Expose method for additional files between index commits (#138133)

This is a simple PR that only exposes a method to return the list of
additional files between 2 lucene commits.

Author: albertzaharovits

server/src/main/java/org/elasticsearch/common/lucene/Lucene.java

@@ -87,9 +87,12 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
 
 public class Lucene {
 
@@ -128,6 +131,14 @@ public static Iterable<String> files(SegmentInfos infos) throws IOException {
         return Iterables.flatten(list);
     }
 
+    /**
+     * Returns the additional files that the {@param current} index commit introduces compared to the {@param previous} one.
+     */
+    public static Set<String> additionalFileNames(IndexCommit previous, IndexCommit current) throws IOException {
+        final Set<String> previousFiles = previous != null ? new HashSet<>(previous.getFileNames()) : Set.of();
+        return current.getFileNames().stream().filter(f -> previousFiles.contains(f) == false).collect(Collectors.toUnmodifiableSet());
+    }
+
     /**
      * Returns the number of documents in the index referenced by this {@link SegmentInfos}
      */

server/src/main/java/org/elasticsearch/index/engine/CombinedDeletionPolicy.java

@@ -29,7 +29,8 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.function.LongSupplier;
-import java.util.stream.Collectors;
+
+import static org.elasticsearch.common.lucene.Lucene.additionalFileNames;
 
 /**
  * An {@link IndexDeletionPolicy} that coordinates between Lucene's commits and the retention of translog generation files,
@@ -130,7 +131,7 @@ public void onCommit(List<? extends IndexCommit> commits) throws IOException {
         assert assertSafeCommitUnchanged(safeCommit);
         if (commitsListener != null) {
             if (newCommit != null) {
-                final Set<String> additionalFiles = listOfNewFileNames(previousLastCommit, newCommit);
+                final Set<String> additionalFiles = additionalFileNames(previousLastCommit, newCommit);
                 commitsListener.onNewAcquiredCommit(newCommit, additionalFiles);
             }
             if (deletedCommits != null) {
@@ -303,11 +304,6 @@ private static int indexOfKeptCommits(List<? extends IndexCommit> commits, long
         return 0;
     }
 
-    private static Set<String> listOfNewFileNames(IndexCommit previous, IndexCommit current) throws IOException {
-        final Set<String> previousFiles = previous != null ? new HashSet<>(previous.getFileNames()) : Set.of();
-        return current.getFileNames().stream().filter(f -> previousFiles.contains(f) == false).collect(Collectors.toUnmodifiableSet());
-    }
-
     /**
      * Checks whether the deletion policy is holding on to externally acquired index commits
      */