[Libguestfs] Few libnbd questions/concerns

[Eric Blake]

On 6/25/19 8:14 AM, Richard W.M. Jones wrote:
> On Tue, Jun 25, 2019 at 02:58:34PM +0200, Martin Kletzander wrote:
>> Here are few things I found out when using libnbd that might be perfectly fine
>> or maybe just an oversight, but I wanted to point them out.  It's nothing major.
>>
>> When running a program with `nbdkit -U - --run ...`, the $nbd parameter gets
>> expanded to nbd:unix:/path/to/socket.  When this string is passed to
>> nbd_connect_uri(), it does not return an error (even though it is not a valid
>> URL), but what's more it treats it as "nbd://localhost", which might be a completely different server (that actually happened to me and it was kind of confusing).
> 
> There's a bit of magic in nbdkit about $nbd.  It also predates our
> standardization effort for NBD URLs which is going on upstream.
> 
> That needs to be fixed, but in the meantime just use $unixsocket
> instead (see nbdkit-captive(1) man page for the full details).

In the meantime, I don't mind working on the quick fix for rejecting an
invalid URI.

> 
>> When nbd_block_status() callback returns -1, the nbd_block_status() function
>> reports:
>>
>>  command failed: Protocol error (errno = 71)
>>
>> which is a bit confusing to me.  I could be nicer to have that report that it
>> really was the callback that caused this.
> 
> Not sure about this one - Eric?

Right now, a failed callback will cause the overall nbd_block_status()
to fail with whatever errno the callback left behind, with EPROTO as the
default. I'm guessing you didn't set an errno when you returned -1?  Is
there a better default we should use than EPROTO?  Is there really a way
to diagnose without an errno value that the callback failed but didn't
set errno?

There's also the related question - my patches to add
nbd_pread_structured are set up to call the callback for as many chunks
as the server sends, even if an earlier invocation of the callback
fails, but to still preserve the errno of the first failed callback
(again defaulting to EPROTO). Should nbd_block_status also call the
callback for all chunks from the server, rather than stopping the use of
further callbacks once the first callback fails, if only for consistency
between the two?

>> One last thing is that I could not find any definition for the flags
>> of "base:allocation" metadata (NBD_STATE_HOLE and NBD_STATE_ZERO).
>> It might just be things that are still missing due to an early stage
>> of libnbd, or it might not belong there or it might be me misusing
>> something.
> 
> Have a look at the NBD protocol doc:
> 
> https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md

We could add convenience #defines into libnbd.h if needed, which would
work for the base:allocation context, but it would not help any other
context (such as qemu:dirty-bitmap:FOO) which has other definitions for
the returned bits.  But the fact that you have to call
nbd_add_meta_context(h, "base:allocation") is also awkward; if we add
convenience macros for the results, we probably also want to support
nbd_add_meta_context(h, LIBNBD_CONTEXT_BASE_ALLOCATION).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 488 bytes
Desc: OpenPGP digital signature
URL: <http://listman.redhat.com/archives/libguestfs/attachments/20190625/ab377b20/attachment.sig>