[jira] [Created] (HADOOP-18815) unnecessary NullPointerException encountered when starting HttpServer2 with prometheus enabled

["ConfX (Jira)" <ji...@apache.org>]

ConfX created HADOOP-18815:
------------------------------

             Summary: unnecessary NullPointerException encountered when starting HttpServer2 with prometheus enabled 
                 Key: HADOOP-18815
                 URL: https://issues.apache.org/jira/browse/HADOOP-18815
             Project: Hadoop Common
          Issue Type: Bug
            Reporter: ConfX
         Attachments: reproduce.sh

h2. What happened?

Attempt to start an {{HttpServer2}} failed due to an NPE thrown in {{{}MetricsSystemImpl{}}}.
h2. Where's the bug?

In line 1278 of {{{}HttpServer2{}}}, if the support for prometheus is enabled the server registers a prometheus sink:
{noformat}
        if (prometheusSupport) {
          DefaultMetricsSystem.instance()
              .register("prometheus", "Hadoop metrics prometheus exporter",
                  prometheusMetricsSink);
        }{noformat}
However, a problem is that if the MetricsSystemImpl returned by the DefaultMetricsSystem.instance has not been start nor init, the config of the metric system would be set to null, thus failing the nullity check at the start of MetricsSystemImpl.registerSink. A better way of handling this would be to check in advance if the metric system has been initialized and initialize it if it has not been initialized.How to reproduce?(1) set hadoop.prometheus.endpoint.enabled to true (2) run org.apache.hadoop.http.TestHttpServer#testHttpResonseContainsDenyStacktrace
{noformat}
java.io.IOException: Problem starting http server
        ...
Caused by: java.lang.NullPointerException: config
    at org.apache.hadoop.thirdparty.com.google.common.base.Preconditions.checkNotNull(Preconditions.java:899)
    at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.registerSink(MetricsSystemImpl.java:298)
    at org.apache.hadoop.metrics2.impl.MetricsSystemImpl.register(MetricsSystemImpl.java:277)
    at org.apache.hadoop.http.HttpServer2.start(HttpServer2.java:1279)
    ... 34 more{noformat}
For an easy reproduction, run the reproduce.sh in the attachment.

We are happy to provide a patch if this issue is confirmed.



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