[Libguestfs] [PATCH nbdkit] server: Deprecate the -e/--exportname parameter.

[Richard W.M. Jones]

On Tue, Jul 21, 2020 at 07:51:29PM -0500, Eric Blake wrote:
> On 7/21/20 12:46 PM, Eric Blake wrote:
> 
> >>(2) The --run option no longer sets $exportname (to -e) nor puts the
> >>     export name into the $uri.  However this was always the wrong
> >>     thing to do since export names are per connection, not per server.
> >>     Existing --run scripts will see $exportname expand to "" which is
> >>     most likely what they saw before and overwhelmingly more likely to
> >>     be correct than if the -e option had been used.
> >>
> 
> The more I think about it, the more I disagree with disabling this.
> Or, put another way, I think that the _only_ time -e makes sense is
> _when_ you are using --run.  Consider:
> 
> nbdkit -U - -e foo info --run 'nbdsh -u $uri -c "print(h.pread(3, 0))"'
> nbdkit -U - -e bar info --run 'nbdsh -u $uri -c "print(h.pread(3, 0))"'
> 
> Of course, you can accomplish the same with:
> 
> nbdkit -U - info --run 'nbdsh -u nbd+unix:///foo\?socket=$unixsocket \
>   -c "print(h.pread(3, 0))"'
> 
> but that's a lot more painful to write.
> 
> We _don't_ need to advertise 'foo' or 'bar' over NBD_OPT_INFO (at
> least, not for plugins that don't implement the forthcoming
> .export_list callback), but _do_ need a way for the captive
> application (nbdsh in this case) to know _which_ export the captive
> should connect to.
> 
> And, if we reinstate _just_ that usage of -e,...
> 
> >>(4) I have temporarily disabled tests/test-long-name.sh.  This is
> >>     still a valid test, but we will have to rewrite it to use
> >>     (probably) sh or eval plugins once we have an implementation of
> >>     list_exports.
> >>---
> 
> ...we may not have to disable this test after all.

I feel for the end user this is all pretty confusing and requires them
to have some insight into what the plugin is expecting and even
understanding of the protocol.

Perhaps we should have a rule something like this:

 - plugins should serve default content on the "" export

 - unless they implement .list_exports, in which case the
   first export returned is the default export

Then we could implement --run transparently by having it set
$exportname/$uri to the first entry in .list_exports or "" if
.list_exports is not implemented.

Here's another idea: Is it possible that if the --run script itself
overrides exportname it could be used by $uri.  Not sure if there's
some kind of delayed shell variable expansion that would make this
possible:

  nbdkit ... --run 'exportname=foo; nbdsh -u $uri ...'

Rich.

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