Add Marker support for donut, also add a testing code

pkg/storage/donut/objectstorage.go

@@ -134,9 +134,22 @@ func (d donut) ListObjects(bucket, prefix, marker, delimiter string, maxkeys int
 		actualObjects = donutObjects
 	}
 
+	sort.Strings(actualObjects)
+	var newActualObjects []string
+	switch {
+	case marker != "":
+		for _, objectName := range actualObjects {
+			if objectName > marker {
+				newActualObjects = append(newActualObjects, objectName)
+			}
+		}
+	default:
+		newActualObjects = actualObjects
+	}
+
 	var results []string
 	var commonPrefixes []string
-	for _, objectName := range actualObjects {
+	for _, objectName := range newActualObjects {
 		if len(results) >= maxkeys {
 			isTruncated = true
 			break

pkg/storage/drivers/api_testsuite.go

@@ -181,10 +181,25 @@ func testPaging(c *check.C, create func() Driver) {
 		c.Assert(resources.CommonPrefixes[0], check.Equals, "this/")
 	}
 
+	// check results with Marker
+	{
+		var prefixes []string
+		resources.CommonPrefixes = prefixes // allocate new everytime
+		resources.Prefix = ""
+		resources.Marker = "newPrefix"
+		resources.Delimiter = "/"
+		resources.Maxkeys = 3
+		objects, resources, err = drivers.ListObjects("bucket", resources)
+		c.Assert(objects[0].Key, check.Equals, "newPrefix2")
+		c.Assert(objects[1].Key, check.Equals, "obj0")
+		c.Assert(objects[2].Key, check.Equals, "obj1")
+		c.Assert(resources.CommonPrefixes[0], check.Equals, "this/")
+	}
 	// check ordering of results with prefix
 	{
 		resources.Prefix = "obj"
 		resources.Delimiter = ""
+		resources.Marker = ""
 		resources.Maxkeys = 1000
 		objects, resources, err = drivers.ListObjects("bucket", resources)
 		c.Assert(objects[0].Key, check.Equals, "obj0")
@@ -196,6 +211,7 @@ func testPaging(c *check.C, create func() Driver) {
 	// check ordering of results with prefix and no paging
 	{
 		resources.Prefix = "new"
+		resources.Marker = ""
 		resources.Maxkeys = 5
 		objects, resources, err = drivers.ListObjects("bucket", resources)
 		c.Assert(objects[0].Key, check.Equals, "newPrefix")