Introduce XContentParser#namedObject (#22003)

Introduces `XContentParser#namedObject which works a little like
`StreamInput#readNamedWriteable`: on startup components register
parsers under names and a superclass. At runtime we look up the
parser and call it to parse the object.

Right now the parsers take a context object they use to help with
the parsing but I hope to be able to eliminate the need for this
context as most what it is used for at this point is to move
around parser registries which should be replaced by this method
eventually. I make no effort to do so in this PR because it is
big enough already. This is meant to the a start down a road that
allows us to remove classes like `QueryParseContext`,
`AggregatorParsers`, `IndicesQueriesRegistry`, and
`ParseFieldRegistry`.

The goal here is to reduce the amount of plumbing required to
allow parsing pluggable things. With this you don't have to pass
registries all over the place. Instead you must pass a super
registry to fewer places and use it to wrap the reader. This is
the same tradeoff that we use for NamedWriteable and it allows
much, much simpler binary serialization. We think we want that
same thing for xcontent serialization.

The only parsing actually converted to this method is parsing
`ScoreFunctions` inside of `FunctionScoreQuery`. I chose this
because it is relatively self contained.

1 files changed, 3 insertions, 6 deletions

diff --git a/core/src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggestionBuilder.java b/core/src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggestionBuilder.java
index 09382d9aaf..0fd3726384 100644
--- a/core/src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggestionBuilder.java
+++ b/core/src/main/java/org/elasticsearch/search/suggest/completion/CompletionSuggestionBuilder.java
@@ -42,7 +42,6 @@ import org.elasticsearch.search.suggest.completion.context.ContextMapping;
 import org.elasticsearch.search.suggest.completion.context.ContextMappings;
 
 import java.io.IOException;
-import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -233,10 +232,7 @@ public class CompletionSuggestionBuilder extends SuggestionBuilder<CompletionSug
             regexOptions.toXContent(builder, params);
         }
         if (contextBytes != null) {
-            try (XContentParser contextParser = XContentFactory.xContent(XContentType.JSON).createParser(contextBytes)) {
-                builder.field(CONTEXTS_FIELD.getPreferredName());
-                builder.copyCurrentStructure(contextParser);
-            }
+            builder.rawField(CONTEXTS_FIELD.getPreferredName(), contextBytes);
         }
         return builder;
     }
@@ -270,7 +266,8 @@ public class CompletionSuggestionBuilder extends SuggestionBuilder<CompletionSug
             CompletionFieldMapper.CompletionFieldType type = (CompletionFieldMapper.CompletionFieldType) mappedFieldType;
             suggestionContext.setFieldType(type);
             if (type.hasContextMappings() && contextBytes != null) {
-                try (XContentParser contextParser = XContentFactory.xContent(contextBytes).createParser(contextBytes)) {
+                try (XContentParser contextParser = XContentFactory.xContent(contextBytes).createParser(context.getXContentRegistry(),
+                        contextBytes)) {
                     if (type.hasContextMappings() && contextParser != null) {
                         ContextMappings contextMappings = type.getContextMappings();
                         contextParser.nextToken();