Re: [Qemu-devel] [Qemu-block] [PATCH RFC] qemu-io: Prefer stderr for error messages

[Kevin Wolf <kwolf@redhat.com>]

Am 13.12.2018 um 11:47 hat Daniel P. Berrangé geschrieben:
> On Thu, Dec 13, 2018 at 01:52:29AM +0200, Nir Soffer wrote:
> > On Thu, Dec 13, 2018 at 12:13 AM Eric Blake <eblake@redhat.com> wrote:
> > >
> > > When a qemu-io command fails, it's best if the failure message
> > > goes to stderr rather than stdout.
> > 
> > This makes sense, but it will break users like this:
> > 
> > https://github.com/oVirt/vdsm/blob/a2836b1d58ffaa0f48cc9c814b6002161a81f044/tests/storage/qemuio.py#L45
> > 
> > We need a way to detect qemu-io verification failures, maybe a special
> > exit code?
> > 
> > 0 - verification succeeded
> > 1 - verification failed
> > 2 - other error (e.g no such file)
> 
> This makes sense. We should *never* expect applications to parse the
> messages on stdout/err, because we reserve the right to change text
> arbitrarily at any time. So we need to use exit status IMHO.

qemu-io processes more than just a single command. What would the exit
code be if one of the commands succeeds, one gets an I/O error, and the
third one succeeds for I/O, but fails pattern verification?

The things is, qemu-io was never meant to be used by other
applications that need to process the results, it's a tool for testing
and debugging. If we had meant it to be used by other programs, we would
have given it a machine-friendly interface.

The machine-friendly interface to the QEMU block layer is qemu-nbd.

> > Or, if qemu-io had a way to read data and write it to stdout, we could
> > compare the data and avoid the need for special exit code.
> 
> That should be trivial to do, and quite desirable too IMHO - libvirt would
> in fact quite like such a feature, as it would let us support format
> conversions when using our upload/download APIs, without having to create
> intermediate files.  Alternatively 'qemu-img convert' could allow for
> /dev/stdin and /dev/stdout as raw files, but that looks considerably
> harder to implement.
> 
> For your usecase that feels rather inefficient as you're introducing
> multiple data copies, which will be bad for large images. Much better
> if we just make qemu-io set good exit codes.

'read -v' produces a hex dump on stdout, but you still need to separate
it from the other output and then parse the hexdump.

The human interface of qemu-io is honestly just not the right tool for
the job, and adding one-off tweaks to make it a little bit less horrible
to use for machines isn't the right approach because it's still not a
proper machine protocol.

Kevin