Add optimized updateSingleProject on Metadata (#129585)

For legacy reasons, `Metadata.Builder` contains a map of
`ProjectMetadata.Builder`s instead of a map of `ProjectMetadata`s. While
the plan is still to change that, there is a significant amount of
refactoring that needs to be done before we can make that change.

We have seen regressions in benchmarks due to this extra conversion of
project metadata -> builder -> project metadata. We add a method that
allows updating a single project without having to convert all the
projects to builders first.

Author: nielsbauman

server/src/main/java/org/elasticsearch/cluster/ClusterState.java

@@ -1005,7 +1005,7 @@ public ClusterState copyAndUpdateMetadata(Consumer<Metadata.Builder> updater) {
     }
 
     public ClusterState copyAndUpdateProject(ProjectId projectId, Consumer<ProjectMetadata.Builder> updater) {
-        return copyAndUpdate(builder -> builder.putProjectMetadata(metadata().getProject(projectId).copyAndUpdate(updater)));
+        return copyAndUpdate(builder -> builder.metadata(metadata.copyAndUpdateProject(projectId, updater)));
     }
 
     @SuppressForbidden(reason = "directly reading ClusterState#clusterFeatures")

server/src/main/java/org/elasticsearch/cluster/ProjectState.java

@@ -73,14 +73,14 @@ public int hashCode() {
     public ClusterState updatedState(Consumer<ProjectMetadata.Builder> projectBuilderConsumer) {
         ProjectMetadata.Builder projectBuilder = ProjectMetadata.builder(metadata());
         projectBuilderConsumer.accept(projectBuilder);
-        return ClusterState.builder(cluster).putProjectMetadata(projectBuilder).build();
+        return updatedState(projectBuilder.build());
     }
 
     /**
      * Build a new {@link ClusterState} with the updated project.
      */
     public ClusterState updatedState(ProjectMetadata updatedProject) {
-        return ClusterState.builder(cluster).putProjectMetadata(updatedProject).build();
+        return ClusterState.builder(cluster).metadata(cluster.metadata().withUpdatedProject(updatedProject)).build();
     }
 
     /**
@@ -100,6 +100,6 @@ public ProjectState updateProject(ProjectMetadata updatedProject) {
                 )
             );
         }
-        return new ProjectState(ClusterState.builder(cluster).putProjectMetadata(updatedProject).build(), project);
+        return new ProjectState(updatedState(updatedProject), project);
     }
 }

server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java

@@ -404,6 +404,44 @@ private Metadata updateSingleProject(Function<ProjectMetadata, ProjectMetadata>
         }
     }
 
+    /**
+     * Updates a single project in the metadata. This offers a more performant way of updating a single project compared to the Builder.
+     */
+    @FixForMultiProject // We should reconsider whether this method is valuable once we update Metadata.Builder to hold constructed projects
+    // instead of project builders.
+    public Metadata withUpdatedProject(ProjectMetadata updatedProject) {
+        final var existingProject = projectMetadata.get(updatedProject.id());
+        if (existingProject == null) {
+            throw new IllegalArgumentException(
+                "Can only update existing project, cannot add a new project [" + updatedProject.id() + "]. Use the builder instead"
+            );
+        }
+        if (updatedProject == existingProject) {
+            return this;
+        }
+        final Map<ProjectId, ProjectMetadata> updatedMap;
+        if (projects().size() == 1) {
+            updatedMap = Map.of(updatedProject.id(), updatedProject);
+        } else {
+            final var hashMap = new HashMap<>(projectMetadata);
+            hashMap.put(updatedProject.id(), updatedProject);
+            updatedMap = Collections.unmodifiableMap(hashMap);
+        }
+        return new Metadata(
+            clusterUUID,
+            clusterUUIDCommitted,
+            version,
+            coordinationMetadata,
+            updatedMap,
+            transientSettings,
+            persistentSettings,
+            settings,
+            hashesOfConsistentSettings,
+            customs,
+            reservedStateMetadata
+        );
+    }
+
     public long version() {
         return this.version;
     }
@@ -1487,6 +1525,18 @@ public Metadata copyAndUpdate(Consumer<Builder> updater) {
         return builder.build();
     }
 
+    public Metadata copyAndUpdateProject(ProjectId projectId, Consumer<ProjectMetadata.Builder> updater) {
+        final var existingProject = projectMetadata.get(projectId);
+        if (existingProject == null) {
+            throw new IllegalArgumentException(
+                "Can only update existing project, cannot add a new project [" + projectId + "]. Use the builder instead"
+            );
+        }
+        final var builder = ProjectMetadata.builder(existingProject);
+        updater.accept(builder);
+        return withUpdatedProject(builder.build());
+    }
+
     public static class Builder {
 
         private String clusterUUID;