Allow project_routing param in _search and _async_sync under feature flag only (elastic#134240)

Allowing a project_routing query param in _search and _async_search under a serverless feature flag
unblocks work that Kibana needs to do. The param is read but thrown away, so that searches do not
behave any differently for now. Later work will implement functionality around this parameter.

Author: quux00

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