Load FieldInfos from store if not yet initialised through a refresh on IndexShard (#125650)

Load field caps from store if they haven't been initialised through a refresh yet.
Keep the plain reads to not mess with performance characteristics too much on the good path but protect against confusing races when loading field infos now (that probably should have been ordered stores in the first place but this was safe due to other locks/volatiles on the refresh path).

Closes #125483

docs/changelog/125650.yaml

@@ -0,0 +1,7 @@
+pr: 125650
+summary: Load `FieldInfos` from store if not yet initialised through a refresh on
+  `IndexShard`
+area: Search
+type: bug
+issues:
+ - 125483

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

@@ -158,6 +158,8 @@ import org.elasticsearch.transport.Transports;
 import java.io.Closeable;
 import java.io.IOException;
 import java.io.PrintStream;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
 import java.nio.channels.ClosedByInterruptException;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
@@ -427,7 +429,6 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
         this.refreshFieldHasValueListener = new RefreshFieldHasValueListener();
         this.relativeTimeInNanosSupplier = relativeTimeInNanosSupplier;
         this.indexCommitListener = indexCommitListener;
-        this.fieldInfos = FieldInfos.EMPTY;
     }
 
     public ThreadPool getThreadPool() {
@@ -1029,12 +1030,26 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
         return index(engine, operation);
     }
 
-    public void setFieldInfos(FieldInfos fieldInfos) {
-        this.fieldInfos = fieldInfos;
+    private static final VarHandle FIELD_INFOS;
+
+    static {
+        try {
+            FIELD_INFOS = MethodHandles.lookup().findVarHandle(IndexShard.class, "fieldInfos", FieldInfos.class);
+        } catch (Exception e) {
+            throw new ExceptionInInitializerError(e);
+        }
     }
 
     public FieldInfos getFieldInfos() {
-        return fieldInfos;
+        var res = fieldInfos;
+        if (res == null) {
+            // don't replace field infos loaded via the refresh listener to avoid overwriting the field with an older version of the
+            // field infos when racing with a refresh
+            var read = loadFieldInfos();
+            var existing = (FieldInfos) FIELD_INFOS.compareAndExchange(this, null, read);
+            return existing == null ? read : existing;
+        }
+        return res;
     }
 
     public static Engine.Index prepareIndex(
@@ -4266,16 +4281,21 @@ public class IndexShard extends AbstractIndexShardComponent implements IndicesCl
 
         @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)) {
+                FIELD_INFOS.setRelease(IndexShard.this, loadFieldInfos());
             }
         }
     }
 
+    private FieldInfos loadFieldInfos() {
+        try (Engine.Searcher hasValueSearcher = getEngine().acquireSearcher("field_has_value")) {
+            return FieldInfos.getMergedFieldInfos(hasValueSearcher.getIndexReader());
+        } catch (AlreadyClosedException ignore) {
+            // engine is closed - no update to 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 class SearchableSnapshotsSearchIntegTests extends BaseFrozenSearchableSna
             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"));
     }
 }