always check max_seq_no = gcp in readonly engine

Author: dnhatn

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

@@ -30,7 +30,6 @@
 import org.apache.lucene.search.SearcherManager;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.Lock;
-import org.elasticsearch.Version;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
 import org.elasticsearch.common.util.concurrent.ReleasableLock;
@@ -123,19 +122,9 @@ public ReadOnlyEngine(EngineConfig config, SeqNoStats seqNoStats, TranslogStats
     }
 
     protected void ensureMaxSeqNoEqualsToGlobalCheckpoint(final SeqNoStats seqNoStats) {
-        // Before 8.0 the global checkpoint is not known and up to date when the engine is created after
-        // peer recovery, so we only check the max seq no / global checkpoint coherency when the global
-        // checkpoint is different from the unassigned sequence number value.
-        // In addition to that we only execute the check if the index the engine belongs to has been
-        // created after the refactoring of the Close Index API and its TransportVerifyShardBeforeCloseAction
-        // that guarantee that all operations have been flushed to Lucene.
-        final Version indexVersionCreated = engineConfig.getIndexSettings().getIndexVersionCreated();
-        if (indexVersionCreated.onOrAfter(Version.V_7_2_0) ||
-            (seqNoStats.getGlobalCheckpoint() != SequenceNumbers.UNASSIGNED_SEQ_NO && indexVersionCreated.onOrAfter(Version.V_6_7_0))) {
-            if (seqNoStats.getMaxSeqNo() != seqNoStats.getGlobalCheckpoint()) {
-                throw new IllegalStateException("Maximum sequence number [" + seqNoStats.getMaxSeqNo()
-                    + "] from last commit does not match global checkpoint [" + seqNoStats.getGlobalCheckpoint() + "]");
-            }
+        if (seqNoStats.getMaxSeqNo() != seqNoStats.getGlobalCheckpoint()) {
+            throw new IllegalStateException("Maximum sequence number [" + seqNoStats.getMaxSeqNo()
+                + "] from last commit does not match global checkpoint [" + seqNoStats.getGlobalCheckpoint() + "]");
         }
         assert assertMaxSeqNoEqualsToGlobalCheckpoint(seqNoStats.getMaxSeqNo(), seqNoStats.getGlobalCheckpoint());
     }