Re: [PATCH] libxl: new xlu_disk_parse function

[Ian Jackson <Ian.Jackson@eu.citrix.com>]

Gianni Tedesco writes ("Re: [Xen-devel] [PATCH] libxl: new xlu_disk_parse function"):
> On Thu, 2011-03-24 at 17:49 +0000, Ian Jackson wrote:
> > +       return;
> 
>        case 1: ??

Deliberately omitted.

> > +    case 2:
> > +    case 3:
> > +    case 4:
> > +       break;
> 
> Or does it belong here? In which case aborting on a parse error is bad
> juju.
>        case 1:
> > +    default:
> > +       abort();

I could add it there for clarity.  The regexp will always match
capturing with 2, 3 or 4 parens.  None of the other errors from
dfa_exec are applicable.  So anything other than 2,3,4 or "did not
match" is due to a bug in the code, not merely bogus input.  Perhaps
this should be mentioned in a comment.

> Hmm, I'm not sure this is nicer than the code it's replacing, it's
> certainly a lot longer. I don't like the idea of it being full of things
> comparing for ",w" or ":cdrom" etc, rather than tokenising it fully and
> evaluating what the tokens are separately.

Perhaps you're right.  Unfortunately the nasty multi-level nature of
this parsing problem makes this a bit awkward.

But I think I can remove the delimiters into the regexp which perhaps
will help.

> > +raw:           { DSET(format,FORMAT,RAW); }
> > +qcow:          { DSET(format,FORMAT,QCOW); }
> > +qcow2:         { DSET(format,FORMAT,QCOW2); }
> > +vhd:           { DSET(format,FORMAT,QCOW2); }
> > +
> > +phy:           { DSET(format,FORMAT,RAW); DSET(backend,BACKEND,PHY); }
> > +file:          { DSET(format,FORMAT,RAW); DSET(backend,BACKEND,TAP); }
> > +tapdisk:|tap2?:        { DSET(backend,BACKEND,TAP); }
> > +aio:           { }
> > +ioemu:         { }
> 
> This bit is quite nice though. We could probably just tidy up the
> existing parser using arrays of values and things rather than a lot of
> if/else statements though.

I wanted to avoid parsing with pointer arithmetic, which is very easy
to write bugs in - particularly when new features are added.

Ian.