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 <ebl...@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
