Fix updateMinNode condition (#80403)

Today the `BalancedShardsAllocator` doesn't correctly compute the best
rebalance operation when running in `explain` mode. This commit
addresses that.

Closes #41194

Author: ViggoC

server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

@@ -1013,7 +1013,7 @@ private AllocateUnassignedDecision decideAllocateUnassigned(final ShardRouting s
                             updateMinNode = currentDecision.type() == Type.YES;
                         }
                     } else {
-                        updateMinNode = true;
+                        updateMinNode = currentWeight < minWeight;
                     }
                     if (updateMinNode) {
                         minNode = node;

server/src/test/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocatorTests.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.cluster.routing.allocation.allocator;
+
+import org.elasticsearch.Version;
+import org.elasticsearch.action.support.replication.ClusterStateCreationUtils;
+import org.elasticsearch.cluster.ClusterInfo;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.ESAllocationTestCase;
+import org.elasticsearch.cluster.metadata.IndexMetadata;
+import org.elasticsearch.cluster.metadata.Metadata;
+import org.elasticsearch.cluster.routing.RoutingNodes;
+import org.elasticsearch.cluster.routing.RoutingTable;
+import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.ShardRoutingState;
+import org.elasticsearch.cluster.routing.allocation.AllocateUnassignedDecision;
+import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
+import org.elasticsearch.cluster.routing.allocation.decider.AllocationDeciders;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.snapshots.SnapshotShardSizeInfo;
+
+import java.util.Collections;
+import java.util.List;
+
+public class BalancedShardsAllocatorTests extends ESAllocationTestCase {
+
+    public void testDecideShardAllocation() {
+        BalancedShardsAllocator allocator = new BalancedShardsAllocator(Settings.EMPTY);
+        ClusterState clusterState = ClusterStateCreationUtils.state("idx", false, ShardRoutingState.STARTED);
+        assertEquals(clusterState.nodes().getSize(), 3);
+
+        // add new index
+        String index = "idx_new";
+        Metadata metadata = Metadata.builder(clusterState.metadata())
+            .put(IndexMetadata.builder(index).settings(settings(Version.CURRENT)).numberOfShards(1).numberOfReplicas(0))
+            .build();
+        RoutingTable initialRoutingTable = RoutingTable.builder(clusterState.routingTable()).addAsNew(metadata.index(index)).build();
+        clusterState = ClusterState.builder(clusterState).metadata(metadata).routingTable(initialRoutingTable).build();
+
+        ShardRouting shard = clusterState.routingTable().index("idx_new").shard(0).primaryShard();
+        RoutingAllocation allocation = new RoutingAllocation(
+            new AllocationDeciders(Collections.emptyList()),
+            new RoutingNodes(clusterState, false),
+            clusterState,
+            ClusterInfo.EMPTY,
+            SnapshotShardSizeInfo.EMPTY,
+            System.nanoTime()
+        );
+
+        allocation.debugDecision(false);
+        AllocateUnassignedDecision allocateDecision = allocator.decideShardAllocation(shard, allocation).getAllocateDecision();
+        allocation.debugDecision(true);
+        AllocateUnassignedDecision allocateDecisionWithExplain = allocator.decideShardAllocation(shard, allocation).getAllocateDecision();
+        // the allocation decision should have same target node no matter the debug is on or off
+        assertEquals(allocateDecision.getTargetNode().getId(), allocateDecisionWithExplain.getTargetNode().getId());
+
+        allocator.allocate(allocation);
+        List<ShardRouting> assignedShards = allocation.routingNodes().assignedShards(shard.shardId());
+        assertEquals(1, assignedShards.size());
+        // the allocation result be consistent with allocation decision
+        assertNotNull(allocateDecision.getTargetNode().getId(), assignedShards.get(0).currentNodeId());
+    }
+
+}