Advance split target primary term to match source

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

@@ -893,15 +893,25 @@ public IndexMetadata withInSyncAllocationIds(int shardId, Set<String> inSyncSet)
      * @return updated instance with incremented primary term
      */
     public IndexMetadata withIncrementedPrimaryTerm(int shardId) {
-        final long[] incremented = this.primaryTerms.clone();
-        incremented[shardId]++;
+        return withSetPrimaryTerm(shardId, this.primaryTerms[shardId] + 1);
+    }
+
+    /**
+     * Creates a copy of this instance that has the primary term for the given shard id set to the value provided.
+     * @param shardId shard id to set primary term for
+     * @param primaryTerm primary term to set
+     * @return updated instance with set primary term
+     */
+    public IndexMetadata withSetPrimaryTerm(int shardId, long primaryTerm) {
+        final long[] newPrimaryTerms = this.primaryTerms.clone();
+        newPrimaryTerms[shardId] = primaryTerm;
         return new IndexMetadata(
             this.index,
             this.version,
             this.mappingVersion,
             this.settingsVersion,
             this.aliasesVersion,
-            incremented,
+            newPrimaryTerms,
             this.state,
             this.numberOfShards,
             this.numberOfReplicas,

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

@@ -12,13 +12,15 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.xcontent.ConstructingObjectParser;
 import org.elasticsearch.xcontent.ParseField;
 import org.elasticsearch.xcontent.ToXContentFragment;
 import org.elasticsearch.xcontent.XContentBuilder;
 import org.elasticsearch.xcontent.XContentParser;
 
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.Objects;
 
 /**
@@ -205,6 +207,18 @@ public static IndexReshardingMetadata newSplitByMultiple(int shardCount, int mul
         return new IndexReshardingMetadata(IndexReshardingState.Split.newSplitByMultiple(shardCount, multiple));
     }
 
+    public IndexReshardingMetadata transitionSplitTargetToHandoff(ShardId shardId) {
+        assert state instanceof IndexReshardingState.Split;
+        IndexReshardingState.Split splitState = (IndexReshardingState.Split) state;
+        IndexReshardingState.Split.TargetShardState[] newTargets = Arrays.copyOf(
+            splitState.targetShards(),
+            splitState.targetShards().length
+        );
+        int i = shardId.getId() - state.shardCountBefore();
+        newTargets[i] = IndexReshardingState.Split.TargetShardState.HANDOFF;
+        return new IndexReshardingMetadata(new IndexReshardingState.Split(splitState.sourceShards(), newTargets));
+    }
+
     /**
      * @return the split state of this metadata block, or throw IllegalArgumentException if this metadata doesn't represent a split
      */
@@ -215,6 +229,10 @@ public IndexReshardingState.Split getSplit() {
         };
     }
 
+    public boolean isSplit() {
+        return state instanceof IndexReshardingState.Split;
+    }
+
     /**
      * @return the number of shards the index has at the start of this operation
      */

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

@@ -250,6 +250,10 @@ TargetShardState[] targetShards() {
             return targetShards.clone();
         }
 
+        public int sourceShard(int targetShard) {
+            return targetShard % shardCountBefore();
+        }
+
         /**
          * Create resharding metadata representing a new split operation
          * Split only supports updating an index to a multiple of its current shard count
@@ -345,6 +349,10 @@ public SourceShardState getSourceShardState(int shardNum) {
             return sourceShards[shardNum];
         }
 
+        public boolean isTargetShard(int shardId) {
+            return shardId >= shardCountBefore();
+        }
+
         /**
          * Get the current target state of a shard
          * @param shardNum an index into shards greater than or equal to the old shard count and less than the new shard count

server/src/main/java/org/elasticsearch/cluster/routing/allocation/IndexMetadataUpdater.java

@@ -12,6 +12,7 @@
 import org.apache.logging.log4j.Logger;
 import org.elasticsearch.cluster.ClusterState;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
+import org.elasticsearch.cluster.metadata.IndexReshardingMetadata;
 import org.elasticsearch.cluster.metadata.Metadata;
 import org.elasticsearch.cluster.metadata.ProjectMetadata;
 import org.elasticsearch.cluster.routing.GlobalRoutingTable;
@@ -134,8 +135,17 @@ public Metadata applyChanges(Metadata oldMetadata, GlobalRoutingTable newRouting
                         shardId,
                         updates
                     );
+                    IndexReshardingMetadata reshardingMetadata = updatedIndexMetadata.getReshardingMetadata();
+                    boolean splitTarget = reshardingMetadata != null
+                        && reshardingMetadata.isSplit()
+                        && reshardingMetadata.getSplit().isTargetShard(shardId.id());
                     updatedIndexMetadata = updates.increaseTerm
-                        ? updatedIndexMetadata.withIncrementedPrimaryTerm(shardId.id())
+                        ? splitTarget
+                            ? updatedIndexMetadata.withSetPrimaryTerm(
+                                shardId.id(),
+                                splitPrimaryTerm(updatedIndexMetadata, reshardingMetadata, shardId)
+                            )
+                            : updatedIndexMetadata.withIncrementedPrimaryTerm(shardId.id())
                         : updatedIndexMetadata;
                 }
                 if (updatedIndexMetadata != oldIndexMetadata) {
@@ -147,6 +157,15 @@ public Metadata applyChanges(Metadata oldMetadata, GlobalRoutingTable newRouting
         return updatedMetadata.build();
     }
 
+    private static long splitPrimaryTerm(IndexMetadata updatedIndexMetadata, IndexReshardingMetadata reshardingMetadata, ShardId shardId) {
+        // We take the max of the source and target primary terms. This guarantees that the target primary term stays
+        // greater than or equal to the source.
+        return Math.max(
+            updatedIndexMetadata.primaryTerm(reshardingMetadata.getSplit().sourceShard(shardId.id())),
+            updatedIndexMetadata.primaryTerm(shardId.id()) + 1
+        );
+    }
+
     /**
      * Updates in-sync allocations with routing changes that were made to the routing table.
      */