Mustache: Add util functions to render JSON and join array values

This pull request adds two util functions to the Mustache templating engine:
- {{#toJson}}my_map{{/toJson}} to render a Map parameter as a JSON string
- {{#join}}my_iterable{{/join}} to render any iterable (including arrays) as a comma separated list of values like `1, 2, 3`. It's also possible de change the default delimiter (comma) to something else.

closes #18970, #19119

Author: tlrx

core/src/main/java/org/elasticsearch/script/mustache/CustomMustacheFactory.java

@@ -0,0 +1,289 @@
+/*
+ * 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.script.mustache;
+
+import com.fasterxml.jackson.core.io.JsonStringEncoder;
+import com.github.mustachejava.Code;
+import com.github.mustachejava.DefaultMustacheFactory;
+import com.github.mustachejava.DefaultMustacheVisitor;
+import com.github.mustachejava.Mustache;
+import com.github.mustachejava.MustacheException;
+import com.github.mustachejava.MustacheVisitor;
+import com.github.mustachejava.TemplateContext;
+import com.github.mustachejava.codes.IterableCode;
+import com.github.mustachejava.codes.WriteCode;
+import com.google.common.base.Function;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentType;
+
+import java.io.IOException;
+import java.io.StringWriter;
+import java.io.Writer;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Objects;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class CustomMustacheFactory extends DefaultMustacheFactory {
+
+    private final Encoder encoder;
+
+    public CustomMustacheFactory(boolean escaping) {
+        super();
+        setObjectHandler(new CustomReflectionObjectHandler());
+        if (escaping) {
+            this.encoder = new JsonEscapeEncoder();
+        } else {
+            this.encoder = new NoEscapeEncoder();
+        }
+    }
+
+    @Override
+    public void encode(String value, Writer writer) {
+        encoder.accept(value, writer);
+    }
+
+    @Override
+    public MustacheVisitor createMustacheVisitor() {
+        return new CustomMustacheVisitor(this);
+    }
+
+    class CustomMustacheVisitor extends DefaultMustacheVisitor {
+
+        public CustomMustacheVisitor(DefaultMustacheFactory df) {
+            super(df);
+        }
+
+        @Override
+        public void iterable(TemplateContext templateContext, String variable, Mustache mustache) {
+            if (ToJsonCode.match(variable)) {
+                list.add(new ToJsonCode(templateContext, df, mustache, variable));
+            } else if (JoinerCode.match(variable)) {
+                list.add(new JoinerCode(templateContext, df, mustache));
+            } else if (CustomJoinerCode.match(variable)) {
+                list.add(new CustomJoinerCode(templateContext, df, mustache, variable));
+            } else {
+                list.add(new IterableCode(templateContext, df, mustache, variable));
+            }
+        }
+    }
+
+    /**
+     * Base class for custom Mustache functions
+     */
+    static abstract class CustomCode extends IterableCode {
+
+        private final String code;
+
+        public CustomCode(TemplateContext tc, DefaultMustacheFactory df, Mustache mustache, String code) {
+            super(tc, df, mustache, extractVariableName(code, mustache, tc));
+            this.code = Objects.requireNonNull(code);
+        }
+
+        @Override
+        public Writer execute(Writer writer, Object[] scopes) {
+            Object resolved = get(scopes);
+            writer = handle(writer, createFunction(resolved), scopes);
+            appendText(writer);
+            return writer;
+        }
+
+        @Override
+        protected void tag(Writer writer, String tag) throws IOException {
+            writer.write(tc.startChars());
+            writer.write(tag);
+            writer.write(code);
+            writer.write(tc.endChars());
+        }
+
+        protected abstract Function createFunction(Object resolved);
+
+        /**
+         * At compile time, this function extracts the name of the variable:
+         * {{#toJson}}variable_name{{/toJson}}
+         */
+        protected static String extractVariableName(String fn, Mustache mustache, TemplateContext tc) {
+            Code[] codes = mustache.getCodes();
+            if (codes == null || codes.length != 1) {
+                throw new MustacheException("Mustache function [" + fn + "] must contain one and only one identifier");
+            }
+
+            try (StringWriter capture = new StringWriter()) {
+                // Variable name is in plain text and has type WriteCode
+                if (codes[0] instanceof WriteCode) {
+                    codes[0].execute(capture, Collections.emptyList());
+                    return capture.toString();
+                } else {
+                    codes[0].identity(capture);
+                    return capture.toString();
+                }
+            } catch (IOException e) {
+                throw new MustacheException("Exception while parsing mustache function [" + fn + "] at line " + tc.line(), e);
+            }
+        }
+    }
+
+    /**
+     * This function renders {@link Iterable} and {@link Map} as their JSON representation
+     */
+    static class ToJsonCode extends CustomCode {
+
+        private static final String CODE = "toJson";
+
+        public ToJsonCode(TemplateContext tc, DefaultMustacheFactory df, Mustache mustache, String variable) {
+            super(tc, df, mustache, CODE);
+            if (CODE.equalsIgnoreCase(variable) == false) {
+                throw new MustacheException("Mismatch function code [" + CODE + "] cannot be applied to [" + variable + "]");
+            }
+        }
+
+        @Override
+        @SuppressWarnings("unchecked")
+        protected Function<String, String>  createFunction(final Object resolved) {
+            return new Function() {
+                @Override
+                public String apply(Object s) {
+                    if (resolved == null) {
+                        return null;
+                    }
+                    try (XContentBuilder builder = XContentBuilder.builder(XContentType.JSON.xContent())) {
+                        if (resolved instanceof Iterable) {
+                            builder.startArray();
+                            for (Object o : (Iterable) resolved) {
+                                builder.value(o);
+                            }
+                            builder.endArray();
+                        } else if (resolved instanceof Map) {
+                            builder.map((Map<String, ?>) resolved);
+                        } else {
+                            // Do not handle as JSON
+                            return oh.stringify(resolved);
+                        }
+                        return builder.string();
+                    } catch (IOException e) {
+                        throw new MustacheException("Failed to convert object to JSON", e);
+                    }
+                }
+            };
+        }
+        static boolean match(String variable) {
+            return CODE.equalsIgnoreCase(variable);
+        }
+    }
+
+    /**
+     * This function concatenates the values of an {@link Iterable} using a given delimiter
+     */
+    static class JoinerCode extends CustomCode {
+
+        protected static final String CODE = "join";
+        private static final String DEFAULT_DELIMITER = ",";
+
+        private final String delimiter;
+
+        public JoinerCode(TemplateContext tc, DefaultMustacheFactory df, Mustache mustache, String delimiter) {
+            super(tc, df, mustache, CODE);
+            this.delimiter = delimiter;
+        }
+
+        public JoinerCode(TemplateContext tc, DefaultMustacheFactory df, Mustache mustache) {
+            this(tc, df, mustache, DEFAULT_DELIMITER);
+        }
+
+        @Override
+        @SuppressWarnings("unchecked")
+        protected Function<String, String> createFunction(final Object resolved) {
+            return new Function() {
+                @Override
+                public Object apply(Object s) {
+                    if (s == null) {
+                        return null;
+                    } else if (resolved instanceof Iterable) {
+                        StringBuilder joiner = new StringBuilder();
+                        Iterator it = ((Iterable) resolved).iterator();
+                        while (it.hasNext()) {
+                            joiner.append(oh.stringify(it.next()));
+                            if (it.hasNext()) {
+                                joiner.append(delimiter);
+                            }
+                        }
+                        return joiner.toString();
+                    }
+                    return s;
+                }
+            };
+        }
+
+        static boolean match(String variable) {
+            return CODE.equalsIgnoreCase(variable);
+        }
+    }
+
+    static class CustomJoinerCode extends JoinerCode {
+
+        private static final Pattern PATTERN = Pattern.compile("^(?:" + CODE + " delimiter='(.*)')$");
+
+        public CustomJoinerCode(TemplateContext tc, DefaultMustacheFactory df, Mustache mustache, String variable) {
+            super(tc, df, mustache, extractDelimiter(variable));
+        }
+
+        private static String extractDelimiter(String variable) {
+            Matcher matcher = PATTERN.matcher(variable);
+            if (matcher.find()) {
+                return matcher.group(1);
+            }
+            throw new MustacheException("Failed to extract delimiter for join function");
+        }
+
+        static boolean match(String variable) {
+            return PATTERN.matcher(variable).matches();
+        }
+    }
+
+    class NoEscapeEncoder implements Encoder {
+
+        @Override
+        public void accept(String s, Writer writer) {
+            try {
+                writer.write(s);
+            } catch (IOException e) {
+                throw new MustacheException("Failed to encode value: " + s);
+            }
+        }
+    }
+
+    class JsonEscapeEncoder implements Encoder {
+
+        @Override
+        public void accept(String s, Writer writer) {
+            try {
+                writer.write(JsonStringEncoder.getInstance().quoteAsString(s));
+            } catch (IOException e) {
+                throw new MustacheException("Failed to escape and encode value: " + s);
+            }
+        }
+    }
+
+    interface Encoder {
+        void accept(String s, Writer writer);
+    }
+}

core/src/main/java/org/elasticsearch/script/mustache/JsonEscapingMustacheFactory.java

@@ -18,11 +18,8 @@
  */
 package org.elasticsearch.script.mustache;
 
-import java.io.Reader;
-import java.lang.ref.SoftReference;
-import java.util.Collections;
-import java.util.Map;
-
+import com.github.mustachejava.Mustache;
+import com.github.mustachejava.MustacheFactory;
 import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.component.AbstractComponent;
 import org.elasticsearch.common.inject.Inject;
@@ -37,8 +34,10 @@
 import org.elasticsearch.script.SearchScript;
 import org.elasticsearch.search.lookup.SearchLookup;
 
-import com.github.mustachejava.Mustache;
-import com.github.mustachejava.DefaultMustacheFactory;
+import java.io.Reader;
+import java.lang.ref.SoftReference;
+import java.util.Collections;
+import java.util.Map;
 
 /**
  * Main entry point handling template registration, compilation and
@@ -88,25 +87,14 @@ public MustacheScriptEngineService(Settings settings) {
      * */
     @Override
     public Object compile(String template, Map<String, String> params) {
-        String contentType = params.get(CONTENT_TYPE_PARAM);
-        if (contentType == null) {
-            contentType = JSON_CONTENT_TYPE;
-        }
-
-        final DefaultMustacheFactory mustacheFactory;
-        switch (contentType){
-            case PLAIN_TEXT_CONTENT_TYPE:
-                mustacheFactory = new NoneEscapingMustacheFactory();
-                break;
-            case JSON_CONTENT_TYPE:
-            default:
-                // assume that the default is json encoding:
-                mustacheFactory = new JsonEscapingMustacheFactory();
-                break;
-        }
-        mustacheFactory.setObjectHandler(new CustomReflectionObjectHandler());
+        final MustacheFactory factory = new CustomMustacheFactory(isJsonEscapingEnabled(params));
         Reader reader = new FastStringReader(template);
-        return mustacheFactory.compile(reader, "query-template");
+        return factory.compile(reader, "query-template");
+    }
+
+    private boolean isJsonEscapingEnabled(Map<String, String> params) {
+        String contentType = params.get(CONTENT_TYPE_PARAM);
+        return contentType == null || JSON_CONTENT_TYPE.equals(contentType);
     }
 
     @Override