Add seperate interface for UIAM authentication

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

@@ -43,7 +43,27 @@ default boolean includeDataStreams() {
         return false;
     }
 
-    interface Replaceable extends IndicesRequest {
+    /**
+     * Interface for indicating potential work related to cross-project authentication and authorization.
+     */
+    interface CrossProjectCandidate {
+
+        /**
+         * Determines whether the request type can support cross-project processing. Cross-project processing entails
+         * 1. UIAM authentication and authorization projects resolution.
+         * 2. If applicable, cross-project flat-world index resolution and error handling
+         * Note: this method only determines in the request _supports_ cross-project. Whether cross-project processing
+         * is actually performed depends on other factors such as:
+         * - Whether CPS is enabled which impacts both 1 and 2.
+         * - Whether {@link IndicesOptions} supports it when the request is an {@link IndicesRequest}. This only impacts 2.
+         * See also {@link org.elasticsearch.search.crossproject.CrossProjectModeDecider}.
+         */
+        default boolean allowsCrossProject() {
+            return false;
+        }
+    }
+
+    interface Replaceable extends IndicesRequest, CrossProjectCandidate {
         /**
          * Sets the indices that the action relates to.
          */
@@ -81,15 +101,6 @@ default boolean allowsRemoteIndices() {
             return false;
         }
 
-        /**
-         * Determines whether the request type allows cross-project processing. Cross-project processing entails cross-project search
-         * index resolution and error handling. Note: this method only determines in the request _supports_ cross-project.
-         * Whether cross-project processing is actually performed is determined by {@link IndicesOptions}.
-         */
-        default boolean allowsCrossProject() {
-            return false;
-        }
-
         @Nullable // if no routing is specified
         default String getProjectRouting() {
             return null;
@@ -103,7 +114,7 @@ default String getProjectRouting() {
      *
      * This may change with https://github.com/elastic/elasticsearch/issues/105598
      */
-    interface SingleIndexNoWildcards extends IndicesRequest {
+    interface SingleIndexNoWildcards extends IndicesRequest, CrossProjectCandidate {
         default boolean allowsRemoteIndices() {
             return true;
         }

server/src/main/java/org/elasticsearch/search/crossproject/CrossProjectModeDecider.java

@@ -44,11 +44,18 @@ public boolean crossProjectEnabled() {
         return crossProjectEnabled;
     }
 
-    public boolean resolvesCrossProject(IndicesRequest.Replaceable request) {
+    public boolean resolvesCrossProject(IndicesRequest.CrossProjectCandidate request) {
         if (crossProjectEnabled == false) {
             return false;
         }
+
         // TODO: The following check can be an method on the request itself
-        return request.allowsCrossProject() && request.indicesOptions().resolveCrossProjectIndexExpression();
+        if (request.allowsCrossProject() == false) {
+            return false;
+        }
+        if (request instanceof IndicesRequest indicesRequest) {
+            return indicesRequest.indicesOptions().resolveCrossProjectIndexExpression();
+        }
+        return true;
     }
 }

server/src/test/java/org/elasticsearch/search/crossproject/CrossProjectModeDeciderTests.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.search.crossproject;
+
+import org.elasticsearch.action.IndicesRequest;
+import org.elasticsearch.action.support.IndicesOptions;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.is;
+
+public class CrossProjectModeDeciderTests extends ESTestCase {
+
+    public void testResolvesCrossProject() {
+        doTestResolvesCrossProject(new CrossProjectModeDecider(Settings.builder().build()), false);
+        doTestResolvesCrossProject(
+            new CrossProjectModeDecider(Settings.builder().put("serverless.cross_project.enabled", true).build()),
+            true
+        );
+    }
+
+    private void doTestResolvesCrossProject(CrossProjectModeDecider crossProjectModeDecider, boolean expected) {
+        final var cpsIndicesOptions = IndicesOptions.builder(org.elasticsearch.action.support.IndicesOptions.DEFAULT)
+            .crossProjectModeOptions(new IndicesOptions.CrossProjectModeOptions(true))
+            .build();
+
+        final var candidateButNotAllowed = randomFrom(
+            new CrossProjectCandidateImpl(false),
+            new IndicesRequestImpl(false, randomFrom(IndicesOptions.DEFAULT, cpsIndicesOptions))
+        );
+        final var candidateAndAllowed = new CrossProjectCandidateImpl(true);
+
+        final var indicesRequestNoCpsOption = new IndicesRequestImpl(true, IndicesOptions.DEFAULT);
+        final var indicesRequestWithCpsOption = new IndicesRequestImpl(true, cpsIndicesOptions);
+
+        assertFalse(crossProjectModeDecider.resolvesCrossProject(candidateButNotAllowed));
+        assertFalse(crossProjectModeDecider.resolvesCrossProject(indicesRequestNoCpsOption));
+
+        assertThat(crossProjectModeDecider.resolvesCrossProject(candidateAndAllowed), is(expected));
+        assertThat(crossProjectModeDecider.resolvesCrossProject(indicesRequestWithCpsOption), is(expected));
+    }
+
+    private static class CrossProjectCandidateImpl implements IndicesRequest.CrossProjectCandidate {
+
+        private boolean allowsCrossProject;
+
+        CrossProjectCandidateImpl(boolean allowsCrossProject) {
+            this.allowsCrossProject = allowsCrossProject;
+        }
+
+        @Override
+        public boolean allowsCrossProject() {
+            return allowsCrossProject;
+        }
+    }
+
+    private static class IndicesRequestImpl extends CrossProjectCandidateImpl implements IndicesRequest {
+
+        private IndicesOptions indicesOptions;
+
+        IndicesRequestImpl(boolean allowsCrossProject, IndicesOptions indicesOptions) {
+            super(allowsCrossProject);
+            this.indicesOptions = indicesOptions;
+        }
+
+        @Override
+        public String[] indices() {
+            return new String[0];
+        }
+
+        @Override
+        public IndicesOptions indicesOptions() {
+            return indicesOptions;
+        }
+    }
+}

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authz/IndicesAndAliasesResolver.java

@@ -167,7 +167,8 @@ ResolvedIndices tryResolveWithoutWildcards(String action, TransportRequest trans
     }
 
     boolean resolvesCrossProject(TransportRequest request) {
-        return request instanceof IndicesRequest.Replaceable replaceable && crossProjectModeDecider.resolvesCrossProject(replaceable);
+        return request instanceof IndicesRequest.CrossProjectCandidate crossProjectCandidate
+            && crossProjectModeDecider.resolvesCrossProject(crossProjectCandidate);
     }
 
     private static boolean requiresWildcardExpansion(IndicesRequest indicesRequest) {
@@ -560,6 +561,7 @@ private static void setResolvedIndexExpressionsIfUnset(IndicesRequest.Replaceabl
                 + "]";
             logger.debug(message);
             // we are excepting `*,-*` below since we've observed this already -- keeping this assertion to catch other cases
+            // If more exceptions are found, we can add a comment to above linked issue and relax this check further
             assert replaceable.indices() == null || isNoneExpression(replaceable.indices()) : message;
         }
     }