Re: [PATCH v2 3/3] block: switch to use qemu_open_err for improved errors

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> Currently at startup if using cache=none on a filesystem lacking
> O_DIRECT such as tmpfs, at startup QEMU prints
>
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: file system may not 
> support O_DIRECT
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: Could not open 
> '/tmp/foo.img': Invalid argument
>
> while at QMP level the hint is missing, so QEMU reports just
>
>   "error": {
>       "class": "GenericError",
>       "desc": "Could not open '/tmp/foo.img': Invalid argument"
>   }
>
> which is close to useless for the end user trying to figure out what
> they did wrong
>
> With this change at startup QEMU prints
>
> qemu-system-x86_64: -drive file=/tmp/foo.img,cache=none: Unable to open 
> '/tmp/foo.img' flags 0x4000: filesystem does not support O_DIRECT
>
> while at the QMP level QEMU reports a massively more informative
>
>   "error": {
>      "class": "GenericError",
>      "desc": "Unable to open '/tmp/foo.img' flags 0x4002: filesystem does not 
> support O_DIRECT"
>   }
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Narrowing down the error's cause takes extra code, but the error message
improvement is worth it.

Reviewed-by: Markus Armbruster <armbru@redhat.com>