Make sure explanation is populated when .not_preferred logging is turned on

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

@@ -862,17 +862,14 @@ public boolean moveShards() {
             for (var storedShardMovement : bestNonPreferredShardMovementsTracker.getBestShardMovements()) {
                 final var shardRouting = storedShardMovement.shardRouting();
                 final var index = projectIndex(shardRouting);
-                // If `shardMoved` is true, there may have been moves that have made our previous move decision
-                // invalid, so we must call `decideMove` again. If not, we know we haven't made any moves, and we
-                // can use the cached decision.
-                final var moveDecision = shardMoved ? decideMove(index, shardRouting) : storedShardMovement.moveDecision();
+                final var moveDecision = reassessDecisionIfRequired(index, storedShardMovement, shardMoved);
                 if (moveDecision.isDecisionTaken() && moveDecision.cannotRemainAndCanMove()) {
                     if (notPreferredLogger.isDebugEnabled()) {
                         notPreferredLogger.debug(
-                            "Moving shard [{}] to [{}] from a NOT_PREFERRED allocation, explanation is [{}]",
+                            "Moving shard [{}] to [{}] from a NOT_PREFERRED allocation: {}",
                             shardRouting,
                             moveDecision.getTargetNode().getName(),
-                            moveDecision.getCanRemainDecision().getExplanation()
+                            moveDecision.getCanRemainDecision()
                         );
                     }
                     executeMove(shardRouting, index, moveDecision, "move-non-preferred");
@@ -886,6 +883,36 @@ public boolean moveShards() {
             return shardMoved;
         }
 
+        /**
+         * If a shard has moved we need to re-run the decision to check that it's still valid. If {@link #notPreferredLogger} is
+         * set to debug, we re-run the decision with explain turned on so we can populate the explanation in our log message
+         *
+         * @param index The index of the shard
+         * @param storedShardMovement The existing shard movement decision
+         * @param shardMoved True if a shard moved in this balancing round, false otherwise
+         * @return The move decision to act on
+         */
+        private MoveDecision reassessDecisionIfRequired(
+            ProjectIndex index,
+            BestShardMovementsTracker.StoredShardMovement storedShardMovement,
+            boolean shardMoved
+        ) {
+            final boolean needToReassess = notPreferredLogger.isDebugEnabled() || shardMoved;
+            if (needToReassess == false) {
+                return storedShardMovement.moveDecision();
+            }
+
+            final var oldDebugMode = allocation.getDebugMode();
+            if (notPreferredLogger.isDebugEnabled()) {
+                allocation.setDebugMode(RoutingAllocation.DebugMode.EXCLUDE_YES_DECISIONS);
+            }
+            try {
+                return decideMove(index, storedShardMovement.shardRouting());
+            } finally {
+                allocation.setDebugMode(oldDebugMode);
+            }
+        }
+
         private void executeMove(ShardRouting shardRouting, ProjectIndex index, MoveDecision moveDecision, String reason) {
             final ModelNode sourceNode = nodes.get(shardRouting.currentNodeId());
             final ModelNode targetNode = nodes.get(moveDecision.getTargetNode().getId());

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

@@ -1027,7 +1027,7 @@ public Decision canRemain(
                 RoutingNode node,
                 RoutingAllocation allocation
             ) {
-                return new Decision.Single(Decision.Type.NOT_PREFERRED, "test_decider", "Always NOT_PREFERRED");
+                return allocation.decision(Decision.NOT_PREFERRED, "test_decider", "Always NOT_PREFERRED");
             }
         })), clusterState.getRoutingNodes().mutableCopy(), clusterState, ClusterInfo.EMPTY, SnapshotShardSizeInfo.EMPTY, 0L);
 
@@ -1039,7 +1039,7 @@ public Decision canRemain(
                 "moved a NOT_PREFERRED allocation",
                 notPreferredLoggerName,
                 Level.DEBUG,
-                "Moving shard [*] to [*] from a NOT_PREFERRED allocation, explanation is [Always NOT_PREFERRED]"
+                "Moving shard [*] to [*] from a NOT_PREFERRED allocation: [NOT_PREFERRED(Always NOT_PREFERRED)]"
             )
         );
     }