rbd: image Open function is non-idiomatic Go about go-ceph

[phlogistonjohn]

I've started working on this. PR coming soon(ish).

[sungjunyoung]

I'm not familiar about librbd :( but I think I can tell about what I think.

I'm agreed about your opinion, I'm confused about Open method's parameter when I use go-ceph first
According to the tests and sources, c_snap_name (string) and read_only (boolean) are the only required parameters for the Open method, And they are can be optional parameter. ("", false).

I think there are some options for idiomatic Go

1. We can create two function Open, OpenReadOnly for the feature because golang doesn't support function overloading. Maybe two function have string parameter for c_snap_name
2. Create struct for Open parameter
   • in gophercloud, there are various option struct for support openstack api parameters

   server, err := servers.Create(client, servers.CreateOpts{
     Name:      "My new server!",
     FlavorRef: "flavor_id",
     ImageRef:  "image_id",
   }).Extract()

3. Apply more declarative parameters. like Open(readOnly bool, snapName string)

[phlogistonjohn]

Anyone interested in this topic please look at #146 and provide feedback. We intend to change the typical workflow such that new open functions return the rbd Image type and Create functions only return an error (like the ceph apis).