Switch AggregationSpec to ContextParser (#50871)

We seem to have settled on the `ContextParser` interface for parsing
stuff, mostly because `ObjectParser` implements it. We don't really
*need* the old `Aggregator.Parser` interface any more because it
duplicates `ContextParser` but with the arguments reversed.

This adds support to `AggregationSpec` to declare aggregation parsers
using `ContextParser`. This should integrate cleanly with
`ObjectParser`. It doesn't drop support for `Aggregator.Parser` or
change the plugin intrface at all so it *should* be safe to backport to
7.x. And we can remove `Aggregator.Parser` in a follow up which is only
targeted to 8.0.

server/src/main/java/org/elasticsearch/plugins/SearchPlugin.java

@@ -26,6 +26,7 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.Writeable;
 import org.elasticsearch.common.lucene.search.function.ScoreFunction;
+import org.elasticsearch.common.xcontent.ContextParser;
 import org.elasticsearch.common.xcontent.XContent;
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.index.query.QueryBuilder;
@@ -248,7 +249,7 @@ public QuerySpec(String name, Writeable.Reader<T> reader, QueryParser<T> parser)
     /**
      * Specification for an {@link Aggregation}.
      */
-    class AggregationSpec extends SearchExtensionSpec<AggregationBuilder, Aggregator.Parser> {
+    class AggregationSpec extends SearchExtensionSpec<AggregationBuilder, ContextParser<String, ? extends AggregationBuilder>> {
         private final Map<String, Writeable.Reader<? extends InternalAggregation>> resultReaders = new TreeMap<>();
 
         /**
@@ -261,7 +262,8 @@ class AggregationSpec extends SearchExtensionSpec<AggregationBuilder, Aggregator
          *        {@link StreamInput}
          * @param parser the parser the reads the aggregation builder from xcontent
          */
-        public AggregationSpec(ParseField name, Writeable.Reader<? extends AggregationBuilder> reader, Aggregator.Parser parser) {
+        public <T extends AggregationBuilder> AggregationSpec(ParseField name, Writeable.Reader<T> reader,
+                ContextParser<String, T> parser) {
             super(name, reader, parser);
         }
 
@@ -274,10 +276,41 @@ public AggregationSpec(ParseField name, Writeable.Reader<? extends AggregationBu
          *        {@link StreamInput}
          * @param parser the parser the reads the aggregation builder from xcontent
          */
-        public AggregationSpec(String name, Writeable.Reader<? extends AggregationBuilder> reader, Aggregator.Parser parser) {
+        public <T extends AggregationBuilder> AggregationSpec(String name, Writeable.Reader<T> reader, ContextParser<String, T> parser) {
             super(name, reader, parser);
         }
 
+        /**
+         * Specification for an {@link Aggregation}.
+         *
+         * @param name holds the names by which this aggregation might be parsed. The {@link ParseField#getPreferredName()} is special as it
+         *        is the name by under which the reader is registered. So it is the name that the {@link AggregationBuilder} should return
+         *        from {@link NamedWriteable#getWriteableName()}.
+         * @param reader the reader registered for this aggregation's builder. Typically a reference to a constructor that takes a
+         *        {@link StreamInput}
+         * @param parser the parser the reads the aggregation builder from xcontent
+         * @deprecated Use the ctor that takes a {@link ContextParser} instead
+         */
+        @Deprecated
+        public AggregationSpec(ParseField name, Writeable.Reader<? extends AggregationBuilder> reader, Aggregator.Parser parser) {
+            super(name, reader, (p, aggName) -> parser.parse(aggName, p));
+        }
+
+        /**
+         * Specification for an {@link Aggregation}.
+         *
+         * @param name the name by which this aggregation might be parsed or deserialized. Make sure that the {@link AggregationBuilder}
+         *        returns this from {@link NamedWriteable#getWriteableName()}.
+         * @param reader the reader registered for this aggregation's builder. Typically a reference to a constructor that takes a
+         *        {@link StreamInput}
+         * @param parser the parser the reads the aggregation builder from xcontent
+         * @deprecated Use the ctor that takes a {@link ContextParser} instead
+         */
+        @Deprecated
+        public AggregationSpec(String name, Writeable.Reader<? extends AggregationBuilder> reader, Aggregator.Parser parser) {
+            super(name, reader, (p, aggName) -> parser.parse(aggName, p));
+        }
+
         /**
          * Add a reader for the shard level results of the aggregation with {@linkplain #getName}'s {@link ParseField#getPreferredName()} as
          * the {@link NamedWriteable#getWriteableName()}.

server/src/main/java/org/elasticsearch/search/SearchModule.java

@@ -422,16 +422,16 @@ private void registerAggregations(List<SearchPlugin> plugins) {
         registerAggregation(new AggregationSpec(GeoCentroidAggregationBuilder.NAME, GeoCentroidAggregationBuilder::new,
                 GeoCentroidAggregationBuilder::parse).addResultReader(InternalGeoCentroid::new));
         registerAggregation(new AggregationSpec(ScriptedMetricAggregationBuilder.NAME, ScriptedMetricAggregationBuilder::new,
-                (name, p) -> ScriptedMetricAggregationBuilder.PARSER.parse(p, name)).addResultReader(InternalScriptedMetric::new));
+                ScriptedMetricAggregationBuilder.PARSER).addResultReader(InternalScriptedMetric::new));
         registerAggregation((new AggregationSpec(CompositeAggregationBuilder.NAME, CompositeAggregationBuilder::new,
-                (name, p) -> CompositeAggregationBuilder.PARSER.parse(p, name)).addResultReader(InternalComposite::new)));
+                CompositeAggregationBuilder.PARSER).addResultReader(InternalComposite::new)));
         registerFromPlugin(plugins, SearchPlugin::getAggregations, this::registerAggregation);
     }
 
     private void registerAggregation(AggregationSpec spec) {
         namedXContents.add(new NamedXContentRegistry.Entry(BaseAggregationBuilder.class, spec.getName(), (p, c) -> {
             String name = (String) c;
-            return spec.getParser().parse(name, p);
+            return spec.getParser().parse(p, name);
         }));
         namedWriteables.add(
                 new NamedWriteableRegistry.Entry(AggregationBuilder.class, spec.getName().getPreferredName(), spec.getReader()));