Honor max segment size during only_expunge_deletes (opensearch-project#10036)

Author: msfroh

CHANGELOG.md

@@ -104,6 +104,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add instrumentation in rest and network layer. ([#9415](https://github.com/opensearch-project/OpenSearch/pull/9415))
 - Allow parameterization of tests with OpenSearchIntegTestCase.SuiteScopeTestCase annotation ([#9916](https://github.com/opensearch-project/OpenSearch/pull/9916))
 - Mute the query profile IT with concurrent execution ([#9840](https://github.com/opensearch-project/OpenSearch/pull/9840))
+- Force merge with `only_expunge_deletes` honors max segment size ([#10036](https://github.com/opensearch-project/OpenSearch/pull/10036))
 - Add instrumentation in transport service. ([#10042](https://github.com/opensearch-project/OpenSearch/pull/10042))
 
 ### Deprecated

server/src/main/java/org/opensearch/index/OpenSearchTieredMergePolicy.java

@@ -42,7 +42,7 @@
 
 /**
  * Wrapper around {@link TieredMergePolicy} which doesn't respect
- * {@link TieredMergePolicy#setMaxMergedSegmentMB(double)} on forced merges.
+ * {@link TieredMergePolicy#setMaxMergedSegmentMB(double)} on forced merges, but DOES respect it on only_expunge_deletes.
  * See https://issues.apache.org/jira/browse/LUCENE-7976.
  *
  * @opensearch.internal
@@ -71,7 +71,7 @@ public MergeSpecification findForcedMerges(
 
     @Override
     public MergeSpecification findForcedDeletesMerges(SegmentInfos infos, MergeContext mergeContext) throws IOException {
-        return forcedMergePolicy.findForcedDeletesMerges(infos, mergeContext);
+        return regularMergePolicy.findForcedDeletesMerges(infos, mergeContext);
     }
 
     public void setForceMergeDeletesPctAllowed(double forceMergeDeletesPctAllowed) {
@@ -80,7 +80,7 @@ public void setForceMergeDeletesPctAllowed(double forceMergeDeletesPctAllowed) {
     }
 
     public double getForceMergeDeletesPctAllowed() {
-        return forcedMergePolicy.getForceMergeDeletesPctAllowed();
+        return regularMergePolicy.getForceMergeDeletesPctAllowed();
     }
 
     public void setFloorSegmentMB(double mbFrac) {

server/src/test/java/org/opensearch/index/OpenSearchTieredMergePolicyTests.java

@@ -32,9 +32,18 @@
 
 package org.opensearch.index;
 
+import org.apache.lucene.index.MergePolicy;
+import org.apache.lucene.index.SegmentCommitInfo;
+import org.apache.lucene.index.SegmentInfos;
 import org.apache.lucene.index.TieredMergePolicy;
+import org.apache.lucene.util.InfoStream;
+import org.apache.lucene.util.Version;
 import org.opensearch.test.OpenSearchTestCase;
 
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Set;
+
 public class OpenSearchTieredMergePolicyTests extends OpenSearchTestCase {
 
     public void testDefaults() {
@@ -80,4 +89,32 @@ public void testSetDeletesPctAllowed() {
         policy.setDeletesPctAllowed(42);
         assertEquals(42, policy.regularMergePolicy.getDeletesPctAllowed(), 0);
     }
+
+    public void testFindDeleteMergesReturnsNullOnEmptySegmentInfos() throws IOException {
+        MergePolicy.MergeSpecification mergeSpecification = new OpenSearchTieredMergePolicy().findForcedDeletesMerges(
+            new SegmentInfos(Version.LATEST.major),
+            new MergePolicy.MergeContext() {
+                @Override
+                public int numDeletesToMerge(SegmentCommitInfo info) {
+                    return 0;
+                }
+
+                @Override
+                public int numDeletedDocs(SegmentCommitInfo info) {
+                    return 0;
+                }
+
+                @Override
+                public InfoStream getInfoStream() {
+                    return InfoStream.NO_OUTPUT;
+                }
+
+                @Override
+                public Set<SegmentCommitInfo> getMergingSegments() {
+                    return Collections.emptySet();
+                }
+            }
+        );
+        assertNull(mergeSpecification);
+    }
 }