Re: [Nbd] Thoughts about the handshake protocol

[Wouter Verhelst <w@...112...>]

On Tue, May 31, 2011 at 01:38:44PM +0200, Goswin von Brederlow wrote:
> Wouter Verhelst <w@...112...> writes:
> 
> > On Tue, May 31, 2011 at 10:28:38AM +0200, Goswin von Brederlow wrote:
> >> C: NBD_OPT_EXPORT_NAME foo
> >> S: size+flags
> >> C: NBD_OPT_SYNC
> >> -- wait --
> >> C: NBD_OPT_END
> >> -- wait --
> >> C - switching to nbd mode
> >> S: error, NBD_OPT_SYNC not known
> >> 
> >> At that point the kernel will be quite confused and kill the connection.
> >
> > No, not that way.
> >
> > C: NBD_OPT_EXPORT_NAME foo
> > S: size + flags
> > C: NBD_OPT_FOO
> > C: NBD_OPT_END
> > S: error, NBD_OPT_SYNC not known
> > S: NBD_OPT_END
> > C: disconnect or switch to kernel
> >
> > [...]
> 
> So NBD_OPT_END would allways have a reply. That would work in this case
> too.
> 
> But what if you have conflicting options A | B? The client first tries
> to set option A. If the server doesn't know that it wants to fall back
> to B. But it doesn't want option B unless A fails.

Let's say that the server must issue out error messages as soon as
possible (i.e., it must not wait until the NBD_OPT_END message has been
received, but must issue them as soon as it finds that there would be an
error condition). That way, the client has two options:

- select() on the socket with a timeout to see whether the server issues
  an error about an unknown option; if not, assume the option has been
  accepted
- deliberately issue an invalid option. That way, the server would
  need to issue an error message, and the client can see whether another
  error message appears before that and doesn't need to wait for an
  indefinite amount of time.

> >> > Mmm. I'd prefer a newline-separated list of export names, that's going
> >> > to be easier -- and there's no need to support complex export names.
> >> 
> >> Works too. What can an export name be? Any UTF-8 string not containing 0
> >> and \n? Only printable ascii chars? [-_0-9a-zA-Z.:#+]?
> >
> > I'd say letters and numbers, and must not start with a number.
> 
> The new style calls for them to be the full path,

Wrong. Whatever gave you that idea?

There is no relation whatsoever between the name of an export and the
name of the exported file.

[...]

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