Scripting: get context names REST API (elastic#48026)

Adds `GET /_script_context`, returning a `contexts` object with each
available context as a key whose value is an empty object. eg.
```
{
  "contexts": {
    "aggregation_selector": {},
    "aggs": {},
    "aggs_combine": {},
...
  }
}
```

refs: elastic#47411

client/rest-high-level/src/test/java/org/elasticsearch/client/RestHighLevelClientTests.java

@@ -760,6 +760,7 @@ public void testApiNamingConventions() throws Exception {
         String[] notYetSupportedApi = new String[]{
             "cluster.remote_info",
             "create",
+            "get_script_context",
             "get_source",
             "indices.delete_alias",
             "indices.exists_type",

rest-api-spec/src/main/resources/rest-api-spec/api/get_script_context.json

@@ -0,0 +1,19 @@
+{
+  "get_script_context":{
+    "documentation":{
+      "description":"Returns all script contexts."
+    },
+    "stability":"experimental",
+    "url":{
+      "paths":[
+        {
+          "path":"/_script_context",
+          "methods":[
+            "GET"
+          ]
+        }
+      ]
+    },
+    "params":{}
+  }
+}

rest-api-spec/src/main/resources/rest-api-spec/test/scripts/20_get_script_context.yml

@@ -0,0 +1,27 @@
+"Action to get all contexts":
+  - skip:
+      version: " - 7.6.0"
+      reason: "get_all_contexts introduced in 7.6.0"
+  - do:
+      get_script_context: {}
+  - match: { contexts.aggregation_selector: {} }
+  - match: { contexts.aggs: {} }
+  - match: { contexts.aggs_combine: {} }
+  - match: { contexts.aggs_init: {} }
+  - match: { contexts.aggs_map: {} }
+  - match: { contexts.aggs_reduce: {} }
+  - match: { contexts.bucket_aggregation: {} }
+  - match: { contexts.field: {} }
+  - match: { contexts.filter: {} }
+  - match: { contexts.ingest: {} }
+  - match: { contexts.interval: {} }
+  - match: { contexts.number_sort: {} }
+  - match: { contexts.processor_conditional: {} }
+  - match: { contexts.score: {} }
+  - match: { contexts.script_heuristic: {} }
+  - match: { contexts.similarity: {} }
+  - match: { contexts.similarity_weight: {} }
+  - match: { contexts.string_sort: {} }
+  - match: { contexts.template: {} }
+  - match: { contexts.terms_set: {} }
+  - match: { contexts.update: {} }

server/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -79,9 +79,11 @@
 import org.elasticsearch.action.admin.cluster.stats.ClusterStatsAction;
 import org.elasticsearch.action.admin.cluster.stats.TransportClusterStatsAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.DeleteStoredScriptAction;
+import org.elasticsearch.action.admin.cluster.storedscripts.GetScriptContextAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.GetStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.PutStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.TransportDeleteStoredScriptAction;
+import org.elasticsearch.action.admin.cluster.storedscripts.TransportGetScriptContextAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.TransportGetStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.TransportPutStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.tasks.PendingClusterTasksAction;
@@ -244,6 +246,7 @@
 import org.elasticsearch.rest.action.admin.cluster.RestDeleteSnapshotAction;
 import org.elasticsearch.rest.action.admin.cluster.RestDeleteStoredScriptAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetRepositoriesAction;
+import org.elasticsearch.rest.action.admin.cluster.RestGetScriptContextAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetSnapshotsAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetStoredScriptAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetTaskAction;
@@ -526,6 +529,7 @@ public <Request extends ActionRequest, Response extends ActionResponse> void reg
         actions.register(PutStoredScriptAction.INSTANCE, TransportPutStoredScriptAction.class);
         actions.register(GetStoredScriptAction.INSTANCE, TransportGetStoredScriptAction.class);
         actions.register(DeleteStoredScriptAction.INSTANCE, TransportDeleteStoredScriptAction.class);
+        actions.register(GetScriptContextAction.INSTANCE, TransportGetScriptContextAction.class);
 
         actions.register(FieldCapabilitiesAction.INSTANCE, TransportFieldCapabilitiesAction.class,
             TransportFieldCapabilitiesIndexAction.class);
@@ -655,6 +659,7 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster) {
         registerHandler.accept(new RestGetStoredScriptAction(restController));
         registerHandler.accept(new RestPutStoredScriptAction(restController));
         registerHandler.accept(new RestDeleteStoredScriptAction(restController));
+        registerHandler.accept(new RestGetScriptContextAction(restController));
 
         registerHandler.accept(new RestFieldCapabilitiesAction(restController));
 

server/src/main/java/org/elasticsearch/action/admin/cluster/storedscripts/GetScriptContextAction.java

@@ -0,0 +1,32 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.storedscripts;
+
+import org.elasticsearch.action.ActionType;
+
+public class GetScriptContextAction extends ActionType<GetScriptContextResponse> {
+
+    public static final GetScriptContextAction INSTANCE = new GetScriptContextAction();
+    public static final String NAME = "cluster:admin/script_context/get";
+
+    private GetScriptContextAction() {
+        super(NAME, GetScriptContextResponse::new);
+    }
+}

server/src/main/java/org/elasticsearch/action/admin/cluster/storedscripts/GetScriptContextRequest.java

@@ -0,0 +1,45 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.action.admin.cluster.storedscripts;
+
+import org.elasticsearch.action.ActionRequest;
+import org.elasticsearch.action.ActionRequestValidationException;
+import org.elasticsearch.common.io.stream.StreamInput;
+
+import java.io.IOException;
+
+public class GetScriptContextRequest extends ActionRequest {
+    public GetScriptContextRequest() {
+        super();
+    }
+
+    GetScriptContextRequest(StreamInput in) throws IOException {
+        super(in);
+    }
+
+    @Override
+    public ActionRequestValidationException validate() {
+        return null;
+    }
+
+    @Override
+    public String toString() {
+        return "get script context";
+    }
+}

server/src/main/java/org/elasticsearch/action/admin/cluster/storedscripts/GetScriptContextResponse.java

@@ -0,0 +1,132 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.storedscripts;
+
+import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.common.ParseField;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.xcontent.ConstructingObjectParser;
+import org.elasticsearch.common.xcontent.StatusToXContentObject;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.rest.RestStatus;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+import static org.elasticsearch.common.xcontent.XContentParser.Token.END_OBJECT;
+import static org.elasticsearch.common.xcontent.XContentParser.Token.START_OBJECT;
+
+public class GetScriptContextResponse extends ActionResponse implements StatusToXContentObject {
+
+    private static final ParseField CONTEXTS = new ParseField("contexts");
+    private final List<String> contextNames;
+
+    @SuppressWarnings("unchecked")
+    public static final ConstructingObjectParser<GetScriptContextResponse,Void> PARSER =
+        new ConstructingObjectParser<>("get_script_context", true,
+            (a) -> {
+                Map<String, Object> contexts = ((List<String>) a[0]).stream().collect(Collectors.toMap(
+                    name -> name, name -> new Object()
+                ));
+                return new GetScriptContextResponse(contexts);
+            }
+        );
+
+    static {
+        PARSER.declareNamedObjects(
+            ConstructingObjectParser.constructorArg(),
+            (p, c, n) ->
+            {
+                // advance empty object
+                assert(p.nextToken() == START_OBJECT);
+                assert(p.nextToken() == END_OBJECT);
+                return n;
+            },
+            CONTEXTS
+        );
+    }
+
+    GetScriptContextResponse(StreamInput in) throws IOException {
+        super(in);
+        int size = in.readInt();
+        ArrayList<String> contextNames = new ArrayList<>(size);
+        for (int i = 0; i < size; i++) {
+            contextNames.add(in.readString());
+        }
+        this.contextNames = Collections.unmodifiableList(contextNames);
+    }
+
+    GetScriptContextResponse(Map<String,Object> contexts) {
+        List<String> contextNames = new ArrayList<>(contexts.keySet());
+        contextNames.sort(String::compareTo);
+        this.contextNames = Collections.unmodifiableList(contextNames);
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        out.writeInt(this.contextNames.size());
+        for (String context: this.contextNames) {
+            out.writeString(context);
+        }
+    }
+
+    @Override
+    public RestStatus status() {
+        return RestStatus.OK;
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject().startObject(CONTEXTS.getPreferredName());
+        for (String contextName: this.contextNames) {
+            builder.startObject(contextName).endObject();
+        }
+        builder.endObject().endObject(); // CONTEXTS
+        return builder;
+    }
+
+    public static GetScriptContextResponse fromXContent(XContentParser parser) throws IOException {
+        return PARSER.apply(parser, null);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        GetScriptContextResponse that = (GetScriptContextResponse) o;
+        return contextNames.equals(that.contextNames);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(contextNames);
+    }
+}

server/src/main/java/org/elasticsearch/action/admin/cluster/storedscripts/TransportGetScriptContextAction.java

@@ -0,0 +1,49 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.elasticsearch.action.admin.cluster.storedscripts;
+
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.support.ActionFilters;
+import org.elasticsearch.action.support.HandledTransportAction;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.script.ScriptService;
+import org.elasticsearch.tasks.Task;
+import org.elasticsearch.transport.TransportService;
+
+import java.util.Map;
+import java.util.stream.Collectors;
+
+public class TransportGetScriptContextAction extends HandledTransportAction<GetScriptContextRequest, GetScriptContextResponse> {
+
+    private final ScriptService scriptService;
+
+    @Inject
+    public TransportGetScriptContextAction(TransportService transportService, ActionFilters actionFilters, ScriptService scriptService) {
+        super(GetScriptContextAction.NAME, transportService, actionFilters, GetScriptContextRequest::new);
+        this.scriptService = scriptService;
+    }
+
+    @Override
+    protected void doExecute(Task task, GetScriptContextRequest request, ActionListener<GetScriptContextResponse> listener) {
+        Map<String,Object> contexts = scriptService.getContextNames().stream().collect(
+            Collectors.toMap(name -> name, name -> new Object())
+        );
+        listener.onResponse(new GetScriptContextResponse(contexts));
+    }
+}