Re: [Qemu-devel] [PATCH] Support 'help' as a synonym for '?' in command line options

[Eric Blake <eblake@redhat.com>]

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On 07/09/2012 05:52 AM, Peter Maydell wrote:
> For command line options which permit '?' meaning 'please list the
> permitted values', add support for 'help' as a synonym, by abstracting
> the check out into a helper function.
> 
> Update the documentation to use 'help' rather than '?', since '?'
> is a shell metacharacter and thus prone to fail confusingly if there
> is a single character filename in the current working directory and
> the '?' has not been escaped. It's therefore better to steer users
> towards 'help', though '?' is retained for backwards compatibility.

I like the idea, but this may cause grief for libvirt, which basically does:

const char *help; // = output of 'qemu -h'
if (strstr(help, "-device driver,?")) {
    /* Cram together all device-related queries into one invocation;
     * the output format makes it possible to distinguish what we
     * need.  With qemu 0.13.0 and later, unrecognized '-device
     * bogus,?' cause an error in isolation, but are silently ignored
     * in combination with '-device ?'.  Upstream qemu 0.12.x doesn't
     * understand '-device name,?', and always exits with status 1 for
     * the simpler '-device ?', so this function is really only useful
     * if -help includes "device driver,?".  */
    cmd = virCommandNewArgList(qemu,
                               "-device", "?",
                               "-device", "pci-assign,?",
                               "-device", "virtio-blk-pci,?",
                               "-device", "virtio-net-pci,?",
                               "-device", "scsi-disk,?",
                               NULL);
}

That is, we are filtering based on the explicit presence of a literal
'?' in the help output to determine whether we can further filter based
on '-device device,?' queries without confusing qemu or libvirt;
changing the 'help' output means that old libvirt with new qemu won't
run the extra queries (as if it had been targeting qemu 0.12.x), even
though the older spelling of the query would still work.

If that is not a concern (that is, if you are willing to state that use
of newer qemu is intended to be coupled with newer libvirt that has been
taught to use 'help' instead of '?'), then this patch is probably fine.
 The alternative is to update 'qemu -h' output to mention both forms,
rather than completely eradicating the ? form from documentation.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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