Fix testForceMergeWithSoftDeletesRetentionAndRecoverySource (#48766)

This test failure manifests the limitation of the recovery source merge
policy explained in #41628. If we already merge down to a single segment
then subsequent force merges will be noop although they can prune
recovery source. We need to adjust this test until we have a fix for the
merge policy.

Relates #41628
Closes #48735

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -1694,18 +1694,26 @@ public void testForceMergeWithSoftDeletesRetentionAndRecoverySource() throws Exc
             settings.put(IndexSettings.INDEX_SOFT_DELETES_RETENTION_OPERATIONS_SETTING.getKey(), 0);
             indexSettings.updateIndexMetaData(IndexMetaData.builder(defaultSettings.getIndexMetaData()).settings(settings).build());
             engine.onSettingsChanged();
-            // If the global checkpoint equals to the local checkpoint, the next force-merge will be a noop
-            // because all deleted documents are expunged in the previous force-merge already. We need to flush
-            // a new segment to make merge happen so that we can verify that all _recovery_source are pruned.
-            if (globalCheckpoint.get() == engine.getLocalCheckpoint() && liveDocs.isEmpty() == false) {
-                String deleteId = randomFrom(liveDocs);
-                engine.delete(new Engine.Delete("test", deleteId, newUid(deleteId), primaryTerm.get()));
-                liveDocsWithSource.remove(deleteId);
-                liveDocs.remove(deleteId);
-                engine.flush();
+            // If we already merged down to 1 segment, then the next force-merge will be a noop. We need to add an extra segment to make
+            // merges happen so we can verify that _recovery_source are pruned. See: https://github.com/elastic/elasticsearch/issues/41628.
+            final int numSegments;
+            try (Engine.Searcher searcher = engine.acquireSearcher("test", Engine.SearcherScope.INTERNAL)) {
+                numSegments = searcher.getDirectoryReader().leaves().size();
+            }
+            if (numSegments == 1) {
+                boolean useRecoverySource = randomBoolean() || omitSourceAllTheTime;
+                ParsedDocument doc = testParsedDocument("dummy", null, testDocument(), B_1, null, useRecoverySource);
+                engine.index(indexForDoc(doc));
+                if (useRecoverySource == false) {
+                    liveDocsWithSource.add(doc.id());
+                }
+                engine.syncTranslog();
+                globalCheckpoint.set(engine.getLocalCheckpoint());
+                engine.flush(randomBoolean(), true);
+            } else {
+                globalCheckpoint.set(engine.getLocalCheckpoint());
+                engine.syncTranslog();
             }
-            globalCheckpoint.set(engine.getLocalCheckpoint());
-            engine.syncTranslog();
             engine.forceMerge(true, 1, false, false, false);
             assertConsistentHistoryBetweenTranslogAndLuceneIndex(engine, mapperService);
             assertThat(readAllOperationsInLucene(engine, mapperService), hasSize(liveDocsWithSource.size()));