chore: refactor retrieving resource metadata (#716)

host logic for getting metadata values for monitored resource in MetadataLoader
refactor MonitoredResourceUitl to use MetadataLoader

Author: minherz

google-cloud-logging/src/main/java/com/google/cloud/logging/MetadataLoader.java

@@ -0,0 +1,204 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * 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 com.google.cloud.logging;
+
+import com.google.cloud.logging.MonitoredResourceUtil.Label;
+import com.google.common.collect.ImmutableMap;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.util.function.Supplier;
+
+public final class MetadataLoader {
+  public static final String ENV_FLEXIBLE = "flex";
+  public static final String ENV_STANDARD = "standard";
+
+  private ResourceTypeEnvironmentGetter getter;
+
+  private final ImmutableMap<Label, Supplier<String>> labelResolvers =
+      ImmutableMap.<Label, Supplier<String>>builder()
+          .put(Label.ClusterName, () -> getClusterName())
+          .put(Label.ConfigurationName, () -> getConfigName())
+          .put(Label.ContainerName, () -> getContainerName())
+          .put(Label.Env, () -> getEnv())
+          .put(Label.FunctionName, () -> getFunctionName())
+          .put(Label.InstanceId, () -> getInstanceId())
+          .put(Label.InstanceName, () -> getInstanceName())
+          .put(Label.CloudRunLocation, () -> getCloudRunLocation())
+          .put(Label.GKELocation, () -> getGKELocation())
+          .put(Label.ModuleId, () -> getModuleId())
+          .put(Label.NamespaceName, () -> getNamespaceName())
+          .put(Label.PodName, () -> getPodName())
+          .put(Label.ProjectId, () -> getProjectId())
+          .put(Label.Region, () -> getRegion())
+          .put(Label.RevisionName, () -> getRevisionName())
+          .put(Label.ServiceName, () -> getServiceName())
+          .put(Label.VersionId, () -> getVersionId())
+          .put(Label.Zone, () -> getZone())
+          .build();
+
+  public MetadataLoader(ResourceTypeEnvironmentGetter getter) {
+    this.getter = getter;
+  }
+
+  /**
+   * Loads metadata value for the {@link label} argument.
+   *
+   * @param label A resource metadata label of type {@see MonitoredResourceUtil.Label}
+   * @return A string with metadata value or {@code null} if the label is not supported.
+   */
+  public String getValue(MonitoredResourceUtil.Label label) {
+    Supplier<String> lambda = labelResolvers.get(label);
+    if (lambda != null) {
+      return lambda.get();
+    }
+    return null;
+  }
+
+  private String getClusterName() {
+    return getter.getAttribute("instance/attributes/cluster-name");
+  }
+
+  private String getConfigName() {
+    return getter.getEnv("K_CONFIGURATION");
+  }
+
+  // due to lack of options to discover container name from within process
+  // allow users to provide the container name via environment variable
+  private String getContainerName() {
+    return getter.getEnv("CONTAINER_NAME");
+  }
+  /**
+   * Distinguish between Standard and Flexible GAE environments. There is no indicator of the
+   * environment. The path to the startup-script in the metadata attribute was selected as one of
+   * the new values that explitly mentioning "flex" and cannot be altered by user (e.g. environment
+   * variable). The method assumes that the resource type is already identified as {@link
+   * Resource.AppEngine}.
+   *
+   * @return {@link MetadataLoader.ENV_FLEXIBLE} for the Flexible environment and {@link
+   *     MetadataLoader.ENV_STANDARD} for the Standard environment.
+   */
+  private String getEnv() {
+    String value = getter.getAttribute("instance/attributes/startup-script");
+    if (value == "/var/lib/flex/startup_script.sh") {
+      return ENV_FLEXIBLE;
+    }
+    return ENV_STANDARD;
+  }
+
+  private String getFunctionName() {
+    String value = getter.getEnv("K_SERVICE");
+    if (value == null) {
+      // keep supporting custom function name if is not provided by default
+      // for backward compatability only; reconsider removing it after Gen2
+      // environment is enrolled for Cloud Function
+      value = getter.getEnv("FUNCTION_NAME");
+    }
+    return value;
+  }
+
+  private String getInstanceId() {
+    return getter.getAttribute("instance/id");
+  }
+
+  private String getInstanceName() {
+    return getter.getAttribute("instance/name");
+  }
+
+  private String getCloudRunLocation() {
+    return getRegion();
+  }
+
+  private String getGKELocation() {
+    return getZone();
+  }
+
+  private String getModuleId() {
+    return getter.getEnv("GAE_SERVICE");
+  }
+  /**
+   * Heuristic to discover the namespace name of the current environment. There is no determenistic
+   * way to discover the namespace name of the process. The name is read from the {@link
+   * K8S_POD_NAMESPACE_PATH} when available or read from a user defined environment variable
+   * "NAMESPACE_NAME"
+   *
+   * @return Namespace name or empty string if the name could not be discovered
+   */
+  private String getNamespaceName() {
+    String value = "";
+    try {
+      value =
+          new String(
+              Files.readAllBytes(
+                  Paths.get("/var/run/secrets/kubernetes.io/serviceaccount/namespace")),
+              StandardCharsets.UTF_8);
+    } catch (IOException e) {
+      // if SA token is not shared the info about namespace is unavailable
+      // allow users to define the namespace name explicitly
+      value = getter.getEnv("NAMESPACE_NAME");
+      if (value == null) {
+        value = "";
+      }
+    }
+    return value;
+  }
+  // Kubernetes set hostname of the pod to the pod name by default, however hostname can be override
+  // in manifest
+  // allow users to provide the container name via environment variable
+  private String getPodName() {
+    String value = getter.getEnv("POD_NAME");
+    if (value != null) {
+      return value;
+    }
+    return getter.getEnv("HOSTNAME");
+  }
+
+  private String getProjectId() {
+    return getter.getAttribute("project/project-id");
+  }
+  /**
+   * Retrieves a region from the qualified region of 'projects/[PROJECT_NUMBER]/regions/[REGION]'
+   *
+   * @return region string id
+   */
+  private String getRegion() {
+    String loc = getter.getAttribute("instance/region");
+    return loc.substring(loc.lastIndexOf('/') + 1);
+  }
+
+  private String getRevisionName() {
+    return getter.getEnv("K_REVISION");
+  }
+
+  private String getServiceName() {
+    return getter.getEnv("K_SERVICE");
+  }
+
+  private String getVersionId() {
+    return getter.getEnv("GAE_VERSION");
+  }
+  /**
+   * Retrieves a zone from the qualified zone of 'projects/[PROJECT_NUMBER]/zones/[ZONE]'
+   *
+   * @return zone string id
+   */
+  private String getZone() {
+    String loc = getter.getAttribute("instance/zone");
+    return loc.substring(loc.lastIndexOf('/') + 1);
+  }
+}