Standard NBD URL syntax (again)

["Richard W.M. Jones" <rjones@redhat.com>]

Previous discussion:
https://lists.debian.org/nbd/2018/07/msg00002.html

Again I'm stuck because we need a URL syntax for NBD.

Various URLs have been proposed before:

  * Library      Hostname+port over TCP/IP
                 Unix domain socket
		 Export name
  - Link for further information
  - Notes

  * libguestfs   nbd://example.com:port      (:port is optional)
                 nbd://?socket=/socket
		 nbd://example.com:port/exportname
                 nbd:///exportname?socket=/socket
  - http://libguestfs.org/guestfish.1.html#a-nbd:-example.com-:port
  - Cannot specify an export name without a leading '/' character

  * QEMU         nbd:example.com:port
                 nbd:unix:/socket
		 nbd:example.com:port:exportname=/export
  - https://qemu.weilnetz.de/doc/qemu-doc.html#Device-URL-Syntax
  - Not a real URL!

  * Curl         nbd://example.com/
                 nbd://example.com/exportname
  - https://github.com/rwmjones/curl/commit/2aba3dd7d8f9b8e03a8c3435e7555ab9e195675a#diff-77ab3188c0ad08b7b9d8d9787c8e9d75
  - Cannot specify an export name without a leading '/' character
  - No Unix domain socket syntax was ever defined.
  - Did not go upstream.

The leading problem with the "obvious" NBD URL format seems to be that
it's impossible to specify empty export names or export names which
don't start with a '/' character.

This is why I'd be inclined to use an extra URL parameter instead,
like:

  nbd://example.com[:port]/?export=exportname

and for Unix domain sockets:

  nbd:///?export=exportname&socket=/socket

The export would be optional, defaulting to empty string.  IPv6
addresses would work in the obvious way for URLs, using "[...]"
around the address part (ie as in RFC 2732).  All escaping issues
would work as for standard URLs.

I think we should standardize this once and for all in a document in
the NBD repository.

Anyway let me know what you think and I will try to write something
formal.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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