Enabling testing against an external cluster (#30885)

Today when executing REST tests we take full responsibility for cluster
configuration. Yet, there are use cases for brining your own cluster to
the REST tests. This commit is a small first step towards that effort by
skipping creating the cluster if the tests.rest.cluster and test.cluster
system properties are set. In this case, the user takes full
responsibility for configuring the cluster as expected by the REST
tests. This step is by no means meant to be perfect or complete, only a
baby step.

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/RestIntegTestTask.groovy

@@ -70,31 +70,44 @@ public class RestIntegTestTask extends DefaultTask {
         runner.parallelism = '1'
         runner.include('**/*IT.class')
         runner.systemProperty('tests.rest.load_packaged', 'false')
-        // we pass all nodes to the rest cluster to allow the clients to round-robin between them
-        // this is more realistic than just talking to a single node
-        runner.systemProperty('tests.rest.cluster', "${-> nodes.collect{it.httpUri()}.join(",")}")
-        runner.systemProperty('tests.config.dir', "${-> nodes[0].pathConf}")
-        // TODO: our "client" qa tests currently use the rest-test plugin. instead they should have their own plugin
-        // that sets up the test cluster and passes this transport uri instead of http uri. Until then, we pass
-        // both as separate sysprops
-        runner.systemProperty('tests.cluster', "${-> nodes[0].transportUri()}")
-
-        // dump errors and warnings from cluster log on failure
-        TaskExecutionAdapter logDumpListener = new TaskExecutionAdapter() {
-            @Override
-            void afterExecute(Task task, TaskState state) {
-                if (state.failure != null) {
-                    for (NodeInfo nodeInfo : nodes) {
-                        printLogExcerpt(nodeInfo)
+
+        if (System.getProperty("tests.rest.cluster") == null) {
+            if (System.getProperty("tests.cluster") != null) {
+                throw new IllegalArgumentException("tests.rest.cluster and tests.cluster must both be null or non-null")
+            }
+            // we pass all nodes to the rest cluster to allow the clients to round-robin between them
+            // this is more realistic than just talking to a single node
+            runner.systemProperty('tests.rest.cluster', "${-> nodes.collect{it.httpUri()}.join(",")}")
+            runner.systemProperty('tests.config.dir', "${-> nodes[0].pathConf}")
+            // TODO: our "client" qa tests currently use the rest-test plugin. instead they should have their own plugin
+            // that sets up the test cluster and passes this transport uri instead of http uri. Until then, we pass
+            // both as separate sysprops
+            runner.systemProperty('tests.cluster', "${-> nodes[0].transportUri()}")
+
+            // dump errors and warnings from cluster log on failure
+            TaskExecutionAdapter logDumpListener = new TaskExecutionAdapter() {
+                @Override
+                void afterExecute(Task task, TaskState state) {
+                    if (state.failure != null) {
+                        for (NodeInfo nodeInfo : nodes) {
+                            printLogExcerpt(nodeInfo)
+                        }
                     }
                 }
             }
-        }
-        runner.doFirst {
-            project.gradle.addListener(logDumpListener)
-        }
-        runner.doLast {
-            project.gradle.removeListener(logDumpListener)
+            runner.doFirst {
+                project.gradle.addListener(logDumpListener)
+            }
+            runner.doLast {
+                project.gradle.removeListener(logDumpListener)
+            }
+        } else {
+            if (System.getProperty("tests.cluster") == null) {
+                throw new IllegalArgumentException("tests.rest.cluster and tests.cluster must both be null or non-null")
+            }
+            // an external cluster was specified and all responsibility for cluster configuration is taken by the user
+            runner.systemProperty('tests.rest.cluster', System.getProperty("tests.rest.cluster"))
+            runner.systemProperty('test.cluster', System.getProperty("tests.cluster"))
         }
 
         // copy the rest spec/tests into the test resources
@@ -109,7 +122,10 @@ public class RestIntegTestTask extends DefaultTask {
                 clusterInit.enabled = false
                 return // no need to add cluster formation tasks if the task won't run!
             }
-            nodes = ClusterFormationTasks.setup(project, "${name}Cluster", runner, clusterConfig)
+            // only create the cluster if needed as otherwise an external cluster to use was specified
+            if (System.getProperty("tests.rest.cluster") == null) {
+                nodes = ClusterFormationTasks.setup(project, "${name}Cluster", runner, clusterConfig)
+            }
             super.dependsOn(runner.finalizedBy)
         }
     }