Simplify loading sequence number stats from commit

This commit simplifies the loading of sequence number stats from a
commit point by combining all the logic for loading into a single method
that returns an instance of SeqNoStats rather than three separate
methods that return longs.

Author: jasontedor

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

@@ -58,6 +58,7 @@
 import org.elasticsearch.index.mapper.internal.SeqNoFieldMapper;
 import org.elasticsearch.index.merge.MergeStats;
 import org.elasticsearch.index.merge.OnGoingMerge;
+import org.elasticsearch.index.seqno.SeqNoStats;
 import org.elasticsearch.index.seqno.SequenceNumbersService;
 import org.elasticsearch.index.shard.DocsStats;
 import org.elasticsearch.index.shard.ElasticsearchMergePolicy;
@@ -80,6 +81,8 @@
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.function.Function;
 
+import static org.elasticsearch.index.seqno.SequenceNumbersService.NO_OPS_PERFORMED;
+
 /**
  *
  */
@@ -142,17 +145,21 @@ public InternalEngine(EngineConfig engineConfig) throws EngineException {
             this.searcherFactory = new SearchFactory(logger, isClosed, engineConfig);
             try {
                 writer = createWriter(openMode == EngineConfig.OpenMode.CREATE_INDEX_AND_TRANSLOG);
-                final long localCheckpoint = loadLocalCheckpointFromCommit(writer);
-                final long globalCheckpoint = loadGlobalCheckpointFromCommit(writer);
-                final long maxSeqNo = loadMaxSeqNoFromCommit(writer);
+                final SeqNoStats seqNoStats = loadSeqNoStatsFromCommit(writer);
                 if (logger.isTraceEnabled()) {
                     logger.trace(
-                            "recovering local checkpoint: [{}], global checkpoint [{}], max sequence number [{}]",
-                            localCheckpoint,
-                            globalCheckpoint,
-                            maxSeqNo);
+                            "recovering max sequence number: [{}], local checkpoint: [{}], global checkpoint: [{}]",
+                            seqNoStats.getMaxSeqNo(),
+                            seqNoStats.getLocalCheckpoint(),
+                            seqNoStats.getGlobalCheckpoint());
                 }
-                seqNoService = new SequenceNumbersService(shardId, engineConfig.getIndexSettings(), maxSeqNo, localCheckpoint, globalCheckpoint);
+                seqNoService =
+                        new SequenceNumbersService(
+                                shardId,
+                                engineConfig.getIndexSettings(),
+                                seqNoStats.getMaxSeqNo(),
+                                seqNoStats.getLocalCheckpoint(),
+                                seqNoStats.getGlobalCheckpoint());
                 indexWriter = writer;
                 translog = openTranslog(engineConfig, writer);
                 assert translog.getGeneration() != null;
@@ -303,33 +310,34 @@ private Translog.TranslogGeneration loadTranslogIdFromCommit(IndexWriter writer)
         return null;
     }
 
-    private long loadLocalCheckpointFromCommit(IndexWriter writer) {
+    private SeqNoStats loadSeqNoStatsFromCommit(IndexWriter writer) throws IOException {
+        final long maxSeqNo;
+        try (IndexReader reader = DirectoryReader.open(writer)) {
+            final FieldStats stats = SeqNoFieldMapper.Defaults.FIELD_TYPE.stats(reader);
+            if (stats != null) {
+                maxSeqNo = (long) stats.getMaxValue();
+            } else {
+                maxSeqNo = NO_OPS_PERFORMED;
+            }
+        }
+
         final Map<String, String> commitUserData = writer.getCommitData();
+
+        final long localCheckpoint;
         if (commitUserData.containsKey(LOCAL_CHECKPOINT_KEY)) {
-            return Long.parseLong(commitUserData.get(LOCAL_CHECKPOINT_KEY));
+            localCheckpoint = Long.parseLong(commitUserData.get(LOCAL_CHECKPOINT_KEY));
         } else {
-            return SequenceNumbersService.NO_OPS_PERFORMED;
+            localCheckpoint = SequenceNumbersService.NO_OPS_PERFORMED;
         }
-    }
 
-    private long loadGlobalCheckpointFromCommit(IndexWriter writer) {
-        final Map<String, String> commitUserData = writer.getCommitData();
+        final long globalCheckpoint;
         if (commitUserData.containsKey(GLOBAL_CHECKPOINT_KEY)) {
-            return Long.parseLong(commitUserData.get(GLOBAL_CHECKPOINT_KEY));
+            globalCheckpoint = Long.parseLong(commitUserData.get(GLOBAL_CHECKPOINT_KEY));
         } else {
-            return SequenceNumbersService.UNASSIGNED_SEQ_NO;
+            globalCheckpoint = SequenceNumbersService.UNASSIGNED_SEQ_NO;
         }
-    }
 
-    private long loadMaxSeqNoFromCommit(IndexWriter writer) throws IOException {
-        try (IndexReader reader = DirectoryReader.open(writer)) {
-            final FieldStats stats = SeqNoFieldMapper.Defaults.FIELD_TYPE.stats(reader);
-            if (stats != null) {
-                return (long) stats.getMaxValue();
-            } else {
-                return SequenceNumbersService.NO_OPS_PERFORMED;
-            }
-        }
+        return new SeqNoStats(maxSeqNo, localCheckpoint, globalCheckpoint);
     }
 
     private SearcherManager createSearcherManager() throws EngineException {