Add initial implementation of YAML config file

This implements the basic framework that allows global YAML
configuration, per-environment custom configuration, and unit-
test-specific configuration.

TESTED=I deployed to alpha, ran some EPP commands through the
nomulus tool, and verified no errors.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=145422680

java/google/registry/config/YamlUtils.java

@@ -0,0 +1,122 @@
+// Copyright 2016 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed 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 google.registry.config;
+
+import static google.registry.config.RegistryEnvironment.UNITTEST;
+import static google.registry.util.FormattingLogger.getLoggerForCallerClass;
+import static google.registry.util.ResourceUtils.readResourceUtf8;
+
+import com.google.common.io.CharStreams;
+import google.registry.util.FormattingLogger;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Map;
+import org.yaml.snakeyaml.Yaml;
+
+/** Utility methods for dealing with YAML. */
+public final class YamlUtils {
+
+  private static final FormattingLogger logger = getLoggerForCallerClass();
+
+  private static final String CUSTOM_CONFIG_PATH = "WEB-INF/nomulus-config.yaml";
+  private static final String YAML_CONFIG_PROD =
+      readResourceUtf8(RegistryConfig.class, "default-config.yaml");
+  private static final String YAML_CONFIG_UNITTEST =
+      readResourceUtf8(RegistryConfig.class, "unittest-config.yaml");
+
+  /**
+   * Loads the {@link RegistryConfigSettings} POJO from the YAML configuration file(s).
+   *
+   * <p>The {@code default-config.yaml} file in this directory is loaded first, and a fatal error is
+   * thrown if it cannot be found or if there is an error parsing it. Separately, the custom config
+   * file located in {@code WEB-INF/nomulus-config.yaml} is also loaded and those values merged into
+   * the POJO. If the custom config file does not exist then an info notice is logged, but if it
+   * does exist and is invalid then a fatal error is thrown.
+   *
+   * <p>Unit tests load the {@code unittest-config.yaml} file for custom config.
+   */
+  static RegistryConfigSettings getConfigSettings() {
+    String yaml = YAML_CONFIG_PROD;
+    if (RegistryEnvironment.get() == UNITTEST) {
+      yaml = mergeYaml(yaml, YAML_CONFIG_UNITTEST);
+    } else {
+      try {
+        // We have to load the file this way because App Engine does not allow loading files in the
+        // WEB-INF directory using a class loader.
+        FileInputStream fin = new FileInputStream(new File(CUSTOM_CONFIG_PATH));
+        String customYaml = CharStreams.toString(new InputStreamReader(fin, "UTF-8"));
+        yaml = mergeYaml(yaml, customYaml);
+      } catch (IOException e) {
+        logger.warningfmt(
+            "There was no custom configuration file to load at %s", CUSTOM_CONFIG_PATH);
+      }
+    }
+    try {
+      return new Yaml().loadAs(yaml, RegistryConfigSettings.class);
+    } catch (Exception e) {
+      throw new IllegalStateException("Fatal error: Custom YAML configuration file is invalid", e);
+    }
+  }
+
+  /**
+   * Recursively merges two YAML documents together.
+   *
+   * <p>Any fields that are specified in customYaml will override fields of the same path in
+   * defaultYaml. Additional fields in customYaml that aren't specified in defaultYaml will be
+   * ignored. The schemas of all fields that are present must be identical, e.g. it is an error to
+   * override a field that has a Map value in the default YAML with a field of any other type in the
+   * custom YAML.
+   *
+   * <p>Only maps are handled recursively; lists are simply overridden in place as-is.
+   */
+  static String mergeYaml(String defaultYaml, String customYaml) {
+    try {
+      Yaml yaml = new Yaml();
+      Map<String, Object> defaultObj = loadAsMap(yaml, defaultYaml);
+      Map<String, Object> customObj = loadAsMap(yaml, customYaml);
+      Object mergedObj = mergeMaps(defaultObj, customObj);
+      logger.infofmt("Successfully loaded custom YAML configuration file.");
+      return yaml.dump(mergedObj);
+    } catch (Exception e) {
+      throw new IllegalStateException("Fatal error: Custom YAML configuration file is invalid", e);
+    }
+  }
+
+  private static Object mergeMaps(Map<String, Object> defaultMap, Map<String, Object> customMap) {
+    for (String key : defaultMap.keySet()) {
+      if (!customMap.containsKey(key)) {
+        continue;
+      }
+      Object defaultValue = defaultMap.get(key);
+      @SuppressWarnings("unchecked")
+      Object newValue =
+          (defaultValue instanceof Map)
+              ? mergeMaps(
+                  (Map<String, Object>) defaultValue, (Map<String, Object>) customMap.get(key))
+              : customMap.get(key);
+      defaultMap.put(key, newValue);
+    }
+    return defaultMap;
+  }
+
+  @SuppressWarnings("unchecked")
+  private static Map<String, Object> loadAsMap(Yaml yaml, String yamlString) {
+    return (Map<String, Object>) yaml.load(yamlString);
+  }
+
+  private YamlUtils() {}
+}