From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48345) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gXRcL-0000OE-Gv for qemu-devel@nongnu.org; Thu, 13 Dec 2018 09:05:34 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gXRcH-0008L5-IE for qemu-devel@nongnu.org; Thu, 13 Dec 2018 09:05:33 -0500 Date: Thu, 13 Dec 2018 15:05:13 +0100 From: Kevin Wolf Message-ID: <20181213140513.GD5427@linux.fritz.box> References: <20181212220410.569069-1-eblake@redhat.com> <20181213104704.GD5171@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20181213104704.GD5171@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [Qemu-block] [PATCH RFC] qemu-io: Prefer stderr for error messages List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Cc: Nir Soffer , Eric Blake , QEMU Developers , qemu-block , Max Reitz Am 13.12.2018 um 11:47 hat Daniel P. Berrang=E9 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 wrote= : > > > > > > When a qemu-io command fails, it's best if the failure message > > > goes to stderr rather than stdout. > >=20 > > This makes sense, but it will break users like this: > >=20 > > https://github.com/oVirt/vdsm/blob/a2836b1d58ffaa0f48cc9c814b6002161a= 81f044/tests/storage/qemuio.py#L45 > >=20 > > We need a way to detect qemu-io verification failures, maybe a specia= l > > exit code? > >=20 > > 0 - verification succeeded > > 1 - verification failed > > 2 - other error (e.g no such file) >=20 > 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 coul= d > > compare the data and avoid the need for special exit code. >=20 > That should be trivial to do, and quite desirable too IMHO - libvirt wo= uld > in fact quite like such a feature, as it would let us support format > conversions when using our upload/download APIs, without having to crea= te > intermediate files. Alternatively 'qemu-img convert' could allow for > /dev/stdin and /dev/stdout as raw files, but that looks considerably > harder to implement. >=20 > 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