Use singleton instance for default project-id

server/src/main/java/org/elasticsearch/cluster/block/ClusterBlocks.java

@@ -438,7 +438,7 @@ private static void writeBlockSet(Set<ClusterBlock> blocks, StreamOutput out) th
     public static ClusterBlocks readFrom(StreamInput in) throws IOException {
         if (in.getTransportVersion().onOrAfter(TransportVersions.MULTI_PROJECT)) {
             final Set<ClusterBlock> global = readBlockSet(in);
-            final Map<ProjectId, ProjectBlocks> projectBlocksMap = in.readImmutableMap(ProjectId::new, ProjectBlocks::readFrom);
+            final Map<ProjectId, ProjectBlocks> projectBlocksMap = in.readImmutableMap(ProjectId::readFrom, ProjectBlocks::readFrom);
             if (global.isEmpty()
                 && noProjectOrDefaultProjectOnly(projectBlocksMap)
                 && projectBlocksMap.getOrDefault(Metadata.DEFAULT_PROJECT_ID, ProjectBlocks.EMPTY).indices().isEmpty()) {

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

@@ -1154,7 +1154,7 @@ public static Metadata readFrom(StreamInput in) throws IOException {
                 builder.put(ReservedStateMetadata.readFrom(in));
             }
 
-            builder.projectMetadata(in.readMap(ProjectId::new, ProjectMetadata::readFrom));
+            builder.projectMetadata(in.readMap(ProjectId::readFrom, ProjectMetadata::readFrom));
         }
         return builder.build();
     }

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

@@ -22,7 +22,7 @@
 
 public record ProjectId(String id) implements Writeable, ToXContent {
 
-    public static final Reader<ProjectId> READER = ProjectId::new;
+    public static final Reader<ProjectId> READER = ProjectId::readFrom;
     private static final int MAX_LENGTH = 128;
 
     public ProjectId {
@@ -53,8 +53,9 @@ private static boolean isValidIdChar(char c) {
         return (c >= '0' && c <= '9') || (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z') || c == '_' || c == '-';
     }
 
-    public ProjectId(StreamInput in) throws IOException {
-        this(in.readString());
+    public static ProjectId readFrom(StreamInput in) throws IOException {
+        final var id = in.readString();
+        return Metadata.DEFAULT_PROJECT_ID.id.equals(id) ? Metadata.DEFAULT_PROJECT_ID : new ProjectId(id);
     }
 
     @Override

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

@@ -2021,7 +2021,7 @@ public Iterator<? extends ToXContent> toXContentChunked(ToXContent.Params p) {
     }
 
     public static ProjectMetadata readFrom(StreamInput in) throws IOException {
-        ProjectId id = new ProjectId(in);
+        ProjectId id = ProjectId.readFrom(in);
         Builder builder = builder(id);
         Function<String, MappingMetadata> mappingLookup;
         Map<String, MappingMetadata> mappingMetadataMap = in.readMapValues(MappingMetadata::new, MappingMetadata::getSha256);

server/src/main/java/org/elasticsearch/cluster/routing/GlobalRoutingTable.java

@@ -178,7 +178,7 @@ public static Diff<GlobalRoutingTable> readDiffFrom(StreamInput in) throws IOExc
     }
 
     public static GlobalRoutingTable readFrom(StreamInput in) throws IOException {
-        final var table = in.readImmutableOpenMap(ProjectId::new, RoutingTable::readFrom);
+        final var table = in.readImmutableOpenMap(ProjectId::readFrom, RoutingTable::readFrom);
         return new GlobalRoutingTable(table);
     }
 

server/src/main/java/org/elasticsearch/cluster/routing/allocation/command/AbstractAllocateAllocationCommand.java

@@ -109,7 +109,7 @@ protected AbstractAllocateAllocationCommand(StreamInput in) throws IOException {
         shardId = in.readVInt();
         node = in.readString();
         if (in.getTransportVersion().onOrAfter(TransportVersions.MULTI_PROJECT)) {
-            projectId = new ProjectId(in);
+            projectId = ProjectId.readFrom(in);
         } else {
             projectId = Metadata.DEFAULT_PROJECT_ID;
         }

server/src/main/java/org/elasticsearch/cluster/routing/allocation/command/CancelAllocationCommand.java

@@ -84,7 +84,7 @@ public CancelAllocationCommand(StreamInput in) throws IOException {
         node = in.readString();
         allowPrimary = in.readBoolean();
         if (in.getTransportVersion().onOrAfter(TransportVersions.MULTI_PROJECT)) {
-            projectId = new ProjectId(in);
+            projectId = ProjectId.readFrom(in);
         } else {
             projectId = Metadata.DEFAULT_PROJECT_ID;
         }

server/src/main/java/org/elasticsearch/cluster/routing/allocation/command/MoveAllocationCommand.java

@@ -69,7 +69,7 @@ public MoveAllocationCommand(StreamInput in) throws IOException {
         fromNode = in.readString();
         toNode = in.readString();
         if (in.getTransportVersion().onOrAfter(TransportVersions.MULTI_PROJECT)) {
-            projectId = new ProjectId(in);
+            projectId = ProjectId.readFrom(in);
         } else {
             projectId = Metadata.DEFAULT_PROJECT_ID;
         }

test/external-modules/multi-project/src/main/java/org/elasticsearch/multiproject/action/DeleteProjectAction.java

@@ -147,7 +147,7 @@ public Request(TimeValue masterNodeTimeout, TimeValue ackTimeout, ProjectId proj
 
         public Request(StreamInput in) throws IOException {
             super(in);
-            this.projectId = new ProjectId(in);
+            this.projectId = ProjectId.readFrom(in);
         }
 
         @Override

test/external-modules/multi-project/src/main/java/org/elasticsearch/multiproject/action/PutProjectAction.java

@@ -132,7 +132,7 @@ public Request(TimeValue masterNodeTimeout, TimeValue ackTimeout, ProjectId proj
 
         public Request(StreamInput in) throws IOException {
             super(in);
-            this.projectId = new ProjectId(in);
+            this.projectId = ProjectId.readFrom(in);
         }
 
         @Override