aliyun: make image replication idempotent

There is a history of failure copying images to other regions on
aliyun. Upstream code that calls the CopyImage code more than once
will get a DuplicateImage error when an image with the same name
already exists. Let's check to see if the image name exists in the
region before attempting to copy the image to the region and return
early if it already exists.

mantle/platform/api/aliyun/api.go

@@ -134,6 +134,20 @@ func (a *API) CopyImage(source_id, dest_name, dest_region, dest_description, kms
 			Value: "mantle",
 		},
 	}
+
+	// Check if an image with the name has already been uploaded. This can
+	// happen when replication to a region fails which happens often on aliyun
+	images, err := a.GetImagesInRegion(dest_name, dest_region)
+	if err != nil {
+		return "", fmt.Errorf("getting image: %v", err)
+	}
+
+	// return early if there is already an image with that tag
+	if len(images.Images.Image) > 0 {
+		plog.Infof("image with name %v in %v region already exists--skipping copy", dest_name, dest_region)
+		return images.Images.Image[0].ImageId, nil
+	}
+
 	response, err := a.ecs.CopyImage(request)
 	if err != nil {
 		return "", fmt.Errorf("copying image: %v", err)
@@ -296,6 +310,16 @@ func (a *API) finishImportImageTask(importImageResponse *ecs.ImportImageResponse
 	return importImageResponse.ImageId, nil
 }
 
+// GetImagesInRegion retrieves a list of images by ImageName in a specified region
+func (a *API) GetImagesInRegion(name string, region string) (*ecs.DescribeImagesResponse, error) {
+	request := ecs.CreateDescribeImagesRequest()
+	request.SetConnectTimeout(defaultConnectTimeout)
+	request.SetReadTimeout(defaultReadTimeout)
+	request.ImageName = name
+	request.RegionId = region
+	return a.ecs.DescribeImages(request)
+}
+
 // GetImages retrieves a list of images by ImageName
 func (a *API) GetImages(name string) (*ecs.DescribeImagesResponse, error) {
 	request := ecs.CreateDescribeImagesRequest()