Use static constant for "segments_stats" source (elastic#129406)

So that we more easily know which engine reference it.

Author: tlrx

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

@@ -132,6 +132,7 @@ public abstract class Engine implements Closeable {
     public static final String SEARCH_SOURCE = "search"; // TODO: Make source of search enum?
     public static final String CAN_MATCH_SEARCH_SOURCE = "can_match";
     protected static final String DOC_STATS_SOURCE = "doc_stats";
+    protected static final String SEGMENTS_STATS_SOURCE = "segments_stats";
     public static final long UNKNOWN_PRIMARY_TERM = -1L;
     public static final String ROOT_DOC_FIELD_NAME = "__root_doc_for_nested";
 
@@ -1136,15 +1137,15 @@ public SegmentsStats segmentsStats(boolean includeSegmentFileSizes, boolean incl
         ensureOpen();
         Set<String> segmentName = new HashSet<>();
         SegmentsStats stats = new SegmentsStats();
-        try (Searcher searcher = acquireSearcher("segments_stats", SearcherScope.INTERNAL)) {
+        try (Searcher searcher = acquireSearcher(SEGMENTS_STATS_SOURCE, SearcherScope.INTERNAL)) {
             for (LeafReaderContext ctx : searcher.getIndexReader().getContext().leaves()) {
                 SegmentReader segmentReader = Lucene.segmentReader(ctx.reader());
                 fillSegmentStats(segmentReader, includeSegmentFileSizes, stats);
                 segmentName.add(segmentReader.getSegmentName());
             }
         }
 
-        try (Searcher searcher = acquireSearcher("segments_stats", SearcherScope.EXTERNAL)) {
+        try (Searcher searcher = acquireSearcher(SEGMENTS_STATS_SOURCE, SearcherScope.EXTERNAL)) {
             for (LeafReaderContext ctx : searcher.getIndexReader().getContext().leaves()) {
                 SegmentReader segmentReader = Lucene.segmentReader(ctx.reader());
                 if (segmentName.contains(segmentReader.getSegmentName()) == false) {

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

@@ -521,7 +521,7 @@ final boolean assertSearcherIsWarmedUp(String source, SearcherScope scope) {
             switch (source) {
                 // we can access segment_stats while a shard is still in the recovering state.
                 case "segments":
-                case "segments_stats":
+                case SEGMENTS_STATS_SOURCE:
                     break;
                 default:
                     assert externalReaderManager.isWarmedUp : "searcher was not warmed up yet for source[" + source + "]";

x-pack/plugin/core/src/main/java/org/elasticsearch/index/engine/frozen/FrozenEngine.java

@@ -250,7 +250,7 @@ private Engine.Searcher openSearcher(String source, SearcherScope scope) throws
             case "refresh_needed":
                 assert false : "refresh_needed is always false";
             case "segments":
-            case "segments_stats":
+            case SEGMENTS_STATS_SOURCE:
             case "completion_stats":
             case FIELD_RANGE_SEARCH_SOURCE: // special case for field_range - we use the cached point values reader
             case CAN_MATCH_SEARCH_SOURCE: // special case for can_match phase - we use the cached point values reader