Extract synthetic source logic from DocumentParser (elastic#116049)

modules/data-streams/src/test/java/org/elasticsearch/datastreams/DataStreamGetWriteIndexTests.java

@@ -241,7 +241,7 @@ public void setup() throws Exception {
                 new MetadataFieldMapper[] { dtfm },
                 Collections.emptyMap()
             );
-            MappingLookup mappingLookup = MappingLookup.fromMappers(mapping, List.of(dtfm, dateFieldMapper), List.of());
+            MappingLookup mappingLookup = MappingLookup.fromMappers(mapping, List.of(dtfm, dateFieldMapper), List.of(), null);
             indicesService = DataStreamTestHelper.mockIndicesServices(mappingLookup);
         }
 

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

@@ -0,0 +1,88 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.mapper;
+
+import org.elasticsearch.core.Nullable;
+import org.elasticsearch.index.IndexSettings;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Contains lookup information needed to perform custom synthetic source logic.
+ * For example fields that use fallback synthetic source implementation or fields that preserve array ordering
+ * in synthetic source;
+ */
+public class CustomSyntheticSourceFieldLookup {
+    private final Map<String, Reason> fieldsWithCustomSyntheticSourceHandling;
+
+    public CustomSyntheticSourceFieldLookup(Mapping mapping, @Nullable IndexSettings indexSettings, boolean isSourceSynthetic) {
+        var fields = new HashMap<String, Reason>();
+        if (isSourceSynthetic && indexSettings != null) {
+            populateFields(fields, mapping.getRoot(), indexSettings.sourceKeepMode());
+        }
+        this.fieldsWithCustomSyntheticSourceHandling = Map.copyOf(fields);
+    }
+
+    private void populateFields(Map<String, Reason> fields, ObjectMapper currentLevel, Mapper.SourceKeepMode defaultSourceKeepMode) {
+        if (currentLevel.isEnabled() == false) {
+            fields.put(currentLevel.fullPath(), Reason.DISABLED_OBJECT);
+            return;
+        }
+        if (sourceKeepMode(currentLevel, defaultSourceKeepMode) == Mapper.SourceKeepMode.ALL) {
+            fields.put(currentLevel.fullPath(), Reason.SOURCE_KEEP_ALL);
+            return;
+        }
+        if (currentLevel.isNested() == false && sourceKeepMode(currentLevel, defaultSourceKeepMode) == Mapper.SourceKeepMode.ARRAYS) {
+            fields.put(currentLevel.fullPath(), Reason.SOURCE_KEEP_ARRAYS);
+        }
+
+        for (Mapper child : currentLevel) {
+            if (child instanceof ObjectMapper objectMapper) {
+                populateFields(fields, objectMapper, defaultSourceKeepMode);
+            } else if (child instanceof FieldMapper fieldMapper) {
+                // The order here is important.
+                // If fallback logic is used, it should be always correctly marked as FALLBACK_SYNTHETIC_SOURCE.
+                // This allows us to apply an optimization for SOURCE_KEEP_ARRAYS and don't store arrays that have one element.
+                // If this order is changed and a field that both has SOURCE_KEEP_ARRAYS and FALLBACK_SYNTHETIC_SOURCE
+                // is marked as SOURCE_KEEP_ARRAYS we would lose data for this field by applying such an optimization.
+                if (fieldMapper.syntheticSourceMode() == FieldMapper.SyntheticSourceMode.FALLBACK) {
+                    fields.put(fieldMapper.fullPath(), Reason.FALLBACK_SYNTHETIC_SOURCE);
+                } else if (sourceKeepMode(fieldMapper, defaultSourceKeepMode) == Mapper.SourceKeepMode.ALL) {
+                    fields.put(fieldMapper.fullPath(), Reason.SOURCE_KEEP_ALL);
+                } else if (sourceKeepMode(fieldMapper, defaultSourceKeepMode) == Mapper.SourceKeepMode.ARRAYS) {
+                    fields.put(fieldMapper.fullPath(), Reason.SOURCE_KEEP_ARRAYS);
+                }
+            }
+        }
+    }
+
+    private Mapper.SourceKeepMode sourceKeepMode(ObjectMapper mapper, Mapper.SourceKeepMode defaultSourceKeepMode) {
+        return mapper.sourceKeepMode().orElse(defaultSourceKeepMode);
+    }
+
+    private Mapper.SourceKeepMode sourceKeepMode(FieldMapper mapper, Mapper.SourceKeepMode defaultSourceKeepMode) {
+        return mapper.sourceKeepMode().orElse(defaultSourceKeepMode);
+    }
+
+    public Map<String, Reason> getFieldsWithCustomSyntheticSourceHandling() {
+        return fieldsWithCustomSyntheticSourceHandling;
+    }
+
+    /**
+     * Specifies why this field needs custom handling.
+     */
+    public enum Reason {
+        SOURCE_KEEP_ARRAYS,
+        SOURCE_KEEP_ALL,
+        FALLBACK_SYNTHETIC_SOURCE,
+        DISABLED_OBJECT
+    }
+}

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

@@ -49,6 +49,7 @@ public static DocumentMapper createEmpty(MapperService mapperService) {
             mapping,
             mapping.toCompressedXContent(),
             IndexVersion.current(),
+            mapperService.getIndexSettings(),
             mapperService.getMapperMetrics(),
             mapperService.index().getName()
         );
@@ -59,12 +60,13 @@ public static DocumentMapper createEmpty(MapperService mapperService) {
         Mapping mapping,
         CompressedXContent source,
         IndexVersion version,
+        IndexSettings indexSettings,
         MapperMetrics mapperMetrics,
         String indexName
     ) {
         this.documentParser = documentParser;
         this.type = mapping.getRoot().fullPath();
-        this.mappingLookup = MappingLookup.fromMapping(mapping);
+        this.mappingLookup = MappingLookup.fromMapping(mapping, indexSettings);
         this.mappingSource = source;
         this.mapperMetrics = mapperMetrics;
         this.indexVersion = version;