Re: [Qemu-devel] [PATCH v5 06/28] qapi: Add some union tests

[Kevin Wolf <kwolf@redhat.com>]

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Am 26.03.2015 um 16:04 hat Eric Blake geschrieben:
> On 03/26/2015 07:18 AM, Markus Armbruster wrote:
> > Eric Blake <eblake@redhat.com> writes:
> >>                     Meanwhile, it would be nice to allow
> >> 'discriminator':'EnumType' without 'base' for creating a simple union
> >> that is type-safe rather than open-coded to a generated enum (right
> >> now, we are only type-safe when using a flat union, but that uses
> >> a different syntax of 'discriminator':'member-name' which requires
> >> a base class containing a 'member-name' enum field).
> > 
> > I'm afraid I don't get you.  Can you give examples?
> 
> Using this common code with the appropriate union for each example:
> { 'command':'foo', 'data':UNION }
> 
> Right now, we have flat unions which are required to be type-safe (all
> branches MUST map back to the enum type of the discriminator, enforced
> by the generator, so that if the enum later adds a member, the union
> must also be updated to match):
> 
> [1]
> { 'union':'Safe', 'base':'Base', 'discriminator':'switch',
>   'data':{ 'one':'str', 'two':'int' }}
> 
> {"execute":"foo", "arguments":{'switch':'one', 'data':'hello'}}
> 
> and simple unions which cannot be typesafe (the branches of the union
> are open-coded - even if they correlate to an existing enum, there is
> nothing enforcing that extensions to the enum be reflected into the union):
> 
> [2]
> { 'union':'SimpleButOpenCoded',
>   'data':{ 'one': 'str', 'two':'int' }}
> 
> {"execute":"foo", "arguments":{'type':'one', 'data':'hello'}}
> 
> I'm hoping to add as a followup series a variant of simple unions that
> is type-safe:
> 
> [3]
> { 'union':'SimpleAndSafe', 'discriminator':'MyEnum',
>   'data':{ 'one':'str', 'two':'int' }}
> 
> {"execute":"foo", "arguments":{'type':'one', 'data':'hello'}}

This would overload 'discriminator' with two different meanings:

* In case [1] it refers to one key in the JSON object which contains the
  name of the union branch to select. That is, it is the _name_ of the
  key used as a discriminator.

* In case [3], the 'type' key is used in the JSON object to select a
  union branch. 'discriminator' describes the _valid values_ of it, i.e.
  the branch names.

We shouldn't mix these meanings. If you need [3], we could call it
'discriminator-type' or something like that. If both options are
consistently used for simple and flat unions, you end up with these
rules:

* 'discriminator' is the key that is used to select the union branch.

  - For flat unions it is required and must refer to an explicitly
    declared key in the base type.

  - For simple unions it is optional and defaults to 'type'. The key is
    implicitly created in the union type.

* 'discrimiator-type' describes the valid values of 'discriminator',
  either by referring to an enum type or to 'str'.

  - For flat unions, this must match the type of the explicit
    declaration of the discriminator field. It is optional and defauls
    to the only valid value.

  - For simple unions it is optional, too, and defaults to 'str'.

  - If it is the name of an enum type, that enum type is reused and the
    declared union branches must match the valid values of the enum.

  - If it is 'str', a new enum is generated, and all the declared union
    branches are used as valid values.

There's quite some duplication in it where we need to make sure that the
schema matches in all places, but without an explicit declaration of the
disriminator field in simple unions, this seems to be the best I can
come up with.

> But the existing, unused-except-in-testsuite, notion of a simple union
> with a base class looks like:
> 
> [4]
> { 'type':'Shared', 'data':{'common':'int'}}
> { 'union':'SimpleWithBase', 'base':'Shared',
>   'data':{ 'one':'str', 'two':'int' }}
> 
> {"execute":"foo", "arguments":{'common':1, 'type':'one', 'data':'hello'}}
> 
> If we were to allow the addition of 'discriminator':'EnumType' to a
> simple union [3], but then add that discriminator to an existing case of
> a simple union with base [4], it would look like:
> 
> { 'type':'Shared', 'data':{'common':'int'}}
> { 'union':'SimpleWithBaseAndDiscriminator', 'base':'Shared',
>   'discriminator':'MyEnum',
>   'data':{ 'one':'str', 'two':'int' }}
> 
> Yuck.  That is indistinguishable from flat unions [1], except by whether
> discriminator names an enum type or a member of the base class.

Which could even be ambiguous, couldn't it?

> > In particular, I can define simple unions in terms of flat ones by
> > restricting all union cases to a single member named 'data'.  They're
> > not implemented that way, but that's a historical accident.  Simple
> > unions are a redundant concept.
> 
> Cool.  Or more concretely,
> 
> { 'union': 'Simple', 'data': { 'one': 'str', 'two': 'int' } }
> 
> is identical on the wire to:
> 
> { 'enum': 'MyEnum', 'data': ['one', 'two'] }
> { 'type': 'Base', 'data': { 'type': 'MyEnum' } }
> { 'type': 'Branch1', 'data': { 'data': 'str' } }
> { 'type': 'Branch2', 'data': { 'data': 'int' } }
> { 'union': 'Flat', 'base': 'Base', 'discriminator': 'type',
>   'data': { 'one': 'Branch1', 'two': 'Branch2' } }

Perhaps we should expose all unions for schema introspection in a way
similar to this (possibly not splitting out Branch1 and Branch2 as
independent types, though). We would just have to give a name to
implicitly generated enums and base types.

Kevin

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