Add marker loop detection

s3-bucket-list

@@ -62,6 +62,9 @@ while True:
 	if not body.startswith(b'<?xml version="1.0" encoding="UTF-8"?>\n<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">'):
 		raise RuntimeError(f'Invalid body: {body[:200]}...')
 
+	if b'<Marker></Marker>' in body[:200] and 'marker' in params:
+		raise RuntimeError('Marker loop (empty marker in response despite providing one)')
+
 	# No risk, no fun!
 	contents = body.split(b'<Contents>')
 	assert all(content.startswith(b'<Key>') for content in contents[1:])
@@ -91,4 +94,6 @@ while True:
 
 	if not truncated:
 		break
+	if 'marker' in params and params['marker'] == lastKey:
+		raise RuntimeError('Marker loop (same last key as previous marker)')
 	params['marker'] = lastKey