Check relocating node id when finding source for reshard recovery (elastic#137848)

* Check relocating node id when finding source for reshard recovery

* Explicit return null

Author: jxie-1

server/src/main/java/org/elasticsearch/indices/cluster/IndicesClusterStateService.java

@@ -1046,6 +1046,18 @@ private static DiscoveryNode findSourceNodeForReshardSplitRecovery(
             return null;
         }
 
+        if (sourceShardRouting.relocatingNodeId() != null) {
+            DiscoveryNode sourceNode = nodes.get(sourceShardRouting.relocatingNodeId());
+            if (sourceNode == null) {
+                logger.trace(
+                    "can't find relocation reshard split source node for shard {} because it is assigned to an unknown node [{}].",
+                    sourceShardRouting.shardId(),
+                    sourceShardRouting.relocatingNodeId()
+                );
+                return null;
+            }
+            return sourceNode;
+        }
         DiscoveryNode sourceNode = nodes.get(sourceShardRouting.currentNodeId());
         if (sourceNode == null) {
             assert false : "Source node for reshard does not exist: " + sourceShardRouting.currentNodeId();