Refactor FilterXContentParser and DelegatingXContentParser (#83457)

We have two implementations of XContentParser that both delegate all of its methods to a delegate, either an inner parser provided at construction (FilterXContentParser) or a more dynamic variant that is returned by overriding the delegate method (DelegatingXContentParser).

Effectively the two classes do exactly the same, the only difference being how the delegate parser is provided. While these two are two separate implementations, they could inherit from each other.

With this change we make FilterXContentParser be the previous DelegatingXContentParser, that allows to override the delegate method, and we introduce a new FilterXContentParserWrapper that takes the fixed delegate as a constructor argument.

Additionally, XContentSubParser is rewritten to extend FilterXContentParserWrapper.

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

@@ -17,9 +17,9 @@
  *
  * A fieldname named {@code "foo.bar.baz":...} will be parsed instead as {@code 'foo':{'bar':{'baz':...}}}
  */
-public class DotExpandingXContentParser extends FilterXContentParser {
+public class DotExpandingXContentParser extends FilterXContentParserWrapper {
 
-    private static class WrappingParser extends DelegatingXContentParser {
+    private static final class WrappingParser extends FilterXContentParser {
 
         final Deque<XContentParser> parsers = new ArrayDeque<>();
 
@@ -135,7 +135,7 @@ public Token nextToken() throws IOException {
             assert expandedTokens < subPaths.length * 2;
             if (expandedTokens == subPaths.length * 2 - 1) {
                 state = State.PARSING_ORIGINAL_CONTENT;
-                Token token = in.currentToken();
+                Token token = delegate().currentToken();
                 if (token == Token.START_OBJECT || token == Token.START_ARRAY) {
                     innerLevel++;
                 }
@@ -170,7 +170,7 @@ public Token currentToken() {
         return switch (state) {
             case EXPANDING_START_OBJECT -> expandedTokens % 2 == 1 ? Token.START_OBJECT : Token.FIELD_NAME;
             case ENDING_EXPANDED_OBJECT -> Token.END_OBJECT;
-            case PARSING_ORIGINAL_CONTENT -> in.currentToken();
+            case PARSING_ORIGINAL_CONTENT -> delegate().currentToken();
         };
     }
 
@@ -181,14 +181,14 @@ public String currentName() throws IOException {
             // whenever we are parsing some inner object/array we can easily delegate to the inner parser
             // e.g. field.with.dots: { obj:{ parsing here } }
             if (innerLevel > 0) {
-                return in.currentName();
+                return delegate().currentName();
             }
             Token token = currentToken();
             // if we are parsing the outer object/array, only at the start object/array we need to return
             // e.g. dots instead of field.with.dots otherwise we can simply delegate to the inner parser
             // which will do the right thing
             if (innerLevel == 0 && token != Token.START_OBJECT && token != Token.START_ARRAY) {
-                return in.currentName();
+                return delegate().currentName();
             }
             // note that innerLevel can be -1 if there are no inner object/array e.g. field.with.dots: value
             // as well as while there is and we are parsing their END_OBJECT or END_ARRAY
@@ -199,7 +199,7 @@ public String currentName() throws IOException {
     @Override
     public void skipChildren() throws IOException {
         if (state == State.EXPANDING_START_OBJECT) {
-            in.skipChildren();
+            delegate().skipChildren();
             state = State.ENDING_EXPANDED_OBJECT;
         }
         if (state == State.PARSING_ORIGINAL_CONTENT) {
@@ -231,7 +231,7 @@ public boolean booleanValue() throws IOException {
         return super.booleanValue();
     }
 
-    private static class SingletonValueXContentParser extends FilterXContentParser {
+    private static class SingletonValueXContentParser extends FilterXContentParserWrapper {
 
         protected SingletonValueXContentParser(XContentParser in) {
             super(in);