Re: [Qemu-devel] Ignoring errno makes QMP errors suck

[Anthony Liguori <anthony@codemonkey.ws>]

On 03/26/2012 10:59 AM, Kevin Wolf wrote:
> Am 26.03.2012 17:38, schrieb Anthony Liguori:
>> You can add parameters to Errors in a fully compatible fashion, so just add an
>> filename parameter to PermissionDenied.  Problem solved.
>
> So your error types will end up accumulating optional parameters for all
> contexts in which they can occur?
>
> In the long run, PermissionDenied would have an optional file name (for
> raw), optional host name, port, sheepdog volume ID (for sheepdog),
> optional source and destination block devices (for blkmirror), remote
> host and port, local address and port (for UDP chardevs)...

I don't see the generalization.

>
> I could go on forever. Does this really make sense?

What's the alternative proposal?  If you just add errno to OPEN_FILE_FAILED, 
then you end up with errors for SHEEP_DOG_ATTACH_VOLUME_FAILED, 
NBD_CONNECT_FAILED, etc.

The proper way to design an error is to make it a verb, and have parameters that 
correspond to the direct object and/or indirect object.  The subject is implied 
by the command itself.

So in the case of block_snapshot_sync, the failure is:

The block_snapshot_sync command failed due to insufficient permission when 
creating foo.img.

So the error is "PERMISSION_DENIED", with the data "filename=foo.img"

In the case of NBD, the error would be:

The block_snapshot_sync command failed because the host localhost:42 could not 
be contacted.

The error is "CONNECTION_REFUSED", with the data "hostname=localhost,port=42".

It the block layer returns both of these as EACCESS today, then this is a good 
argument to refactor the block layer to take an Error object instead of 
overloading the meaning of EACCESS.

Regards,

Anthony Liguori

>
> Kevin