Allow project_routing param in _search and _async_sync under feature flag only

server/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -942,7 +942,7 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster, Predicate<
         registerHandler.accept(new RestBulkAction(settings, clusterSettings, bulkService));
         registerHandler.accept(new RestUpdateAction());
 
-        registerHandler.accept(new RestSearchAction(restController.getSearchUsageHolder(), clusterSupportsFeature));
+        registerHandler.accept(new RestSearchAction(restController.getSearchUsageHolder(), clusterSupportsFeature, settings));
         registerHandler.accept(new RestSearchScrollAction());
         registerHandler.accept(new RestClearScrollAction());
         registerHandler.accept(new RestOpenPointInTimeAction());

server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java

@@ -16,6 +16,7 @@
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.client.internal.node.NodeClient;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.core.Booleans;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.features.NodeFeature;
@@ -66,10 +67,16 @@ public class RestSearchAction extends BaseRestHandler {
 
     private final SearchUsageHolder searchUsageHolder;
     private final Predicate<NodeFeature> clusterSupportsFeature;
+    private final Settings settings;
 
     public RestSearchAction(SearchUsageHolder searchUsageHolder, Predicate<NodeFeature> clusterSupportsFeature) {
+        this(searchUsageHolder, clusterSupportsFeature, null);
+    }
+
+    public RestSearchAction(SearchUsageHolder searchUsageHolder, Predicate<NodeFeature> clusterSupportsFeature, Settings settings) {
         this.searchUsageHolder = searchUsageHolder;
         this.clusterSupportsFeature = clusterSupportsFeature;
+        this.settings = settings;
     }
 
     @Override
@@ -101,6 +108,12 @@ public RestChannelConsumer prepareRequest(final RestRequest request, final NodeC
         // access the BwC param, but just drop it
         // this might be set by old clients
         request.param("min_compatible_shard_node");
+
+        if (settings != null && settings.getAsBoolean("serverless.cross_project.enabled", false)) {
+            // accept but drop project_routing param until fully supported
+            request.param("project_routing");
+        }
+
         /*
          * We have to pull out the call to `source().size(size)` because
          * _update_by_query and _delete_by_query uses this same parsing

x-pack/plugin/async-search/src/main/java/org/elasticsearch/xpack/search/AsyncSearch.java

@@ -55,7 +55,7 @@ public List<RestHandler> getRestHandlers(
         Predicate<NodeFeature> clusterSupportsFeature
     ) {
         return Arrays.asList(
-            new RestSubmitAsyncSearchAction(restController.getSearchUsageHolder(), clusterSupportsFeature),
+            new RestSubmitAsyncSearchAction(restController.getSearchUsageHolder(), clusterSupportsFeature, settings),
             new RestGetAsyncSearchAction(),
             new RestGetAsyncStatusAction(),
             new RestDeleteAsyncSearchAction()

x-pack/plugin/async-search/src/main/java/org/elasticsearch/xpack/search/RestSubmitAsyncSearchAction.java

@@ -7,6 +7,7 @@
 package org.elasticsearch.xpack.search;
 
 import org.elasticsearch.client.internal.node.NodeClient;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.rest.BaseRestHandler;
 import org.elasticsearch.rest.RestRequest;
@@ -37,10 +38,20 @@ public final class RestSubmitAsyncSearchAction extends BaseRestHandler {
 
     private final SearchUsageHolder searchUsageHolder;
     private final Predicate<NodeFeature> clusterSupportsFeature;
+    private final Settings settings;
 
     public RestSubmitAsyncSearchAction(SearchUsageHolder searchUsageHolder, Predicate<NodeFeature> clusterSupportsFeature) {
+        this(searchUsageHolder, clusterSupportsFeature, null);
+    }
+
+    public RestSubmitAsyncSearchAction(
+        SearchUsageHolder searchUsageHolder,
+        Predicate<NodeFeature> clusterSupportsFeature,
+        Settings settings
+    ) {
         this.searchUsageHolder = searchUsageHolder;
         this.clusterSupportsFeature = clusterSupportsFeature;
+        this.settings = settings;
     }
 
     @Override
@@ -59,6 +70,12 @@ protected RestChannelConsumer prepareRequest(RestRequest request, NodeClient cli
             client.threadPool().getThreadContext().setErrorTraceTransportHeader(request);
         }
         SubmitAsyncSearchRequest submit = new SubmitAsyncSearchRequest();
+
+        if (settings != null && settings.getAsBoolean("serverless.cross_project.enabled", false)) {
+            // accept but drop project_routing param until fully supported
+            request.param("project_routing");
+        }
+
         IntConsumer setSize = size -> submit.getSearchRequest().source().size(size);
         // for simplicity, we share parsing with ordinary search. That means a couple of unsupported parameters, like scroll
         // and pre_filter_shard_size get set to the search request although the REST spec don't list