Honour max segment size when setting only_expunge_deletes on force merge

This commit changes the es merge policy to apply the maximum segment size
on force merges that only expunge deletes (forceMergeDeletes).
This option is useful for read-write use cases that wants to reclaim deleted docs
more aggressively than the `index.merge.policy.deletes_pct_allowed`.

Relates elastic#77270

Author: jimczi

docs/reference/indices/forcemerge.asciidoc

@@ -123,7 +123,8 @@ If so, executes it.
 --
 (Optional, Boolean)
 If `true`,
-only expunge segments containing document deletions.
+expunge all segments containing more than `index.merge.policy.expunge_deletes_allowed`
+ (default to 10) percents of deleted documents.
 Defaults to `false`.
 
 In Lucene,
@@ -133,8 +134,6 @@ During a merge,
 a new segment is created
 that does not contain those document deletions.
 
-NOTE: This parameter does *not* override the
-`index.merge.policy.expunge_deletes_allowed` setting.
 --
 
 

server/src/main/java/org/elasticsearch/index/EsTieredMergePolicy.java

@@ -41,7 +41,7 @@ public MergeSpecification findForcedMerges(SegmentInfos infos, int maxSegmentCou
 
     @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) {

server/src/test/java/org/elasticsearch/index/EsTieredMergePolicyTests.java

@@ -32,6 +32,7 @@ public void testSetForceMergeDeletesPctAllowed() {
         EsTieredMergePolicy policy = new EsTieredMergePolicy();
         policy.setForceMergeDeletesPctAllowed(42);
         assertEquals(42, policy.forcedMergePolicy.getForceMergeDeletesPctAllowed(), 0);
+        assertEquals(42, policy.regularMergePolicy.getForceMergeDeletesPctAllowed(), 0);
     }
 
     public void testSetFloorSegmentMB() {