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Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaV
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs |
Date: |
Tue, 28 Jul 2015 14:41:27 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 |
On 07/28/2015 12:44 AM, Markus Armbruster wrote:
>>
>>> +def gen_visit_union(name, base, variants):
>>> + ret = ''
>>>
>>> if base:
>>> - assert discriminator
>>> - base_fields = find_struct(base)['data'].copy()
>>> - del base_fields[discriminator]
>>> - ret += generate_visit_struct_fields(name, base_fields)
>>> + members = [m for m in base.members if m != variants.tag_member]
>>> + ret += generate_visit_struct_fields(name, members)
>>
>> Ouch. This hurts. If the same class is used as both the base class of a
>> flat union, and the base class of an ordinary struct, then the struct
>> tries to visit the base class, but no longer parses the field that the
>> union was using as its discriminator.
>>
>> We don't have any code that demonstrates this, but probably should. I
>> ran into it while working up my POC of what it would take to unbox
>> inherited structs (http://thread.gmane.org/gmane.comp.emulators.qemu/353204)
>
> Is this broken in master, or do my patches break it?
>
> Got a reproducer?
Turns out I'm mistaken; we got lucky. The call to
generate_visit_struct_fields() creates a function for 'name' (aka the
union name), and not for 'base' (aka the class name that owns the
fields). So even if we have Base as a common struct between Child and
Union, the code emitted for Child generates visit_Base_fields(), while
the code emitted for Union generates visit_Union_fields().
So there is no reproducer, but _only_ as long as we reject unions as a
base class for any other object. And there is redundancy: we could
reuse visit_Base_fields() for the sake of the union, then avoid
(re-)visiting the discriminator, and we would no longer need to emit
visit_Union_fields(). But I can do that as part of the followup
cleanups; since I don't see anything broken with your patch, we don't
have to worry about it during this series.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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- Re: [Qemu-devel] [PATCH RFC v2 47/47] qapi-introspect: Hide type names, (continued)
- [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/01
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Eric Blake, 2015/07/22
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/27
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Eric Blake, 2015/07/27
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/28
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Eric Blake, 2015/07/28
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/29
Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Eric Blake, 2015/07/27
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/28
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs,
Eric Blake <=
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Markus Armbruster, 2015/07/29
- Re: [Qemu-devel] [PATCH RFC v2 27/47] qapi-visit: Convert to QAPISchemaVisitor, fixing bugs, Eric Blake, 2015/07/29
[Qemu-devel] [PATCH RFC v2 24/47] tests/qapi-schema: Convert test harness to QAPISchemaVisitor, Markus Armbruster, 2015/07/01
[Qemu-devel] [PATCH RFC v2 41/47] qom: Don't use 'gen': false for qom-get, qom-set, object-add, Markus Armbruster, 2015/07/01
[Qemu-devel] [PATCH RFC v2 33/47] qapi: Clean up after recent conversions to QAPISchemaVisitor, Markus Armbruster, 2015/07/01