Re: [PATCH 1/2] qapi: introduce forwarding visitor

[Eric Blake <eblake@redhat.com>]

On Mon, Jul 19, 2021 at 12:40:32PM +0200, Paolo Bonzini wrote:
> This new adaptor visitor takes a single field of the adaptee, and exposes it
> with a different name.
> 
> This will be used for QOM alias properties.  Alias targets can of course
> have a different name than the alias property itself (e.g. a machine's
> pflash0 might be an alias of a property named 'drive').  When the target's
> getter or setter invokes the visitor, it will use a different name than
> what the caller expects, and the visitor will not be able to find it
> (or will consume erroneously).

How does this differ from Kevin's attempt to add QMP aliasing,
https://lists.gnu.org/archive/html/qemu-devel/2021-02/msg04097.html

> 
> The solution is for alias getters and setters to wrap the incoming
> visitor, and forward the sole field that the target is expecting while
> renaming it appropriately.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

> diff --git a/include/qapi/forward-visitor.h b/include/qapi/forward-visitor.h
> new file mode 100644
> index 0000000000..c7002d53e6
> --- /dev/null
> +++ b/include/qapi/forward-visitor.h

> +
> +typedef struct ForwardFieldVisitor ForwardFieldVisitor;
> +
> +/*
> + * The forwarding visitor only expects a single name, @from, to be passed for
> + * toplevel fields.  It is converted to @to and forward to the @target visitor.

and forwarded

> + * Calls within a struct are forwarded without changing the name.
> + */
> +Visitor *visitor_forward_field(Visitor *target, const char *from, const char *to);
> +
> +#endif

> +++ b/qapi/qapi-forward-visitor.c
> @@ -0,0 +1,307 @@
> +/*
> + * Forward Visitor
> + *
> + * Copyright (C) 2021 Red Hat, Inc.
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2.1 or later.
> + * See the COPYING.LIB file in the top-level directory.
> + *
> + */
> +
> +#include "qemu/osdep.h"
> +#include <math.h>

Did you really use math.h?


> +
> +static void forward_field_complete(Visitor *v, void *opaque)
> +{
> +    /*
> +     * Do nothing, the complete method will be called at due time

s/at/in/

> +     * on the target visitor.
> +     */
> +}
> +

> +++ b/tests/unit/test-forward-visitor.c
> @@ -0,0 +1,165 @@
> +/*
> + * QAPI Forwarding Visitor unit-tests.

Otherwise it looks like it does what it advertises.  I don't know if
Markus will have any opinions, but I'm comfortable with:

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org