feat: Introduce a getActiveRevision method

This loads the head object and then calls `listAllObjects` with the new
`until` parameter and stops as soon as it has found an object which
matches the `ETag` of the active revision.

It returns the active revision in the same format that `fetchRevisions`
would - for backward compatibility with use-cases which were depending
on that.

Author: Kelvin Luck

lib/s3.js

@@ -152,6 +152,31 @@ module.exports = CoreObject.extend({
       .then((response) => response.Contents.find((element) => element.Key === revisionPrefix));
   },
 
+  getActiveRevision(options) {
+    var client         = this._client;
+    var bucket         = options.bucket;
+    var prefix         = options.prefix;
+    var revisionPrefix = joinUriSegments(prefix, options.filePattern + ":");
+    var indexKey       = joinUriSegments(prefix, options.filePattern);
+
+    return headObject(client, { Bucket: bucket, Key: indexKey })
+      .then((current) => this.listAllObjects(
+          { Bucket: bucket, Prefix: revisionPrefix }, 
+          (element) => element.ETag === current.ETag
+        )
+        .then((elements) => {
+          let activeRevision = elements.find((element) => element.ETag === current.ETag);
+          if (!activeRevision) {
+            return;
+          }
+          
+          var revision = activeRevision.Key.substr(revisionPrefix.length);
+          return { active: true, revision, timestamp: activeRevision.LastModified };
+        })
+      );
+
+  },
+
   fetchRevisions: function(options) {
     var client         = this._client;
     var bucket         = options.bucket;