Correctly identify parent of copy_to destination field for synthetic source purposes

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

@@ -123,6 +123,7 @@ public int get() {
     private Field version;
     private final SeqNoFieldMapper.SequenceIDFields seqID;
     private final Set<String> fieldsAppliedFromTemplates;
+
     /**
      * Fields that are copied from values of other fields via copy_to.
      * This per-document state is needed since it is possible
@@ -453,26 +454,16 @@ public boolean isFieldAppliedFromTemplate(String name) {
 
     public void markFieldAsCopyTo(String fieldName) {
         copyToFields.add(fieldName);
-        if (mappingLookup.isSourceSynthetic() && indexSettings().getSkipIgnoredSourceWrite() == false) {
-            /*
-            Mark this field as containing copied data meaning it should not be present
-            in synthetic _source (to be consistent with stored _source).
-            Ignored source values take precedence over standard synthetic source implementation
-            so by adding this nothing entry we "disable" field in synthetic source.
-            Otherwise, it would be constructed f.e. from doc_values which leads to duplicate values
-            in copied field after reindexing.
-
-            Note that this applies to fields that are copied from fields using ignored source themselves
-            and therefore we don't check for canAddIgnoredField().
-            */
-            ignoredFieldValues.add(IgnoredSourceFieldMapper.NameValue.fromContext(this, fieldName, XContentDataHelper.nothing()));
-        }
     }
 
     public boolean isCopyToDestinationField(String name) {
         return copyToFields.contains(name);
     }
 
+    public Set<String> getCopyToFields() {
+        return copyToFields;
+    }
+
     /**
      * Add a new mapper dynamically created while parsing.
      *

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

@@ -26,9 +26,8 @@
 import java.io.IOException;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Collections;
-import java.util.Comparator;
-import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Stream;
@@ -113,6 +112,10 @@ NameValue cloneWithValue(BytesRef value) {
             assert value() == null;
             return new NameValue(name, parentOffset, value, doc);
         }
+
+        boolean hasValue() {
+            return XContentDataHelper.isDataPresent(value);
+        }
     }
 
     static final class IgnoredValuesFieldMapperType extends StringFieldType {
@@ -147,11 +150,38 @@ protected String contentType() {
     @Override
     public void postParse(DocumentParserContext context) {
         // Ignored values are only expected in synthetic mode.
-        assert context.getIgnoredFieldValues().isEmpty() || context.mappingLookup().isSourceSynthetic();
-        List<NameValue> ignoredFieldValues = new ArrayList<>(context.getIgnoredFieldValues());
-        // ensure consistent ordering when retrieving synthetic source
-        Collections.sort(ignoredFieldValues, Comparator.comparing(NameValue::name));
-        for (NameValue nameValue : ignoredFieldValues) {
+        if (context.mappingLookup().isSourceSynthetic() == false) {
+            assert context.getIgnoredFieldValues().isEmpty();
+            return;
+        }
+
+        Collection<NameValue> ignoredValuesToWrite = context.getIgnoredFieldValues();
+        if (context.getCopyToFields().isEmpty() == false && indexSettings.getSkipIgnoredSourceWrite() == false) {
+            /*
+            Mark fields as containing copied data meaning they should not be present
+            in synthetic _source (to be consistent with stored _source).
+            Ignored source values take precedence over standard synthetic source implementation
+            so by adding the `XContentDataHelper.voidValue()` entry we disable the field in synthetic source.
+            Otherwise, it would be constructed f.e. from doc_values which leads to duplicate values
+            in copied field after reindexing.
+            */
+            var mutableList = new ArrayList<>(ignoredValuesToWrite);
+            for (String copyToField : context.getCopyToFields()) {
+                ObjectMapper parent = context.parent().findParentMapper(copyToField);
+                if (parent == null) {
+                    // There are scenarios when this can happen:
+                    // 1. all values of the field that is the source of copy_to are null
+                    // 2. copy_to points at a field inside a disabled object
+                    // 3. copy_to points at dynamic field which is not yet applied to mapping, we will process it properly on re-parse.
+                    continue;
+                }
+                int offset = parent.isRoot() ? 0 : parent.fullPath().length() + 1;
+                mutableList.add(new IgnoredSourceFieldMapper.NameValue(copyToField, offset, XContentDataHelper.voidValue(), context.doc()));
+            }
+            ignoredValuesToWrite = mutableList;
+        }
+
+        for (NameValue nameValue : ignoredValuesToWrite) {
             nameValue.doc().add(new StoredField(NAME, encode(nameValue)));
         }
     }

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

@@ -40,7 +40,8 @@ public Set<NodeFeature> getFeatures() {
             Mapper.SYNTHETIC_SOURCE_KEEP_FEATURE,
             SourceFieldMapper.SYNTHETIC_SOURCE_WITH_COPY_TO_AND_DOC_VALUES_FALSE_SUPPORT,
             SourceFieldMapper.SYNTHETIC_SOURCE_COPY_TO_FIX,
-            FlattenedFieldMapper.IGNORE_ABOVE_SUPPORT
+            FlattenedFieldMapper.IGNORE_ABOVE_SUPPORT,
+            SourceFieldMapper.SYNTHETIC_SOURCE_COPY_TO_INSIDE_OBJECTS_FIX
         );
     }
 }

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

@@ -808,6 +808,42 @@ protected void doXContent(XContentBuilder builder, Params params) throws IOExcep
 
     }
 
+    ObjectMapper findParentMapper(String leafFieldPath) {
+        var pathComponents = leafFieldPath.split("\\.");
+        int startPathComponent = 0;
+
+        ObjectMapper current = this;
+        String pathInCurrent = leafFieldPath;
+
+        while (current != null) {
+            if (current.mappers.containsKey(pathInCurrent)) {
+                return current;
+            }
+
+            // Go one level down if possible
+            var parent = current;
+            current = null;
+
+            var childMapperName = new StringBuilder();
+            for (int i = startPathComponent; i < pathComponents.length - 1; i++) {
+                if (childMapperName.isEmpty() == false) {
+                    childMapperName.append(".");
+                }
+                childMapperName.append(pathComponents[i]);
+
+                var childMapper = parent.mappers.get(childMapperName.toString());
+                if (childMapper instanceof ObjectMapper objectMapper) {
+                    current = objectMapper;
+                    startPathComponent = i + 1;
+                    pathInCurrent = pathInCurrent.substring(childMapperName.length() + 1);
+                    break;
+                }
+            }
+        }
+
+        return null;
+    }
+
     protected SourceLoader.SyntheticFieldLoader syntheticFieldLoader(Stream<Mapper> mappers, boolean isFragment) {
         var fields = mappers.sorted(Comparator.comparing(Mapper::fullPath))
             .map(Mapper::syntheticFieldLoader)
@@ -828,10 +864,18 @@ public SourceLoader.SyntheticFieldLoader syntheticFieldLoader() {
     private class SyntheticSourceFieldLoader implements SourceLoader.SyntheticFieldLoader {
         private final List<SourceLoader.SyntheticFieldLoader> fields;
         private final boolean isFragment;
+
         private boolean storedFieldLoadersHaveValues;
         private boolean docValuesLoadersHaveValues;
         private boolean ignoredValuesPresent;
         private List<IgnoredSourceFieldMapper.NameValue> ignoredValues;
+        // If this loader has anything to write.
+        // In special cases this can be false even if doc values loaders or stored field loaders
+        // have values.
+        // F.e. objects that only contain fields that are destinations of copy_to.
+        private boolean writersHaveValues;
+        // Use an ordered map between field names and writers to order writing by field name.
+        private TreeMap<String, FieldWriter> currentWriters;
 
         private SyntheticSourceFieldLoader(List<SourceLoader.SyntheticFieldLoader> fields, boolean isFragment) {
             this.fields = fields;
@@ -882,22 +926,69 @@ public boolean advanceToDoc(int docId) throws IOException {
             }
         }
 
+        @Override
+        public void prepare() {
+            if ((storedFieldLoadersHaveValues || docValuesLoadersHaveValues || ignoredValuesPresent) == false) {
+                writersHaveValues = false;
+                return;
+            }
+
+            for (var loader : fields) {
+                // Currently this logic is only relevant for object loaders.
+                if (loader instanceof ObjectMapper.SyntheticSourceFieldLoader objectSyntheticFieldLoader) {
+                    objectSyntheticFieldLoader.prepare();
+                }
+            }
+
+            currentWriters = new TreeMap<>();
+
+            if (ignoredValues != null && ignoredValues.isEmpty() == false) {
+                for (IgnoredSourceFieldMapper.NameValue value : ignoredValues) {
+                    if (value.hasValue()) {
+                        writersHaveValues |= true;
+                    }
+
+                    var existing = currentWriters.get(value.name());
+                    if (existing == null) {
+                        currentWriters.put(value.name(), new FieldWriter.IgnoredSource(value));
+                    } else if (existing instanceof FieldWriter.IgnoredSource isw) {
+                        isw.mergeWith(value);
+                    }
+                }
+            }
+
+            for (SourceLoader.SyntheticFieldLoader field : fields) {
+                if (field.hasValue()) {
+                    if (currentWriters.containsKey(field.fieldName()) == false) {
+                        writersHaveValues |= true;
+                        currentWriters.put(field.fieldName(), new FieldWriter.FieldLoader(field));
+                    } else {
+                        // Skip if the field source is stored separately, to avoid double-printing.
+                        // Make sure to reset the state of loader so that values stored inside will not
+                        // be used after this document is finished.
+                        field.reset();
+                    }
+                }
+            }
+        }
+
         @Override
         public boolean hasValue() {
-            return storedFieldLoadersHaveValues || docValuesLoadersHaveValues || ignoredValuesPresent;
+            return writersHaveValues;
         }
 
         @Override
         public void write(XContentBuilder b) throws IOException {
             if (hasValue() == false) {
                 return;
             }
+
             if (isRoot() && isEnabled() == false) {
                 // If the root object mapper is disabled, it is expected to contain
                 // the source encapsulated within a single ignored source value.
                 assert ignoredValues.size() == 1 : ignoredValues.size();
                 XContentDataHelper.decodeAndWrite(b, ignoredValues.get(0).value());
-                ignoredValues = null;
+                softReset();
                 return;
             }
 
@@ -907,41 +998,12 @@ public void write(XContentBuilder b) throws IOException {
                 b.startObject(leafName());
             }
 
-            if (ignoredValues != null && ignoredValues.isEmpty() == false) {
-                // Use an ordered map between field names and writer functions, to order writing by field name.
-                Map<String, FieldWriter> orderedFields = new TreeMap<>();
-                for (IgnoredSourceFieldMapper.NameValue value : ignoredValues) {
-                    var existing = orderedFields.get(value.name());
-                    if (existing == null) {
-                        orderedFields.put(value.name(), new FieldWriter.IgnoredSource(value));
-                    } else if (existing instanceof FieldWriter.IgnoredSource isw) {
-                        isw.mergeWith(value);
-                    }
-                }
-                for (SourceLoader.SyntheticFieldLoader field : fields) {
-                    if (field.hasValue()) {
-                        if (orderedFields.containsKey(field.fieldName()) == false) {
-                            orderedFields.put(field.fieldName(), new FieldWriter.FieldLoader(field));
-                        } else {
-                            // Skip if the field source is stored separately, to avoid double-printing.
-                            // Make sure to reset the state of loader so that values stored inside will not
-                            // be used after this document is finished.
-                            field.reset();
-                        }
-                    }
-                }
-
-                for (var writer : orderedFields.values()) {
+            for (var writer : currentWriters.values()) {
+                if (writer.hasValue()) {
                     writer.writeTo(b);
                 }
-                ignoredValues = null;
-            } else {
-                for (SourceLoader.SyntheticFieldLoader field : fields) {
-                    if (field.hasValue()) {
-                        field.write(b);
-                    }
-                }
             }
+
             b.endObject();
             softReset();
         }
@@ -957,6 +1019,8 @@ private void softReset() {
             storedFieldLoadersHaveValues = false;
             docValuesLoadersHaveValues = false;
             ignoredValuesPresent = false;
+            ignoredValues = null;
+            writersHaveValues = false;
         }
 
         @Override
@@ -986,34 +1050,49 @@ public String fieldName() {
         interface FieldWriter {
             void writeTo(XContentBuilder builder) throws IOException;
 
+            boolean hasValue();
+
             record FieldLoader(SourceLoader.SyntheticFieldLoader loader) implements FieldWriter {
                 @Override
                 public void writeTo(XContentBuilder builder) throws IOException {
                     loader.write(builder);
                 }
+
+                @Override
+                public boolean hasValue() {
+                    return loader.hasValue();
+                }
             }
 
             class IgnoredSource implements FieldWriter {
                 private final String fieldName;
                 private final String leafName;
-                private final List<BytesRef> values;
+                private final List<BytesRef> encodedValues;
 
                 IgnoredSource(IgnoredSourceFieldMapper.NameValue initialValue) {
                     this.fieldName = initialValue.name();
                     this.leafName = initialValue.getFieldName();
-                    this.values = new ArrayList<>();
-                    this.values.add(initialValue.value());
+                    this.encodedValues = new ArrayList<>();
+                    if (initialValue.hasValue()) {
+                        this.encodedValues.add(initialValue.value());
+                    }
                 }
 
                 @Override
                 public void writeTo(XContentBuilder builder) throws IOException {
-                    XContentDataHelper.writeMerged(builder, leafName, values);
+                    XContentDataHelper.writeMerged(builder, leafName, encodedValues);
+                }
+
+                @Override
+                public boolean hasValue() {
+                    return encodedValues.isEmpty() == false;
                 }
 
                 public FieldWriter mergeWith(IgnoredSourceFieldMapper.NameValue nameValue) {
                     assert Objects.equals(nameValue.name(), fieldName) : "IgnoredSource is merged with wrong field data";
-
-                    values.add(nameValue.value());
+                    if (nameValue.hasValue()) {
+                        encodedValues.add(nameValue.value());
+                    }
                     return this;
                 }
             }

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

@@ -48,6 +48,9 @@ public class SourceFieldMapper extends MetadataFieldMapper {
         "mapper.source.synthetic_source_with_copy_to_and_doc_values_false"
     );
     public static final NodeFeature SYNTHETIC_SOURCE_COPY_TO_FIX = new NodeFeature("mapper.source.synthetic_source_copy_to_fix");
+    public static final NodeFeature SYNTHETIC_SOURCE_COPY_TO_INSIDE_OBJECTS_FIX = new NodeFeature(
+        "mapper.source.synthetic_source_copy_to_inside_objects_fix"
+    );
 
     public static final String NAME = "_source";
     public static final String RECOVERY_SOURCE_NAME = "_recovery_source";

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

@@ -209,6 +209,9 @@ public void write(LeafStoredFieldLoader storedFieldLoader, int docId, XContentBu
                 if (docValuesLoader != null) {
                     docValuesLoader.advanceToDoc(docId);
                 }
+
+                loader.prepare();
+
                 // TODO accept a requested xcontent type
                 if (loader.hasValue()) {
                     loader.write(b);
@@ -299,6 +302,16 @@ public String fieldName() {
          */
         DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf) throws IOException;
 
+        /**
+         Perform any preprocessing needed before producing synthetic source
+         and deduce whether this mapper (and its children, if any) have values to write.
+         The expectation is for this method to be called before {@link SyntheticFieldLoader#hasValue()}
+         and {@link SyntheticFieldLoader#write(XContentBuilder)} are used.
+         */
+        default void prepare() {
+            // Noop
+        }
+
         /**
          * Has this field loaded any values for this document?
          */