Better support for switching parsers. Relates to ES-13516 (#138722)

Author: mosche

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

@@ -225,4 +225,12 @@ <T> Map<String, T> map(Supplier<Map<String, T>> mapFactory, CheckedFunction<XCon
      * The callback to notify when parsing encounters a deprecated field.
      */
     DeprecationHandler getDeprecationHandler();
+
+    /**
+     * Switch to a different underlying parser.
+     * Typically, that's a noop but some filter parsers might want to wrap the underlying parser again.
+     */
+    default XContentParser switchParser(XContentParser parser) throws IOException {
+        return parser;
+    }
 }

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

@@ -35,10 +35,6 @@
  */
 class DotExpandingXContentParser extends FilterXContentParserWrapper {
 
-    static boolean isInstance(XContentParser parser) {
-        return parser instanceof WrappingParser;
-    }
-
     private static final class WrappingParser extends FilterXContentParser {
 
         private final ContentPath contentPath;
@@ -52,6 +48,11 @@ private static final class WrappingParser extends FilterXContentParser {
             }
         }
 
+        @Override
+        public XContentParser switchParser(XContentParser parser) throws IOException {
+            return new WrappingParser(parser, contentPath);
+        }
+
         @Override
         public Token nextToken() throws IOException {
             Token token;

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

@@ -297,10 +297,9 @@ private static DocumentParserContext cloneDocumentParserContext(
             BytesReference.bytes(builder),
             context.parser().contentType()
         );
-        if (DotExpandingXContentParser.isInstance(context.parser())) {
-            // If we performed dot expanding originally we need to continue to do so when we replace the parser.
-            newParser = DotExpandingXContentParser.expandDots(newParser, context.path());
-        }
+        // If we performed dot expanding originally we need to continue to do so when we replace the parser.
+        // To reliably do that, we need to delegate switching to the parser itself as it might be decorated / wrapped.
+        newParser = context.parser().switchParser(newParser);
 
         DocumentParserContext subcontext = context.switchParser(newParser);
         subcontext.setRecordedSource();  // Avoids double-storing parts of the source for the same parser subtree.