From 9c451252712f0e89d710fe477ef99b8641e899c2 Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Sun, 21 Jan 2018 17:10:10 +0000
Subject: [PATCH] azureblob: fix RangeOption support in Open #1825

---
 backend/azureblob/azureblob.go | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/backend/azureblob/azureblob.go b/backend/azureblob/azureblob.go
index 20d09c49f..113af3567 100644
--- a/backend/azureblob/azureblob.go
+++ b/backend/azureblob/azureblob.go
@@ -854,14 +854,21 @@ func (o *Object) Open(options ...fs.OpenOption) (in io.ReadCloser, err error) {
 	for _, option := range options {
 		switch x := option.(type) {
 		case *fs.RangeOption:
+			start, end := x.Start, x.End
+			if end < 0 {
+				end = 0
+			}
+			if start < 0 {
+				start = o.size - end
+				end = 0
+			}
 			getBlobRangeOptions.Range = &storage.BlobRange{
-				Start: uint64(x.Start),
-				End:   uint64(x.End),
+				Start: uint64(start),
+				End:   uint64(end),
 			}
 		case *fs.SeekOption:
 			getBlobRangeOptions.Range = &storage.BlobRange{
 				Start: uint64(x.Offset),
-				End:   uint64(o.size),
 			}
 		default:
 			if option.Mandatory() {