On Thu, 2011-03-31 at 18:30 +0100, Ian Jackson wrote:
> 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.
Well it is designed so that the pointer arithmetic parts are the main
loop which doesn't need altering. Within the loop it's just a matter of
doing state transitions and that is the essence of the parser. Just a
bit of care and testing when modifying it is all that's required - it's
not *that* subtle.
Gianni
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel
|
Home