Don't rebuild shadowed field lookup on every document (#76023)

#75595 added better checks for fields shadowed by runtime fields, so that we
don't index data that would never be searched. However, the shadow lookup
was being rebuilt for every document, which has caused a noticeable regression
in indexing times. This commit reworks things so that this lookup is built once
per mapping update and lives on MappingLookup.

server/src/main/java/org/elasticsearch/index/mapper/DocumentParserContext.java

@@ -92,7 +92,6 @@ protected void addDoc(LuceneDocument doc) {
     private final Set<String> newFieldsSeen;
     private final Map<String, ObjectMapper> dynamicObjectMappers;
     private final List<RuntimeField> dynamicRuntimeFields;
-    private final Set<String> shadowedFields;
     private Field version;
     private SeqNoFieldMapper.SequenceIDFields seqID;
 
@@ -108,7 +107,6 @@ private DocumentParserContext(DocumentParserContext in) {
         this.newFieldsSeen = in.newFieldsSeen;
         this.dynamicObjectMappers = in.dynamicObjectMappers;
         this.dynamicRuntimeFields = in.dynamicRuntimeFields;
-        this.shadowedFields = in.shadowedFields;
         this.version = in.version;
         this.seqID = in.seqID;
     }
@@ -129,17 +127,6 @@ protected DocumentParserContext(MappingLookup mappingLookup,
         this.newFieldsSeen = new HashSet<>();
         this.dynamicObjectMappers = new HashMap<>();
         this.dynamicRuntimeFields = new ArrayList<>();
-        this.shadowedFields = buildShadowedFields(mappingLookup);
-    }
-
-    private static Set<String> buildShadowedFields(MappingLookup lookup) {
-        Set<String> shadowedFields = new HashSet<>();
-        for (RuntimeField runtimeField : lookup.getMapping().getRoot().runtimeFields()) {
-            for (MappedFieldType mft : runtimeField.asMappedFieldTypes()) {
-                shadowedFields.add(mft.name());
-            }
-        }
-        return shadowedFields;
     }
 
     public final IndexSettings indexSettings() {
@@ -243,7 +230,7 @@ public final List<Mapper> getDynamicMappers() {
     }
 
     public final boolean isShadowed(String field) {
-        return shadowedFields.contains(field);
+        return mappingLookup.isShadowed(field);
     }
 
     public final ObjectMapper getObjectMapper(String name) {

server/src/main/java/org/elasticsearch/index/mapper/MappingLookup.java

@@ -16,6 +16,7 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -51,6 +52,7 @@ private CacheKey() {}
     private final Map<String, NamedAnalyzer> indexAnalyzersMap = new HashMap<>();
     private final List<FieldMapper> indexTimeScriptMappers = new ArrayList<>();
     private final Mapping mapping;
+    private final Set<String> shadowedFields;
 
     /**
      * Creates a new {@link MappingLookup} instance by parsing the provided mapping and extracting its field definitions.
@@ -157,6 +159,13 @@ private MappingLookup(Mapping mapping,
             }
         }
 
+        this.shadowedFields = new HashSet<>();
+        for (RuntimeField runtimeField : mapping.getRoot().runtimeFields()) {
+            for (MappedFieldType mft : runtimeField.asMappedFieldTypes()) {
+                shadowedFields.add(mft.name());
+            }
+        }
+
         this.fieldTypeLookup = new FieldTypeLookup(mappers, aliasMappers, mapping.getRoot().runtimeFields());
         this.indexTimeLookup = new FieldTypeLookup(mappers, aliasMappers, Collections.emptyList());
         this.fieldMappers = Collections.unmodifiableMap(fieldMappers);
@@ -199,6 +208,13 @@ public Iterable<Mapper> fieldMappers() {
         return fieldMappers.values();
     }
 
+    /**
+     * @return {@code true} if the given field is shadowed by a runtime field
+     */
+    public boolean isShadowed(String field) {
+        return shadowedFields.contains(field);
+    }
+
     void checkLimits(IndexSettings settings) {
         checkFieldLimit(settings.getMappingTotalFieldsLimit());
         checkObjectDepthLimit(settings.getMappingDepthLimit());