Fix serialization part 2

Signed-off-by: Peter Alfonsi <[email protected]>

Author: Peter Alfonsi

server/src/main/java/org/opensearch/search/aggregations/metrics/TDigestState.java

@@ -83,20 +83,24 @@ public static TDigestState read(StreamInput in) throws IOException {
             double compression = in.readDouble();
             AVLTreeDigest treeDigest = new AVLTreeDigest(compression);
             int n = in.readVInt();
-            for (int i = 0; i < n; i++) {
-                treeDigest.add(in.readDouble(), in.readVInt());
+            if (n > 0) {
+                for (int i = 0; i < n; i++) {
+                    treeDigest.add(in.readDouble(), in.readVInt());
+                }
+                MergingDigest mergingDigest = new MergingDigest(compression);
+                mergingDigest.add(List.of(treeDigest));
+                return new TDigestState(compression, mergingDigest);
             }
-            MergingDigest mergingDigest = new MergingDigest(compression);
-            mergingDigest.add(List.of(treeDigest));
-            return new TDigestState(compression, mergingDigest);
+            return new TDigestState(compression);
         } else {
             // For MergingDigest, adding the original centroids in ascending order to a new, empty MergingDigest isn't guaranteed
             // to produce a MergingDigest whose centroids are exactly equal to the originals.
             // So, use the library's serialization code to ensure we get the exact same centroids, allowing us to compare with equals().
             // The AVLTreeDigest had the same limitation for equals() where it was only guaranteed to return true if the other object was
             // produced by de/serializing the object, so this should be fine.
             int byteSize = in.readVInt();
-            byte[] bytes = in.readNBytes(byteSize);
+            byte[] bytes = new byte[byteSize];
+            in.readBytes(bytes, 0, byteSize);
             MergingDigest mergingDigest = MergingDigest.fromBytes(ByteBuffer.wrap(bytes));
             if (mergingDigest.centroids().isEmpty()) {
                 return new TDigestState(mergingDigest.compression());

server/src/test/java/org/opensearch/search/aggregations/metrics/InternalMedianAbsoluteDeviationTests.java

@@ -41,6 +41,7 @@
 import java.util.Map;
 
 public class InternalMedianAbsoluteDeviationTests extends InternalAggregationTestCase<InternalMedianAbsoluteDeviation> {
+
     @Override
     protected InternalMedianAbsoluteDeviation createTestInstance(String name, Map<String, Object> metadata) {
         final TDigestState valuesSketch = new TDigestState(randomDoubleBetween(20, 1000, true));