Currently the volumeattach extension uses the following signature for deleting an attachment

func Delete(client *gophercloud.ServiceClient, serverID, attachmentID string) (r DeleteResult)

It implies that an attachment ID is required to perform the delete.

However the implementation uses the os-volume_attachments resource path, which the OpenStack API reference shows takes a volume ID, specifically describing it as "The UUID of the volume to detach".

In my particular case the attachment ID field is an empty string, so following the Delete functions signature doesn't work. Only using the volume ID succeeds.