Scripting: add available languages & contexts API (#49652)

Adds `GET /_script_language` to support Kibana dynamic scripting
language selection.

Response contains whether `inline` and/or `stored` scripts are
enabled as determined by the `script.allowed_types` settings.

For each scripting language registered, such as `painless`,
`expression`, `mustache` or custom, available contexts for the language
are included as determined by the `script.allowed_contexts` setting.

Response format:
```
{
  "types_allowed": [
    "inline",
    "stored"
  ],
  "language_contexts": [
    {
      "language": "expression",
      "contexts": [
        "aggregation_selector",
        "aggs"
        ...
      ]
    },
    {
      "language": "painless",
      "contexts": [
        "aggregation_selector",
        "aggs",
        "aggs_combine",
        ...
      ]
    }
...
  ]
}
```

Fixes: #49463

Author: stu-elastic

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

@@ -765,6 +765,7 @@ public void testApiNamingConventions() throws Exception {
             "cluster.remote_info",
             "create",
             "get_script_context",
+            "get_script_languages",
             "get_source",
             "indices.exists_type",
             "indices.get_upgrade",

modules/lang-expression/src/main/java/org/elasticsearch/script/expression/ExpressionScriptEngine.java

@@ -55,6 +55,8 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
 
 /**
  * Provides the infrastructure for Lucene expressions as a scripting language for Elasticsearch.
@@ -65,6 +67,41 @@ public class ExpressionScriptEngine implements ScriptEngine {
 
     public static final String NAME = "expression";
 
+    private static Map<ScriptContext<?>, Function<Expression,Object>> contexts = Map.of(
+        BucketAggregationScript.CONTEXT,
+            ExpressionScriptEngine::newBucketAggregationScriptFactory,
+
+        BucketAggregationSelectorScript.CONTEXT,
+            (Expression expr) -> {
+                BucketAggregationScript.Factory factory = newBucketAggregationScriptFactory(expr);
+                BucketAggregationSelectorScript.Factory wrappedFactory = parameters -> new BucketAggregationSelectorScript(parameters) {
+                    @Override
+                    public boolean execute() {
+                        return factory.newInstance(getParams()).execute().doubleValue() == 1.0;
+                    }
+                };
+                return wrappedFactory;
+            },
+
+        FilterScript.CONTEXT,
+            (Expression expr) -> (FilterScript.Factory) (p, lookup) -> newFilterScript(expr, lookup, p),
+
+        ScoreScript.CONTEXT,
+            (Expression expr) -> (ScoreScript.Factory) (p, lookup) -> newScoreScript(expr, lookup, p),
+
+        TermsSetQueryScript.CONTEXT,
+            (Expression expr) -> (TermsSetQueryScript.Factory) (p, lookup) -> newTermsSetQueryScript(expr, lookup, p),
+
+        AggregationScript.CONTEXT,
+            (Expression expr) -> (AggregationScript.Factory) (p, lookup) -> newAggregationScript(expr, lookup, p),
+
+        NumberSortScript.CONTEXT,
+            (Expression expr) -> (NumberSortScript.Factory) (p, lookup) -> newSortScript(expr, lookup, p),
+
+        FieldScript.CONTEXT,
+            (Expression expr) -> (FieldScript.Factory) (p, lookup) -> newFieldScript(expr, lookup, p)
+    );
+
     @Override
     public String getType() {
         return NAME;
@@ -102,37 +139,15 @@ protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundE
                 }
             }
         });
-        if (context.instanceClazz.equals(BucketAggregationScript.class)) {
-            return context.factoryClazz.cast(newBucketAggregationScriptFactory(expr));
-        } else if (context.instanceClazz.equals(BucketAggregationSelectorScript.class)) {
-            BucketAggregationScript.Factory factory = newBucketAggregationScriptFactory(expr);
-            BucketAggregationSelectorScript.Factory wrappedFactory = parameters -> new BucketAggregationSelectorScript(parameters) {
-                @Override
-                public boolean execute() {
-                    return factory.newInstance(getParams()).execute().doubleValue() == 1.0;
-                }
-            };
-            return context.factoryClazz.cast(wrappedFactory);
-        } else if (context.instanceClazz.equals(FilterScript.class)) {
-            FilterScript.Factory factory = (p, lookup) -> newFilterScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
-        } else if (context.instanceClazz.equals(ScoreScript.class)) {
-            ScoreScript.Factory factory = (p, lookup) -> newScoreScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
-        } else if (context.instanceClazz.equals(TermsSetQueryScript.class)) {
-            TermsSetQueryScript.Factory factory = (p, lookup) -> newTermsSetQueryScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
-        } else if (context.instanceClazz.equals(AggregationScript.class)) {
-            AggregationScript.Factory factory = (p, lookup) -> newAggregationScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
-        } else if (context.instanceClazz.equals(NumberSortScript.class)) {
-            NumberSortScript.Factory factory = (p, lookup) -> newSortScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
-        } else if (context.instanceClazz.equals(FieldScript.class)) {
-            FieldScript.Factory factory = (p, lookup) -> newFieldScript(expr, lookup, p);
-            return context.factoryClazz.cast(factory);
+        if (contexts.containsKey(context) == false) {
+            throw new IllegalArgumentException("expression engine does not know how to handle script context [" + context.name + "]");
         }
-        throw new IllegalArgumentException("expression engine does not know how to handle script context [" + context.name + "]");
+        return context.factoryClazz.cast(contexts.get(context).apply(expr));
+    }
+
+    @Override
+    public Set<ScriptContext<?>> getSupportedContexts() {
+        return contexts.keySet();
     }
 
     private static BucketAggregationScript.Factory newBucketAggregationScriptFactory(Expression expr) {
@@ -166,7 +181,7 @@ public Double execute() {
         };
     }
 
-    private NumberSortScript.LeafFactory newSortScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
+    private static NumberSortScript.LeafFactory newSortScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
         // NOTE: if we need to do anything complicated with bindings in the future, we can just extend Bindings,
         // instead of complicating SimpleBindings (which should stay simple)
         SimpleBindings bindings = new SimpleBindings();
@@ -193,7 +208,7 @@ private NumberSortScript.LeafFactory newSortScript(Expression expr, SearchLookup
         return new ExpressionNumberSortScript(expr, bindings, needsScores);
     }
 
-    private TermsSetQueryScript.LeafFactory newTermsSetQueryScript(Expression expr, SearchLookup lookup,
+    private static TermsSetQueryScript.LeafFactory newTermsSetQueryScript(Expression expr, SearchLookup lookup,
         @Nullable Map<String, Object> vars) {
         // NOTE: if we need to do anything complicated with bindings in the future, we can just extend Bindings,
         // instead of complicating SimpleBindings (which should stay simple)
@@ -216,7 +231,7 @@ private TermsSetQueryScript.LeafFactory newTermsSetQueryScript(Expression expr,
         return new ExpressionTermSetQueryScript(expr, bindings);
     }
 
-    private AggregationScript.LeafFactory newAggregationScript(Expression expr, SearchLookup lookup,
+    private static AggregationScript.LeafFactory newAggregationScript(Expression expr, SearchLookup lookup,
         @Nullable Map<String, Object> vars) {
         // NOTE: if we need to do anything complicated with bindings in the future, we can just extend Bindings,
         // instead of complicating SimpleBindings (which should stay simple)
@@ -252,7 +267,7 @@ private AggregationScript.LeafFactory newAggregationScript(Expression expr, Sear
         return new ExpressionAggregationScript(expr, bindings, needsScores, specialValue);
     }
 
-    private FieldScript.LeafFactory newFieldScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
+    private static FieldScript.LeafFactory newFieldScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
         SimpleBindings bindings = new SimpleBindings();
         for (String variable : expr.variables) {
             try {
@@ -273,7 +288,7 @@ private FieldScript.LeafFactory newFieldScript(Expression expr, SearchLookup loo
      * This is a hack for filter scripts, which must return booleans instead of doubles as expression do.
      * See https://github.com/elastic/elasticsearch/issues/26429.
      */
-    private FilterScript.LeafFactory newFilterScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
+    private static FilterScript.LeafFactory newFilterScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
         ScoreScript.LeafFactory searchLeafFactory = newScoreScript(expr, lookup, vars);
         return ctx -> {
             ScoreScript script = searchLeafFactory.newInstance(ctx);
@@ -290,7 +305,7 @@ public void setDocument(int docid) {
         };
     }
 
-    private ScoreScript.LeafFactory newScoreScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
+    private static ScoreScript.LeafFactory newScoreScript(Expression expr, SearchLookup lookup, @Nullable Map<String, Object> vars) {
         // NOTE: if we need to do anything complicated with bindings in the future, we can just extend Bindings,
         // instead of complicating SimpleBindings (which should stay simple)
         SimpleBindings bindings = new SimpleBindings();
@@ -327,7 +342,7 @@ private ScoreScript.LeafFactory newScoreScript(Expression expr, SearchLookup loo
     /**
      * converts a ParseException at compile-time or link-time to a ScriptException
      */
-    private ScriptException convertToScriptException(String message, String source, String portion, Throwable cause) {
+    private static ScriptException convertToScriptException(String message, String source, String portion, Throwable cause) {
         List<String> stack = new ArrayList<>();
         stack.add(portion);
         StringBuilder pointer = new StringBuilder();

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/MustacheScriptEngine.java

@@ -41,6 +41,7 @@
 import java.security.PrivilegedAction;
 import java.util.Collections;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * Main entry point handling template registration, compilation and
@@ -79,6 +80,11 @@ public <T> T compile(String templateName, String templateSource, ScriptContext<T
 
     }
 
+    @Override
+    public Set<ScriptContext<?>> getSupportedContexts() {
+        return Set.of(TemplateScript.CONTEXT);
+    }
+
     private CustomMustacheFactory createMustacheFactory(Map<String, String> options) {
         if (options == null || options.isEmpty() || options.containsKey(Script.CONTENT_TYPE_OPTION) == false) {
             return new CustomMustacheFactory();

modules/lang-painless/src/main/java/org/elasticsearch/painless/PainlessScriptEngine.java

@@ -146,6 +146,11 @@ public Loader run() {
         }
     }
 
+    @Override
+    public Set<ScriptContext<?>> getSupportedContexts() {
+        return contextsToCompilers.keySet();
+    }
+
     /**
      * Generates a stateful factory class that will return script instances.  Acts as a middle man between
      * the {@link ScriptContext#factoryClazz} and the {@link ScriptContext#instanceClazz} when used so that

plugins/examples/script-expert-scoring/src/main/java/org/elasticsearch/example/expertscript/ExpertScriptPlugin.java

@@ -35,6 +35,7 @@
 import java.io.UncheckedIOException;
 import java.util.Collection;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * An example script plugin that adds a {@link ScriptEngine} implementing expert scoring.
@@ -76,6 +77,11 @@ public void close() {
             // optionally close resources
         }
 
+        @Override
+        public Set<ScriptContext<?>> getSupportedContexts() {
+            return Set.of(ScoreScript.CONTEXT);
+        }
+
         private static class PureDfLeafFactory implements LeafFactory {
             private final Map<String, Object> params;
             private final SearchLookup lookup;

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

@@ -0,0 +1,19 @@
+{
+  "get_script_languages":{
+    "documentation":{
+      "description":"Returns available script types, languages and contexts"
+    },
+    "stability":"experimental",
+    "url":{
+      "paths":[
+        {
+          "path":"/_script_language",
+          "methods":[
+            "GET"
+          ]
+        }
+      ]
+    },
+    "params":{}
+  }
+}

rest-api-spec/src/main/resources/rest-api-spec/test/scripts/25_get_script_languages.yml

@@ -0,0 +1,9 @@
+"Action to get script languages":
+  - skip:
+      version: " - 7.6.0"
+      reason: "get_script_languages introduced in 7.6.0"
+  - do:
+      get_script_languages: {}
+
+  - match: { types_allowed.0: "inline" }
+  - match: { types_allowed.1: "stored" }

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

@@ -80,10 +80,12 @@
 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.GetScriptLanguageAction;
 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.TransportGetScriptLanguageAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.TransportGetStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.storedscripts.TransportPutStoredScriptAction;
 import org.elasticsearch.action.admin.cluster.tasks.PendingClusterTasksAction;
@@ -249,6 +251,7 @@
 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.RestGetScriptLanguageAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetSnapshotsAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetStoredScriptAction;
 import org.elasticsearch.rest.action.admin.cluster.RestGetTaskAction;
@@ -522,6 +525,7 @@ public <Request extends ActionRequest, Response extends ActionResponse> void reg
         actions.register(GetStoredScriptAction.INSTANCE, TransportGetStoredScriptAction.class);
         actions.register(DeleteStoredScriptAction.INSTANCE, TransportDeleteStoredScriptAction.class);
         actions.register(GetScriptContextAction.INSTANCE, TransportGetScriptContextAction.class);
+        actions.register(GetScriptLanguageAction.INSTANCE, TransportGetScriptLanguageAction.class);
 
         actions.register(FieldCapabilitiesAction.INSTANCE, TransportFieldCapabilitiesAction.class);
         actions.register(TransportFieldCapabilitiesIndexAction.TYPE, TransportFieldCapabilitiesIndexAction.class);
@@ -662,6 +666,7 @@ public void initRestHandlers(Supplier<DiscoveryNodes> nodesInCluster) {
         registerHandler.accept(new RestPutStoredScriptAction(restController));
         registerHandler.accept(new RestDeleteStoredScriptAction(restController));
         registerHandler.accept(new RestGetScriptContextAction(restController));
+        registerHandler.accept(new RestGetScriptLanguageAction(restController));
 
         registerHandler.accept(new RestFieldCapabilitiesAction(restController));
 

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

@@ -0,0 +1,31 @@
+/*
+ * 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 GetScriptLanguageAction extends ActionType<GetScriptLanguageResponse> {
+    public static final GetScriptLanguageAction INSTANCE = new GetScriptLanguageAction();
+    public static final String NAME = "cluster:admin/script_language/get";
+
+    private GetScriptLanguageAction() {
+        super(NAME, GetScriptLanguageResponse::new);
+    }
+}

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

@@ -0,0 +1,42 @@
+/*
+ * 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 GetScriptLanguageRequest extends ActionRequest {
+    public GetScriptLanguageRequest() {
+        super();
+    }
+
+    GetScriptLanguageRequest(StreamInput in) throws IOException {
+        super(in);
+    }
+
+    @Override
+    public ActionRequestValidationException validate() { return null; }
+
+    @Override
+    public String toString() { return "get script languages"; }
+}