Re: [Qemu-devel] [PATCH v11 10/15] qapi: Emit structs used as variants in topological order

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> Right now, we emit the branches of union types as a boxed pointer,
> and it suffices to have a forward declaration of the type.  However,
> a future patch will swap things to directly use the branch type,
> instead of hiding it behind a pointer.  For this to work, the
> compiler needs the full definition of the type, not just a forward
> declaration, prior to the union that is including the branch type.
> This patch just adds topological sorting to hoist all types
> mentioned in a branch of a union to be fully declared before the
> union itself.  The sort is always possible, because we do not
> allow circular union types that include themselves as a direct
> branch

When a patch makes a "can't happen" claim, I normally ask for assertions
to make sure we go down in flames should it happen anyway.  But in this
case, the C compiler will shoot us down then, and that'll do nicely.

>        (it is, however, still possible to include a branch type
> that itself has a pointer to the union, for a type that can
> indirectly recursively nest itself - that remains safe, because
> that the member of the branch type will remain a pointer, and the
> QMP representation of such a type adds another {} for each recurring
> layer of the union type).
>
> Signed-off-by: Eric Blake <eblake@redhat.com>