On Thu, May 04, 2017 at 03:42:59PM +0200, Markus Armbruster wrote: > Eric Blake <ebl...@redhat.com> writes: > > > On 05/04/2017 03:06 AM, Markus Armbruster wrote: > >> Eduardo Habkost <ehabk...@redhat.com> writes: > >> > >>> On Wed, May 03, 2017 at 06:07:43PM +0200, Markus Armbruster wrote: > >>>> Eduardo Habkost <ehabk...@redhat.com> writes: > >>>> > >>>>> When parsing alternates from a string, there are some limitations in > >>>>> what we can do, but it is a valid use case in some situations. We can > >>>>> support booleans, integer types, and enums. > >> > >> By the way, the same restrictions apply to the "keyval" variant of the > >> QObject input visitor. It's a known problem stated here: > >> > >> Message-ID: <8737exuz6u....@dusky.pond.sub.org> > >> https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg00046.html > >> > >> However, I failed to document it properly in the source. > >> > > > >>>> Begs the question what happens when you violate these restrictions. > >>> > >>> Right now, we don't detect those cases and behavior is undefined. > >>> I think it will be a good idea to give start_alternate() enough > >>> information to detect those cases (by adding a 'const char *const > >>> enum_table[]' parameter). > >> > >> Alternate types that won't work with the string input visitor can be > >> detected at compile time (by qapi.py), but not their actual use. Pity. > >> > >> Do we actually use alternates that violate the restrictions? If not, we > >> could simply restrict alternates so they work with *all* visitors. If > >> we ever run into an actual need for alternates that don't, we'll be no > >> worse off than now. > >> > >> Let's review existing alternates outside tests: > >> > >> * Qcow2OverlapChecks: struct + enum > >> * BlockdevRef: struct + str > >> * GuestFileWhence: int + enum (all enum members start with a letter) > >> > >> Restricting alternates looks practical to me. Eric, what do you think? > > > > As in: we forbid the combination of a scalar (whether 'int', 'number', > > 'bool', and perhaps 'null') with a plain 'str' (since there's no way to > > tell whether '1' should parse as an integer or the string "1"); and > > combining a scalar with an 'enum' requires that all enum members be > > distinct from what could otherwise be parsed as a scalar? > > Exactly. > > > I can live > > with such a restriction. > > Then let's do it. > > Eduardo, are you comfortable implementing this, or would you like me to > do it?
I will give it a try and include it in the next version. Thanks! -- Eduardo
