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Re: qapi-schema esotera

From: Eric Blake
Subject: Re: qapi-schema esotera
Date: Mon, 3 Aug 2020 12:25:33 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 8/3/20 11:49 AM, John Snow wrote:
UNION is split into two primary forms:

1. Simple (No discriminator nor base)
2. Flat (Discriminator and base)

In expr.py, I notice that we modify the perceived type of the 'type' expression based on the two union forms.

1a. Simple unions allow Array[T]
1b. Flat unions disallow Array[T]

Rather, branches in a simple unions are syntactic sugar for a wrapper struct that contains a single member 'data'; because of that extra nesting, the type of that single member is unconstrained. In flat unionw, the type MUST be a QAPI struct, because its members will be used inline; as currently coded, this prevents the use of an intrinsic type ('int', 'str') or an array type.

If you need to use an array type in a flat union, you can't do:

{ 'union' ...
  'data': { 'foo': [ 'MyBranch' ] } }

but you can provide a wrapper type yourself:

{ 'struct': 'MyBranch', 'data': { 'array': [ 'MyType' ] } }
{ 'union' ...
  'data': { 'foo': 'MyBranch' } }

 From the docs:

     UNION = { 'union': STRING,
               'data': BRANCHES,
               '*if': COND,
               '*features': FEATURES }
           | { 'union': STRING,
               'data': BRANCHES,
               'base': ( MEMBERS | STRING ),
               'discriminator': STRING,
               '*if': COND,
               '*features': FEATURES }
     BRANCHES = { BRANCH, ... }
            | STRING : { 'type': TYPE-REF, '*if': COND }

Both arms use the same "BRANCHES" grammar production, which both use TYPE-REF.


Implying that List[T] should be allowed for both productions.
Can I ask for a ruling from the judges?

As you found, the docs are a bit misleading; the semantic constraint on flat union branches being a struct (because they will be inlined) prevents the use of type-refs that are valid in simple unions (where those simple types will be wrapped in an implicit struct). A patch to improve the docs would be a reasonable idea.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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