Fix secure repository-hdfs tests on JDK 9

The secure repository-hdfs tests fail on JDK 9 because some Hadoop code
reaches into sun.security.krb5. This commit adds the necessary flags to
open the java.security.jgss module. Note that these flags are actually
needed at runtime as well when using secure repository-hdfs. For now we
will punt on how best to help users obtain this when running on JDK 9
with this plugin.

Relates #25205

plugins/repository-hdfs/build.gradle

@@ -122,10 +122,18 @@ task secureHdfsFixture(type: org.elasticsearch.gradle.test.AntFixture) {
   Path keytabPath = project(':test:fixtures:krb5kdc-fixture').buildDir.toPath().resolve("keytabs").resolve("hdfs_hdfs.build.elastic.co.keytab")
   Path krb5Config = project(':test:fixtures:krb5kdc-fixture').buildDir.toPath().resolve("conf").resolve("krb5.conf")
 
-  args "-Djava.security.krb5.conf=${krb5Config}", 'hdfs.MiniHDFS',
-          baseDir,
-          "hdfs/hdfs.build.elastic.co@${realm}",
-          "${keytabPath}"
+  final List<String> miniHDFSArgs = ["-Djava.security.krb5.conf=${krb5Config}"]
+
+  if (project.rootProject.ext.javaVersion == JavaVersion.VERSION_1_9) {
+    miniHDFSArgs.add('--add-opens=java.security.jgss/sun.security.krb5=ALL-UNNAMED')
+  }
+
+  miniHDFSArgs.add('hdfs.MiniHDFS')
+  miniHDFSArgs.add(baseDir)
+  miniHDFSArgs.add("hdfs/hdfs.build.elastic.co@${realm}")
+  miniHDFSArgs.add("${keytabPath}")
+
+  args miniHDFSArgs.toArray()
 }
 
 boolean fixtureSupported = false
@@ -170,10 +178,19 @@ project.afterEvaluate {
   project.integTestSecureCluster.dependsOn(project.bundlePlugin)
   project.integTestSecure.clusterConfig.plugin(project.path)
   project.integTestSecure.clusterConfig.extraConfigFile("repository-hdfs/krb5.keytab", "${elasticsearchKT}")
-  project.integTestSecure.clusterConfig.jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
+
+  final String baseJvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
           " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
           " " + "-Djava.security.krb5.conf=${krb5conf}" +
           " " + System.getProperty('tests.jvm.argline', '')
+  final String jvmArgs
+  if (project.rootProject.ext.javaVersion == JavaVersion.VERSION_1_9) {
+    jvmArgs = baseJvmArgs + " " + '--add-opens=java.security.jgss/sun.security.krb5=ALL-UNNAMED'
+  } else {
+    jvmArgs = baseJvmArgs
+  }
+
+  project.integTestSecure.clusterConfig.jvmArgs = jvmArgs
 }
 
 RestIntegTestTask integTestSecure = project.tasks.create('integTestSecure', RestIntegTestTask.class) {