Re: [Qemu-devel] [RFC 0/8]: QError v2

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 11 Nov 2009 15:20:30 -0600
Anthony Liguori <anthony@codemonkey.ws> wrote:

> Luiz Capitulino wrote:
> >  Hi,
> >
> >  I can't remember seeing updated versions of a RFC series, but this should
> > prevent Anthony's scripts from merging these patches.
> >
> >  This new QError version has two major changes: the static error table has
> > been dropped and I'm using symbolic names instead of error codes.
> >
> >  Now, a call to:
> >
> > monitor_printf(mon, "husb: host usb device %d.%d is already open\n",
> >                bus_num, addr);
> >
> >  Would become something like:
> >
> > qemu_error_new('DeviceAlreadyOpen', "{ 'bus_num': %d, 'addr': %d }",
> >                bus_num, addr);
> >   
> 
> I mostly like this but this is not what the patches do :-)
> 
> Here's what I would like to see:

 Thinking about this a little more I see some problems with it.

> #define QERR_DEVICE_ALREADY_OPEN "{'class': 'DeviceAlreadyOpen', 'data' 
> : {'bus_num': %d, 'addr': %d}"
> 
> qemu_error_new(QERR_DEVICE_ALREADY_OPEN, bus_num, addr);

 What about DeviceAlreadyOpen errors with a different argument list?

> That gives us a nice simple interface with full error checking on the 
> parameters.

 I've said this is not so simple because people writing those macros
would find out that the 'class' or 'data' _keys_ are missing or incorrect
only at run-time, when the error is triggered.

> For human readable strings, I'd suggest making a table somewhere else 
> that looked like:
> 
> QErrorStringTable qerror_descriptions[] = {
> { QERR_DEVICE_ALREADY_OPEN, "This device at %(bus_num)d.%(addr)d is 
> already open." },
> ...
> };

 How do you suggest we lookup the table? Doing a strcmp() on
QERR_DEVICE_ALREADY_OPEN?

> There are a number of advantages to an approach like this.  The table 
> can be reused by both in the server and by a client.

 My suggestions on both problems makes me willing go back to my initial
series, which had a table indexed by an error number.