Changes

Author: Tim-Brooks

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

@@ -1972,9 +1972,10 @@ public Builder reshardAddShards(int shardCount) {
             }
             settings = Settings.builder().put(settings).put(SETTING_NUMBER_OF_SHARDS, shardCount).build();
             var newPrimaryTerms = new long[shardCount];
+            Arrays.fill(newPrimaryTerms, this.primaryTerms.length, newPrimaryTerms.length, SequenceNumbers.UNASSIGNED_PRIMARY_TERM);
             System.arraycopy(primaryTerms, 0, newPrimaryTerms, 0, this.primaryTerms.length);
             primaryTerms = newPrimaryTerms;
-            routingNumShards = shardCount;
+            routingNumShards = MetadataCreateIndexService.calculateNumRoutingShards(shardCount, indexCreatedVersion(settings));
             return this;
         }
 

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

@@ -341,6 +341,8 @@ public void writeTo(StreamOutput out) throws IOException {
             out.writeByte((byte) Reason.PRIMARY_FAILED.ordinal());
         } else if (reason.equals(Reason.RESHARD_ADDED)
             && out.getTransportVersion().before(TransportVersions.UNASSIGENEDINFO_RESHARD_ADDED)) {
+                // We should have protection to ensure we do not reshard in mixed clusters
+                assert false;
                 out.writeByte((byte) Reason.FORCED_EMPTY_PRIMARY.ordinal());
             } else {
                 out.writeByte((byte) reason.ordinal());