[Libguestfs] [PATCH libnbd 3/4] lib: Add FREE_CALLBACK macro.

[Eric Blake]

On 8/13/19 5:37 PM, Richard W.M. Jones wrote:
> Simple macro encapsulating the process for freeing a callback.
> ---
>  generator/states-reply-simple.c     |  4 +--
>  generator/states-reply-structured.c | 42 +++++++++--------------------
>  generator/states-reply.c            |  4 +--
>  generator/states.c                  |  4 +--
>  lib/aio.c                           | 17 ++++--------
>  lib/debug.c                         |  6 +----
>  lib/internal.h                      | 14 ++++++++++
>  7 files changed, 35 insertions(+), 56 deletions(-)
> 

Handy if we keep lots of .free() sites, but less useful if we only .free
during retirement (see my commentary in 2).

> +++ b/lib/internal.h
> @@ -273,6 +273,20 @@ struct command {
>    uint32_t error; /* Local errno value */
>  };
>  
> +/* Free a callback.
> + *
> + * Note this works for any type of callback because the basic layout
> + * of the struct is the same for all of them.  Therefore casting cb to
> + * nbd_completion_callback does not change the effective code.
> + */
> +#define FREE_CALLBACK(cb)                                               \
> +  do {                                                                  \
> +    nbd_completion_callback *_cb = (nbd_completion_callback *)&(cb);    \
> +    if (_cb->callback != NULL && _cb->free != NULL)                     \
> +      _cb->free (_cb->user_data);                                       \
> +    _cb->callback = NULL;                                               \
> +  } while (0)
> +

Some nasty type-punning (probably violates strict C) but I don't see any
problem with it working in practice, if we still want to keep it.

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

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