Add force single data path option for integ tests (elastic#71868)

Some functionality will no longer work with multiple data paths and in
order to run integration tests for that, we need the capability to
force a single data path for those tests.

Relates elastic#71844

Author: henningandersen

test/framework/src/main/java/org/elasticsearch/test/ESIntegTestCase.java

@@ -1969,7 +1969,7 @@ protected TestCluster buildTestCluster(Scope scope, long seed) throws IOExceptio
         return new InternalTestCluster(seed, createTempDir(), supportsDedicatedMasters, getAutoManageMasterNodes(),
             minNumDataNodes, maxNumDataNodes,
             InternalTestCluster.clusterName(scope.name(), seed) + "-cluster", nodeConfigurationSource, getNumClientNodes(),
-            nodePrefix, mockPlugins, getClientWrapper(), forbidPrivateIndexSettings());
+            nodePrefix, mockPlugins, getClientWrapper(), forbidPrivateIndexSettings(), forceSingleDataPath());
     }
 
     private NodeConfigurationSource getNodeConfigSource() {
@@ -2325,6 +2325,13 @@ protected boolean forbidPrivateIndexSettings() {
         return true;
     }
 
+    /**
+     * Override to return true in tests that cannot handle multiple data paths.
+     */
+    protected boolean forceSingleDataPath() {
+        return false;
+    }
+
     /**
      * Returns an instance of {@link RestClient} pointing to the current test cluster.
      * Creates a new client if the method is invoked for the first time in the context of the current test scope.

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -287,7 +287,8 @@ public InternalTestCluster(
                 nodePrefix,
                 mockPlugins,
                 clientWrapper,
-                true);
+                true,
+                false);
     }
 
     public InternalTestCluster(
@@ -303,7 +304,8 @@ public InternalTestCluster(
             final String nodePrefix,
             final Collection<Class<? extends Plugin>> mockPlugins,
             final Function<Client, Client> clientWrapper,
-            final boolean forbidPrivateIndexSettings) {
+            final boolean forbidPrivateIndexSettings,
+            final boolean forceSingleDataPath) {
         super(clusterSeed);
         this.autoManageMasterNodes = autoManageMasterNodes;
         this.clientWrapper = clientWrapper;
@@ -365,7 +367,8 @@ public InternalTestCluster(
             numSharedDedicatedMasterNodes, numSharedDataNodes, numSharedCoordOnlyNodes,
             autoManageMasterNodes ? "auto-managed" : "manual");
         this.nodeConfigurationSource = nodeConfigurationSource;
-        numDataPaths = random.nextInt(5) == 0 ? 2 + random.nextInt(3) : 1;
+        // use 1 data path if we are forced to, or 80% of the time that we are not, otherwise use between 2 and 4 data paths
+        numDataPaths = forceSingleDataPath || random.nextDouble() < 0.8 ? 1 : RandomNumbers.randomIntBetween(random, 2, 4);
         Builder builder = Settings.builder();
         builder.put(Environment.PATH_HOME_SETTING.getKey(), baseDir);
         builder.put(Environment.PATH_REPO_SETTING.getKey(), baseDir.resolve("repos"));