Re: [Qemu-devel] [PATCH 2/4] string-input-visitor: Support alternate types

[Eric Blake]

On 05/04/2017 03:06 AM, Markus Armbruster wrote:
> Eduardo Habkost <address@hidden> writes:
> 
>> On Wed, May 03, 2017 at 06:07:43PM +0200, Markus Armbruster wrote:
>>> Eduardo Habkost <address@hidden> 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: <address@hidden>
>     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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