add prototype gcp cs storage backend support

storage/gcp.go

@@ -0,0 +1,46 @@
+package storage
+
+import (
+	"context"
+	"io"
+	"log"
+
+	"cloud.google.com/go/storage"
+)
+
+// snapshot upload to gcp cloud storage
+func snapshotCSUpload(csBucket string, snapshotFile io.Reader, snapshotName string) error {
+	// gcp client
+	ctx := context.Background()
+	gcpClient, err := storage.NewClient(ctx)
+	if err != nil {
+		log.Print("unable to initialize GCP client for transfer to cloud storage bucket")
+		return err
+	}
+	defer gcpClient.Close()
+
+	// initialize target in cloud storage bucket
+	uploadTarget := gcpClient.Bucket(csBucket).Object(snapshotName)
+	// only upload if does not exist (TODO feature all below)
+	// uploadTarget = uploadTarget.If(storage.Conditions{DoesNotExist: true})
+	// If the live object already exists in your csBucket, set instead a
+	// generation-match precondition using the live object's generation number.
+	// attrs, err := o.Attrs(ctx)
+	// if err != nil {
+	// 	return fmt.Errorf("object.Attrs: %w", err)
+	// }
+	// o = o.If(storage.Conditions{GenerationMatch: attrs.Generation})
+
+	// write snapshotfile to upload target in cloud storage bucket
+	uploadTransfer := uploadTarget.NewWriter(ctx)
+	if _, err = io.Copy(uploadTransfer, snapshotFile); err != nil {
+		log.Printf("failed to upload snapshot file %s to bucket %s", uploadTarget.ObjectName(), uploadTarget.BucketName())
+		return err
+	}
+	if err := uploadTransfer.Close(); err != nil {
+		log.Printf("failed to close snapshot file %s upload transfer to %s", uploadTarget.ObjectName(), uploadTarget.BucketName())
+		return err
+	}
+
+	return nil
+}