryService.java:198)
at org.apache.sentry.service.thrift.SentryService$1.run(SentryService.java:195)
... 8 more

port 29000 turns out to be taken by another application:

$ netstat -tulpn | grep 29000
tcp 0 0 0.0.0.0:29000 0.0.0.0:* LISTEN 9510/BLAH

. But Sentry still starts as if normal although the clients (Hive, Impala, etc.) cannot run queries. That is extremely confusing. Sentry startup should be corrected to fail to start in those situations with a clear error pointing at the cause.