Gate reading of optional string array for bwc

server/src/main/java/org/elasticsearch/TransportVersions.java

@@ -155,6 +155,7 @@ static TransportVersion def(int id) {
     public static final TransportVersion ML_INFERENCE_EMBEDDING_BYTE_ADDED = def(8_615_00_0);
     public static final TransportVersion ML_INFERENCE_L2_NORM_SIMILARITY_ADDED = def(8_616_00_0);
     public static final TransportVersion SEARCH_NODE_LOAD_AUTOSCALING = def(8_617_00_0);
+    public static final TransportVersion DIMENSIONS_SERIALIZATION_PERSISTENT_TASK = def(8_618_00_0);
 
     /*
      * STOP! READ THIS FIRST! No, really,

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/DownsampleShardIndexer.java

@@ -148,6 +148,17 @@ class DownsampleShardIndexer {
 
     public DownsampleIndexerAction.ShardDownsampleResponse execute() throws IOException {
         final Query initialStateQuery = createQuery();
+        if (this.dimensions == null || this.dimensions.length <= 0) {
+            final String errorMessage = "Downsampling task ["
+                + task.getPersistentTaskId()
+                + "] on shard "
+                + indexShard.shardId()
+                + " failed because of missing dimensions for index ["
+                + task.getDownsampleIndex()
+                + "]";
+            logger.error(errorMessage);
+            throw new DownsampleShardIndexerException(errorMessage, false);
+        }
         if (initialStateQuery instanceof MatchNoDocsQuery) {
             return new DownsampleIndexerAction.ShardDownsampleResponse(indexShard.shardId(), task.getNumIndexed());
         }

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/DownsampleShardTaskParams.java

@@ -71,7 +71,9 @@ public record DownsampleShardTaskParams(
             new ShardId(in),
             in.readStringArray(),
             in.readStringArray(),
-            in.readOptionalStringArray()
+            in.getTransportVersion().onOrAfter(TransportVersions.DIMENSIONS_SERIALIZATION_PERSISTENT_TASK)
+                ? in.readOptionalStringArray()
+                : new String[] {}
         );
     }
 
@@ -108,7 +110,9 @@ public void writeTo(StreamOutput out) throws IOException {
         shardId.writeTo(out);
         out.writeStringArray(metrics);
         out.writeStringArray(labels);
-        out.writeOptionalStringArray(dimensions);
+        if (out.getTransportVersion().onOrAfter(TransportVersions.DIMENSIONS_SERIALIZATION_PERSISTENT_TASK)) {
+            out.writeOptionalStringArray(dimensions);
+        }
     }
 
     public static DownsampleShardTaskParams fromXContent(XContentParser parser) throws IOException {