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? I can live with such a restriction. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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