Add option to ObjectParser to consume unknown fields (#42491)

ObjectParser has two ways of dealing with unknown fields: ignore them entirely,
or throw an error. Sometimes it can be useful instead to gather up these unknown
fields and record them separately, for example as arbitrary entries in a map.

This commit adds the ability to specify an unknown field consumer on an ObjectParser,
called with the field name and parsed value of each unknown field encountered during
parsing. The public API of ObjectParser is largely unchanged, with a single new
constructor method and interface definition.

libs/x-content/src/main/java/org/elasticsearch/common/xcontent/ObjectParser.java

@@ -78,14 +78,63 @@ public final class ObjectParser<Value, Context> extends AbstractObjectParser<Val
         };
     }
 
+    private interface UnknownFieldParser<Value, Context> {
+
+        void acceptUnknownField(String parserName, String field, XContentLocation location, XContentParser parser,
+                                Value value, Context context) throws IOException;
+    }
+
+    private static <Value, Context> UnknownFieldParser<Value, Context> ignoreUnknown() {
+      return (n, f, l, p, v, c) -> p.skipChildren();
+    }
+
+    private static <Value, Context> UnknownFieldParser<Value, Context> errorOnUnknown() {
+        return (n, f, l, p, v, c) -> {
+            throw new XContentParseException(l, "[" + n + "] unknown field [" + f + "], parser not found");
+        };
+    }
+
+    /**
+     * Defines how to consume a parsed undefined field
+     */
+    public interface UnknownFieldConsumer<Value> {
+        void accept(Value target, String field, Object value);
+    }
+
+    private static <Value, Context> UnknownFieldParser<Value, Context> consumeUnknownField(UnknownFieldConsumer<Value> consumer) {
+        return (parserName, field, location, parser, value, context) -> {
+            XContentParser.Token t = parser.currentToken();
+            switch (t) {
+                case VALUE_STRING:
+                    consumer.accept(value, field, parser.text());
+                    break;
+                case VALUE_NUMBER:
+                    consumer.accept(value, field, parser.numberValue());
+                    break;
+                case VALUE_BOOLEAN:
+                    consumer.accept(value, field, parser.booleanValue());
+                    break;
+                case VALUE_NULL:
+                    consumer.accept(value, field, null);
+                    break;
+                case START_OBJECT:
+                    consumer.accept(value, field, parser.map());
+                    break;
+                case START_ARRAY:
+                    consumer.accept(value, field, parser.list());
+                    break;
+                default:
+                    throw new XContentParseException(parser.getTokenLocation(),
+                        "[" + parserName + "] cannot parse field [" + field + "] with value type [" + t + "]");
+            }
+        };
+    }
+
     private final Map<String, FieldParser> fieldParserMap = new HashMap<>();
     private final String name;
     private final Supplier<Value> valueSupplier;
-    /**
-     * Should this parser ignore unknown fields? This should generally be set to true only when parsing responses from external systems,
-     * never when parsing requests from users.
-     */
-    private final boolean ignoreUnknownFields;
+
+    private final UnknownFieldParser<Value, Context> unknownFieldParser;
 
     /**
      * Creates a new ObjectParser instance with a name. This name is used to reference the parser in exceptions and messages.
@@ -95,25 +144,45 @@ public final class ObjectParser<Value, Context> extends AbstractObjectParser<Val
     }
 
     /**
-     * Creates a new ObjectParser instance which a name.
+     * Creates a new ObjectParser instance with a name.
      * @param name the parsers name, used to reference the parser in exceptions and messages.
      * @param valueSupplier a supplier that creates a new Value instance used when the parser is used as an inner object parser.
      */
     public ObjectParser(String name, @Nullable Supplier<Value> valueSupplier) {
-        this(name, false, valueSupplier);
+        this(name, errorOnUnknown(), valueSupplier);
     }
 
     /**
-     * Creates a new ObjectParser instance which a name.
+     * Creates a new ObjectParser instance with a name.
      * @param name the parsers name, used to reference the parser in exceptions and messages.
      * @param ignoreUnknownFields Should this parser ignore unknown fields? This should generally be set to true only when parsing
      *      responses from external systems, never when parsing requests from users.
      * @param valueSupplier a supplier that creates a new Value instance used when the parser is used as an inner object parser.
      */
     public ObjectParser(String name, boolean ignoreUnknownFields, @Nullable Supplier<Value> valueSupplier) {
+        this(name, ignoreUnknownFields ? ignoreUnknown() : errorOnUnknown(), valueSupplier);
+    }
+
+    /**
+     * Creates a new ObjectParser instance with a name.
+     * @param name the parsers name, used to reference the parser in exceptions and messages.
+     * @param unknownFieldConsumer how to consume parsed unknown fields
+     * @param valueSupplier a supplier that creates a new Value instance used when the parser is used as an inner object parser.
+     */
+    public ObjectParser(String name, UnknownFieldConsumer<Value> unknownFieldConsumer, @Nullable Supplier<Value> valueSupplier) {
+        this(name, consumeUnknownField(unknownFieldConsumer), valueSupplier);
+    }
+
+    /**
+     * Creates a new ObjectParser instance with a name.
+     * @param name the parsers name, used to reference the parser in exceptions and messages.
+     * @param unknownFieldParser how to parse unknown fields
+     * @param valueSupplier a supplier that creates a new Value instance used when the parser is used as an inner object parser.
+     */
+    private ObjectParser(String name, UnknownFieldParser<Value, Context> unknownFieldParser, @Nullable Supplier<Value> valueSupplier) {
         this.name = name;
         this.valueSupplier = valueSupplier;
-        this.ignoreUnknownFields = ignoreUnknownFields;
+        this.unknownFieldParser = unknownFieldParser;
     }
 
     /**
@@ -152,17 +221,18 @@ public final class ObjectParser<Value, Context> extends AbstractObjectParser<Val
 
         FieldParser fieldParser = null;
         String currentFieldName = null;
+        XContentLocation currentPosition = null;
         while ((token = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
             if (token == XContentParser.Token.FIELD_NAME) {
                 currentFieldName = parser.currentName();
-                fieldParser = getParser(currentFieldName, parser);
+                currentPosition = parser.getTokenLocation();
+                fieldParser = fieldParserMap.get(currentFieldName);
             } else {
                 if (currentFieldName == null) {
                     throw new XContentParseException(parser.getTokenLocation(), "[" + name  + "] no field found");
                 }
                 if (fieldParser == null) {
-                    assert ignoreUnknownFields : "this should only be possible if configured to ignore known fields";
-                    parser.skipChildren(); // noop if parser points to a value, skips children if parser is start object or start array
+                    unknownFieldParser.acceptUnknownField(name, currentFieldName, currentPosition, parser, value, context);
                 } else {
                     fieldParser.assertSupports(name, parser, currentFieldName);
                     parseSub(parser, fieldParser, currentFieldName, value, context);
@@ -363,15 +433,6 @@ public final class ObjectParser<Value, Context> extends AbstractObjectParser<Val
         }
     }
 
-    private FieldParser getParser(String fieldName, XContentParser xContentParser) {
-        FieldParser parser = fieldParserMap.get(fieldName);
-        if (parser == null && false == ignoreUnknownFields) {
-            throw new XContentParseException(xContentParser.getTokenLocation(),
-                    "[" + name + "] unknown field [" + fieldName + "], parser not found");
-        }
-        return parser;
-    }
-
     private class FieldParser {
         private final Parser<Value, Context> parser;
         private final EnumSet<XContentParser.Token> supportedTokens;

libs/x-content/src/test/java/org/elasticsearch/common/xcontent/ObjectParserTests.java

@@ -33,7 +33,9 @@ import java.net.URI;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.hamcrest.Matchers.containsString;
@@ -733,4 +735,41 @@ public class ObjectParserTests extends ESTestCase {
             this.foo = foo;
         }
     }
+
+    private static class ObjectWithArbitraryFields {
+        String name;
+        Map<String, Object> fields = new HashMap<>();
+        void setField(String key, Object value) {
+            fields.put(key, value);
+        }
+        void setName(String name) {
+            this.name = name;
+        }
+    }
+
+    public void testConsumeUnknownFields() throws IOException {
+        XContentParser parser = createParser(JsonXContent.jsonXContent,
+            "{\n"
+                + "  \"test\" : \"foo\",\n"
+                + "  \"test_number\" : 2,\n"
+                + "  \"name\" : \"geoff\",\n"
+                + "  \"test_boolean\" : true,\n"
+                + "  \"test_null\" : null,\n"
+                + "  \"test_array\":  [1,2,3,4],\n"
+                + "  \"test_nested\": { \"field\" : \"value\", \"field2\" : [ \"list1\", \"list2\" ] }\n"
+                + "}");
+        ObjectParser<ObjectWithArbitraryFields, Void> op
+            = new ObjectParser<>("unknown", ObjectWithArbitraryFields::setField, ObjectWithArbitraryFields::new);
+        op.declareString(ObjectWithArbitraryFields::setName, new ParseField("name"));
+
+        ObjectWithArbitraryFields o = op.parse(parser, null);
+        assertEquals("geoff", o.name);
+        assertEquals(6, o.fields.size());
+        assertEquals("foo", o.fields.get("test"));
+        assertEquals(2, o.fields.get("test_number"));
+        assertEquals(true, o.fields.get("test_boolean"));
+        assertNull(o.fields.get("test_null"));
+        assertEquals(List.of(1, 2, 3, 4), o.fields.get("test_array"));
+        assertEquals(Map.of("field", "value", "field2", List.of("list1", "list2")), o.fields.get("test_nested"));
+    }
 }