Restrict testing of legacy discovery to tests (#61178)

The 7.x branch preserves the legacy discovery mechanism from 6.x purely
for running internal cluster tests; this mechanism is otherwise
completely untested and unsupported. However it is still technically
possible to use it outside of the test suite if you dig through the
source code to work out what settings need to be set. With this change
we make it impossible to use this mechanism in production.

Closes #61177

server/src/main/java/org/elasticsearch/bootstrap/BootstrapChecks.java

@@ -739,6 +739,11 @@ boolean isAllPermissionGranted() {
     static class DiscoveryConfiguredCheck implements BootstrapCheck {
         @Override
         public BootstrapCheckResult check(BootstrapContext context) {
+            if (DiscoveryModule.ZEN_DISCOVERY_TYPE.equals(DiscoveryModule.DISCOVERY_TYPE_SETTING.get(context.settings()))) {
+                return BootstrapCheckResult.failure(String.format(Locale.ROOT,
+                    "discovery type [%s] is unsuitable for production use", DiscoveryModule.ZEN_DISCOVERY_TYPE));
+            }
+
             if (DiscoveryModule.ZEN2_DISCOVERY_TYPE.equals(DiscoveryModule.DISCOVERY_TYPE_SETTING.get(context.settings())) == false) {
                 return BootstrapCheckResult.success();
             }

server/src/main/java/org/elasticsearch/discovery/DiscoveryModule.java

@@ -19,8 +19,9 @@
 
 package org.elasticsearch.discovery;
 
-import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.elasticsearch.Assertions;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.coordination.Coordinator;
 import org.elasticsearch.cluster.coordination.ElectionStrategy;
@@ -67,7 +68,7 @@
 public class DiscoveryModule {
     private static final Logger logger = LogManager.getLogger(DiscoveryModule.class);
 
-    public static final String ZEN_DISCOVERY_TYPE = "legacy-zen";
+    public static final String ZEN_DISCOVERY_TYPE = "legacy-zen-for-testing-only-do-not-use";
     public static final String ZEN2_DISCOVERY_TYPE = "zen";
 
     public static final String SINGLE_NODE_DISCOVERY_TYPE = "single-node";
@@ -155,7 +156,7 @@ public DiscoveryModule(Settings settings, ThreadPool threadPool, TransportServic
                 transportService, namedWriteableRegistry, allocationService, masterService, gatewayMetaState::getPersistedState,
                 seedHostsProvider, clusterApplier, joinValidators, new Random(Randomness.get().nextLong()), rerouteService,
                 electionStrategy, nodeHealthService);
-        } else if (ZEN_DISCOVERY_TYPE.equals(discoveryType)) {
+        } else if (Assertions.ENABLED && ZEN_DISCOVERY_TYPE.equals(discoveryType)) {
             discovery = new ZenDiscovery(settings, threadPool, transportService, namedWriteableRegistry, masterService, clusterApplier,
                 clusterSettings, seedHostsProvider, allocationService, joinValidators, rerouteService);
         } else {

server/src/test/java/org/elasticsearch/bootstrap/BootstrapChecksTests.java

@@ -723,6 +723,9 @@ public boolean alwaysEnforce() {
     public void testDiscoveryConfiguredCheck() throws NodeValidationException {
         final List<BootstrapCheck> checks = Collections.singletonList(new BootstrapChecks.DiscoveryConfiguredCheck());
 
+        final BootstrapContext zen1Context = createTestContext(Settings.builder()
+                .put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(), ZEN_DISCOVERY_TYPE).build(), Metadata.EMPTY_METADATA);
+
         final BootstrapContext zen2Context = createTestContext(Settings.builder()
             .put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(), ZEN2_DISCOVERY_TYPE).build(), Metadata.EMPTY_METADATA);
 
@@ -731,7 +734,11 @@ public void testDiscoveryConfiguredCheck() throws NodeValidationException {
 
         // not enforced for non-zen2 discovery
         BootstrapChecks.check(createTestContext(Settings.builder().put(DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey(),
-            randomFrom(ZEN_DISCOVERY_TYPE, "single-node", randomAlphaOfLength(5))).build(), Metadata.EMPTY_METADATA), true, checks);
+            randomFrom("single-node", randomAlphaOfLength(5))).build(), Metadata.EMPTY_METADATA), true, checks);
+
+        assertThat(expectThrows(NodeValidationException.class,
+                () -> BootstrapChecks.check(zen1Context, true, checks)),
+                hasToString(containsString("discovery type [legacy-zen-for-testing-only-do-not-use] is unsuitable for production use")));
 
         final NodeValidationException e = expectThrows(NodeValidationException.class,
             () -> BootstrapChecks.check(zen2Context, true, checks));