[7.17] Register reserved routes so we do not register them by accident (

#92893) (#92994)

* Register reserved routes so we do not register them by accident (#92893)

This commits adds four "reserved" paths that Elasticsearch should not register under any cases. By registering these and returning a bad request (we don't actually serve them), we can ensure that if we ever try to register them in the path, Elasticsearch will trip an assertion and fail to build.

(cherry picked from commit 4573c56)

# Conflicts:
#	server/src/main/java/org/elasticsearch/rest/RestController.java
#	server/src/test/java/org/elasticsearch/rest/RestControllerTests.java

* Fix compilation

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

server/src/main/java/org/elasticsearch/rest/RestController.java

@@ -62,6 +62,12 @@ public class RestController implements HttpServerTransport.Dispatcher {
     private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(RestController.class);
     static final String ELASTIC_PRODUCT_HTTP_HEADER = "X-elastic-product";
     static final String ELASTIC_PRODUCT_HTTP_HEADER_VALUE = "Elasticsearch";
+    static final Set<String> RESERVED_PATHS = org.elasticsearch.core.Set.of(
+        "/__elb_health__",
+        "/__elb_health__/zk",
+        "/_health",
+        "/_health/zk"
+    );
 
     private static final BytesReference FAVICON_RESPONSE;
 
@@ -206,6 +212,9 @@ protected void registerHandler(RestRequest.Method method, String path, RestHandl
     }
 
     private void registerHandlerNoWrap(RestRequest.Method method, String path, RestHandler maybeWrappedHandler) {
+        if (RESERVED_PATHS.contains(path)) {
+            throw new IllegalArgumentException("path [" + path + "] is a reserved path and may not be registered");
+        }
         handlers.insertOrUpdate(
             path,
             new MethodHandlers(path).addMethod(method, maybeWrappedHandler),

server/src/test/java/org/elasticsearch/rest/RestControllerTests.java

@@ -712,6 +712,26 @@ public Exception getInboundException() {
         assertThat(channel.getRestResponse().getHeaders().get("Allow"), hasItem(equalTo(GET.toString())));
     }
 
+    public void testRegisterWithReservedPath() {
+        final RestController restController = new RestController(Collections.emptySet(), null, client, circuitBreakerService, usageService);
+        for (String path : RestController.RESERVED_PATHS) {
+            IllegalArgumentException iae = expectThrows(IllegalArgumentException.class, () -> {
+                restController.registerHandler(new Route(randomFrom(RestRequest.Method.values()), path), new RestHandler() {
+                    @Override
+                    public void handleRequest(RestRequest request, RestChannel channel, NodeClient client) {
+                        channel.sendResponse(new BytesRestResponse(RestStatus.OK, BytesRestResponse.TEXT_CONTENT_TYPE, BytesArray.EMPTY));
+                    }
+
+                    @Override
+                    public boolean supportsContentStream() {
+                        return true;
+                    }
+                });
+            });
+            assertThat(iae.getMessage(), containsString("path [" + path + "] is a reserved path and may not be registered"));
+        }
+    }
+
     private static final class TestHttpServerTransport extends AbstractLifecycleComponent implements HttpServerTransport {
 
         TestHttpServerTransport() {}