Add methods to ProjectStateRegistry and its builder

server/src/main/java/org/elasticsearch/cluster/project/ProjectStateRegistry.java

@@ -54,6 +54,10 @@ public class ProjectStateRegistry extends AbstractNamedDiffable<Custom> implemen
     // A counter that is incremented each time one or more projects are marked for deletion.
     private final long projectsMarkedForDeletionGeneration;
 
+    public static ProjectStateRegistry get(ClusterState clusterState) {
+        return clusterState.custom(TYPE, EMPTY);
+    }
+
     public ProjectStateRegistry(StreamInput in) throws IOException {
         if (in.getTransportVersion().onOrAfter(TransportVersions.PROJECT_STATE_REGISTRY_ENTRY)) {
             projectsEntries = in.readMap(ProjectId::readFrom, Entry::readFrom);
@@ -80,6 +84,10 @@ private ProjectStateRegistry(
         this.projectsMarkedForDeletionGeneration = projectsMarkedForDeletionGeneration;
     }
 
+    public boolean hasProject(ProjectId projectId) {
+        return projectsEntries.containsKey(projectId);
+    }
+
     /**
      * Retrieves the settings for a specific project based on its project ID from the specified cluster state without creating a new object.
      * If you need a full state of the project rather than just its setting, please use {@link ClusterState#projectState(ProjectId)}
@@ -97,6 +105,10 @@ public Settings getProjectSettings(ProjectId projectId) {
         return projectsEntries.getOrDefault(projectId, EMPTY_ENTRY).settings;
     }
 
+    public Set<ProjectId> getProjectsMarkedForDeletion() {
+        return projectsMarkedForDeletion;
+    }
+
     public boolean isProjectMarkedForDeletion(ProjectId projectId) {
         return projectsMarkedForDeletion.contains(projectId);
     }
@@ -310,6 +322,12 @@ public Builder markProjectForDeletion(ProjectId projectId) {
             return this;
         }
 
+        public Builder removeProject(ProjectId projectId) {
+            projectsEntries.remove(projectId);
+            projectsMarkedForDeletion.remove(projectId);
+            return this;
+        }
+
         public ProjectStateRegistry build() {
             final var unknownButUnderDeletion = Sets.difference(projectsMarkedForDeletion, projectsEntries.keys());
             if (unknownButUnderDeletion.isEmpty() == false) {
@@ -319,7 +337,7 @@ public ProjectStateRegistry build() {
             }
             return new ProjectStateRegistry(
                 projectsEntries.build(),
-                projectsMarkedForDeletion,
+                Collections.unmodifiableSet(projectsMarkedForDeletion),
                 newProjectMarkedForDeletion ? projectsMarkedForDeletionGeneration + 1 : projectsMarkedForDeletionGeneration
             );
         }

server/src/test/java/org/elasticsearch/cluster/project/ProjectStateRegistryTests.java

@@ -46,6 +46,11 @@ public void testBuilder() {
         var unknownProjectId = randomUniqueProjectId();
         var throwingBuilder = ProjectStateRegistry.builder(projectStateRegistry).markProjectForDeletion(unknownProjectId);
         assertThrows(IllegalArgumentException.class, throwingBuilder::build);
+
+        var projectToRemove = randomFrom(projectStateRegistry.knownProjects());
+        projectStateRegistry = ProjectStateRegistry.builder(projectStateRegistry).removeProject(projectToRemove).build();
+        assertFalse(projectStateRegistry.hasProject(projectToRemove));
+        assertFalse(projectStateRegistry.isProjectMarkedForDeletion(projectToRemove));
     }
 
     public void testDiff() {