TechDocs/GCS: Enable authentication using environemnt variables

GOOGLE_APPLICATION_CREDENTIALS is the environment variable used by the client. Compute Engine, Google Kubernetes Engine, App Engine, Cloud Run, and Cloud Functions automatically provide a default service account and set this env variable.
https://cloud.google.com/docs/authentication/production

packages/techdocs-common/src/stages/publish/googleStorage.ts

@@ -27,57 +27,60 @@ import { getHeadersForFileExtension, getFileTreeRecursively } from './helpers';
 import { PublisherBase, PublishRequest } from './types';
 
 export class GoogleGCSPublish implements PublisherBase {
-  static fromConfig(config: Config, logger: Logger): PublisherBase {
-    let credentials = '';
-    let projectId = '';
+  static async fromConfig(
+    config: Config,
+    logger: Logger,
+  ): Promise<PublisherBase> {
     let bucketName = '';
     try {
-      credentials = config.getString(
-        'techdocs.publisher.googleGcs.credentials',
-      );
-      projectId = config.getString('techdocs.publisher.googleGcs.projectId');
       bucketName = config.getString('techdocs.publisher.googleGcs.bucketName');
     } catch (error) {
       throw new Error(
         "Since techdocs.publisher.type is set to 'googleGcs' in your app config, " +
-          'credentials, projectId and bucketName are required in techdocs.publisher.googleGcs ' +
-          'required to authenticate with Google Cloud Storage.',
+          'techdocs.publisher.googleGcs.bucketName is required.',
       );
     }
 
+    // Credentials is an optional config. If missing, default GCS environment variables will be used.
+    // Read more here https://cloud.google.com/docs/authentication/production
+    const credentials = config.getOptionalString(
+      'techdocs.publisher.googleGcs.credentials',
+    );
     let credentialsJson = {};
-    try {
-      credentialsJson = JSON.parse(credentials);
-    } catch (err) {
-      throw new Error(
-        'Error in parsing techdocs.publisher.googleGcs.credentials config to JSON.',
-      );
+    if (credentials) {
+      try {
+        credentialsJson = JSON.parse(credentials);
+      } catch (err) {
+        throw new Error(
+          'Error in parsing techdocs.publisher.googleGcs.credentials config to JSON.',
+        );
+      }
     }
+    const projectId = config.getOptionalString(
+      'techdocs.publisher.googleGcs.projectId',
+    );
 
     const storageClient = new Storage({
-      credentials: credentialsJson,
-      projectId: projectId,
+      ...(credentials && {
+        credentials: credentialsJson,
+      }),
+      ...(projectId && { projectId }),
     });
 
     // Check if the defined bucket exists. Being able to connect means the configuration is good
     // and the storage client will work.
-    storageClient
-      .bucket(bucketName)
-      .getMetadata()
-      .then(() => {
-        logger.info(
-          `Successfully connected to the GCS bucket ${bucketName} in the GCP project ${projectId}.`,
-        );
-      })
-      .catch(reason => {
-        logger.error(
-          `Could not retrieve metadata about the GCS bucket ${bucketName} in the GCP project ${projectId}. ` +
-            'Make sure the GCP project and the bucket exists and the access key located at the path ' +
-            "techdocs.publisher.googleGcs.credentials defined in app config has the role 'Storage Object Creator'. " +
-            'Refer to https://backstage.io/docs/features/techdocs/using-cloud-storage',
-        );
-        throw new Error(`from GCS client library: ${reason.message}`);
-      });
+    try {
+      await storageClient.bucket(bucketName).getMetadata();
+      logger.info(`Successfully connected to the GCS bucket ${bucketName}.`);
+    } catch (err) {
+      logger.error(
+        `Could not retrieve metadata about the GCS bucket ${bucketName}. ` +
+          'Make sure the bucket exists. Also make sure that authentication is setup either by explicitly defining ' +
+          'techdocs.publisher.googleGcs.credentials in app config or by using environment variables' +
+          'Refer to https://backstage.io/docs/features/techdocs/using-cloud-storage',
+      );
+      throw new Error(err.message);
+    }
 
     return new GoogleGCSPublish(storageClient, bucketName, logger);
   }

packages/techdocs-common/src/stages/publish/publish.ts

@@ -43,7 +43,7 @@ export class Publisher {
     switch (publisherType) {
       case 'googleGcs':
         logger.info('Creating Google Storage Bucket publisher for TechDocs');
-        return GoogleGCSPublish.fromConfig(config, logger);
+        return await GoogleGCSPublish.fromConfig(config, logger);
       case 'awsS3':
         logger.info('Creating AWS S3 Bucket publisher for TechDocs');
         return AwsS3Publish.fromConfig(config, logger);