Core: deletions (alone) should also force a refresh when version map is using too much RAM

We already force a refresh in index/create ops, to clear version map
when it's using too much RAM, but we were failing to do this for
deletes, so an app that does tons of deletes with no indexing, and has
set refresh_interval to -1, would have version map using unbounded
RAM.

Closes elastic#10312

Author: mikemccand

src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -461,6 +461,7 @@ public void delete(Delete delete) throws EngineException {
         }
 
         maybePruneDeletedTombstones();
+        checkVersionMapRefresh();
     }
 
     private void maybePruneDeletedTombstones() {

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

@@ -55,6 +55,7 @@
 import org.elasticsearch.index.deletionpolicy.KeepOnlyLastDeletionPolicy;
 import org.elasticsearch.index.deletionpolicy.SnapshotDeletionPolicy;
 import org.elasticsearch.index.deletionpolicy.SnapshotIndexCommit;
+import org.elasticsearch.index.engine.Engine.Searcher;
 import org.elasticsearch.index.indexing.ShardIndexingService;
 import org.elasticsearch.index.indexing.slowlog.ShardSlowLogIndexingService;
 import org.elasticsearch.index.mapper.ParseContext.Document;
@@ -1644,4 +1645,39 @@ public void testRetryWithAutogeneratedIdsAndWrongOrderWorksAndNoDuplicateDocs()
         replicaSearcher.close();
     }
 
+    // #10312
+    @Test
+    public void testDeletesAloneCanTriggerRefresh() throws IOException {
+        // Tiny indexing buffer:
+        Settings indexSettings = ImmutableSettings.builder().put(defaultSettings)
+            .put(EngineConfig.INDEX_BUFFER_SIZE_SETTING, "1kb").build();
+        IndexSettingsService indexSettingsService = new IndexSettingsService(shardId.index(), indexSettings);
+        try (Store store = createStore();
+            Translog translog = createTranslog();
+            Engine engine = new InternalEngine(config(indexSettingsService, store, translog, createMergeScheduler(indexSettingsService)))) {
+            for(int i=0;i<100;i++) {
+                String id = Integer.toString(i);
+                ParsedDocument doc = testParsedDocument(id, id, "test", null, -1, -1, testDocument(), B_1, false);
+                engine.index(new Engine.Index(null, newUid(id), doc, 2, VersionType.EXTERNAL, Engine.Operation.Origin.PRIMARY, System.nanoTime()));
+            }
+
+            // Force merge so we know all merges are done before we start deleting:
+            engine.forceMerge(true, 1, false, false);
+
+            Searcher s = engine.acquireSearcher("test");
+            long version1 = ((DirectoryReader) s.reader()).getVersion();
+            s.close();
+            for(int i=0;i<100;i++) {
+                String id = Integer.toString(i);
+                engine.delete(new Engine.Delete("test", id, newUid(id), 10, VersionType.EXTERNAL, Engine.Operation.Origin.PRIMARY, System.nanoTime(), false));
+            }
+            s = engine.acquireSearcher("test");
+            long version2 = ((DirectoryReader) s.reader()).getVersion();
+            s.close();
+
+            // 100 buffered deletes will easily exceed 25% of our 1 KB indexing buffer so it should have forced a refresh:
+            assertThat(version2, greaterThan(version1));
+        }
+    }
+
 }