Add shardCountBefore/After operations to IndexReshardingMetadata (#125058)

* Add shardCountBefore/After operations to IndexReshardingMetadata

Any reshard operation will change shard count, so it makes sense
to make this information available here.

* [CI] Auto commit changes from spotless

---------

Co-authored-by: elasticsearchmachine <infra-root+elasticsearchmachine@elastic.co>

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

@@ -205,10 +205,27 @@ public class IndexReshardingMetadata implements ToXContentFragment, Writeable {
         return new IndexReshardingMetadata(IndexReshardingState.Split.newSplitByMultiple(shardCount, multiple));
     }
 
+    /**
+     * @return the split state of this metadata block, or throw IllegalArgumentException if this metadata doesn't represent a split
+     */
     public IndexReshardingState.Split getSplit() {
         return switch (state) {
             case IndexReshardingState.Noop ignored -> throw new IllegalArgumentException("resharding metadata is not a split");
             case IndexReshardingState.Split s -> s;
         };
     }
+
+    /**
+     * @return the number of shards the index has at the start of this operation
+     */
+    public int shardCountBefore() {
+        return state.shardCountBefore();
+    }
+
+    /**
+     * @return the number of shards that the index will have when resharding completes
+     */
+    public int shardCountAfter() {
+        return state.shardCountAfter();
+    }
 }

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

@@ -29,6 +29,16 @@ import java.util.Objects;
  * concrete subclasses for the operations that are currently defined (which is only split for now).
  */
 public abstract sealed class IndexReshardingState implements Writeable, ToXContentFragment {
+    /**
+     * @return the number of shards the index has at the start of this operation
+     */
+    public abstract int shardCountBefore();
+
+    /**
+     * @return the number of shards that the index will have when resharding completes
+     */
+    public abstract int shardCountAfter();
+
     // This class exists only so that tests can check that IndexReshardingMetadata can support more than one kind of operation.
     // When we have another real operation such as Shrink this can be removed.
     public static final class Noop extends IndexReshardingState {
@@ -64,6 +74,16 @@ public abstract sealed class IndexReshardingState implements Writeable, ToXConte
         public int hashCode() {
             return 0;
         }
+
+        @Override
+        public int shardCountBefore() {
+            return 1;
+        }
+
+        @Override
+        public int shardCountAfter() {
+            return 1;
+        }
     }
 
     public static final class Split extends IndexReshardingState {
@@ -210,13 +230,13 @@ public abstract sealed class IndexReshardingState implements Writeable, ToXConte
             return Objects.hash(Arrays.hashCode(sourceShards), Arrays.hashCode(targetShards));
         }
 
-        // visible for testing
-        int oldShardCount() {
+        @Override
+        public int shardCountBefore() {
             return oldShardCount;
         }
 
-        // visible for testing
-        int newShardCount() {
+        @Override
+        public int shardCountAfter() {
             return newShardCount;
         }
 

server/src/test/java/org/elasticsearch/cluster/metadata/IndexReshardingMetadataTests.java

@@ -21,8 +21,8 @@ public class IndexReshardingMetadataTests extends ESTestCase {
         var split = metadata.getSplit();
 
         // starting state is as expected
-        assert split.oldShardCount() == numShards;
-        assert split.newShardCount() == numShards * multiple;
+        assert metadata.shardCountBefore() == numShards;
+        assert metadata.shardCountAfter() == numShards * multiple;
         for (int i = 0; i < numShards; i++) {
             assert split.getSourceShardState(i) == IndexReshardingState.Split.SourceShardState.SOURCE;
         }