Load field infos from directory if not yet cached on IndexShard

If we don't yet have a cached value ready to go for the field infos,
lets load it from the directory.

Fixes elastic#125383

Author: original-brownbear

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -418,7 +418,6 @@ public IndexShard(
         this.refreshFieldHasValueListener = new RefreshFieldHasValueListener();
         this.relativeTimeInNanosSupplier = relativeTimeInNanosSupplier;
         this.indexCommitListener = indexCommitListener;
-        this.fieldInfos = FieldInfos.EMPTY;
     }
 
     public ThreadPool getThreadPool() {
@@ -1025,7 +1024,11 @@ public void setFieldInfos(FieldInfos fieldInfos) {
     }
 
     public FieldInfos getFieldInfos() {
-        return fieldInfos;
+        var res = fieldInfos;
+        if (res == null) {
+            return loadFieldInfos();
+        }
+        return res;
     }
 
     public static Engine.Index prepareIndex(
@@ -4131,16 +4134,23 @@ public void beforeRefresh() {}
 
         @Override
         public void afterRefresh(boolean didRefresh) {
-            if (enableFieldHasValue && (didRefresh || fieldInfos == FieldInfos.EMPTY)) {
-                try (Engine.Searcher hasValueSearcher = getEngine().acquireSearcher("field_has_value")) {
-                    setFieldInfos(FieldInfos.getMergedFieldInfos(hasValueSearcher.getIndexReader()));
-                } catch (AlreadyClosedException ignore) {
-                    // engine is closed - no updated FieldInfos is fine
-                }
+            if (enableFieldHasValue && (didRefresh || fieldInfos == null)) {
+                loadFieldInfos();
             }
         }
     }
 
+    private FieldInfos loadFieldInfos() {
+        try (Engine.Searcher hasValueSearcher = getEngine().acquireSearcher("field_has_value")) {
+            var res = FieldInfos.getMergedFieldInfos(hasValueSearcher.getIndexReader());
+            setFieldInfos(res);
+            return res;
+        } catch (AlreadyClosedException ignore) {
+            // engine is closed - no update to3 FieldInfos is fine
+        }
+        return FieldInfos.EMPTY;
+    }
+
     /**
      * Returns the shard-level field stats, which includes the number of segments in the latest NRT reader of this shard
      * and the total number of fields across those segments.

x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotsSearchIntegTests.java

@@ -7,6 +7,7 @@
 
 package org.elasticsearch.xpack.searchablesnapshots;
 
+import org.elasticsearch.action.fieldcaps.FieldCapabilitiesResponse;
 import org.elasticsearch.action.index.IndexRequestBuilder;
 import org.elasticsearch.action.search.SearchRequest;
 import org.elasticsearch.action.search.SearchType;
@@ -125,5 +126,9 @@ public void testKeywordSortedQueryOnFrozen() throws Exception {
             assertThat(searchResponse.getTotalShards(), equalTo(20));
             assertThat(searchResponse.getHits().getTotalHits().value(), equalTo(4L));
         });
+
+        // check that field_caps empty field filtering works as well
+        FieldCapabilitiesResponse response = client().prepareFieldCaps(mountedIndices).setFields("*").setincludeEmptyFields(false).get();
+        assertNotNull(response.getField("keyword"));
     }
 }