Re: [PATCH v4 4/7] nbd: Update qapi to support exporting multiple bitmaps

[Eric Blake <eblake@redhat.com>]

On 10/20/20 3:51 AM, Markus Armbruster wrote:

>> #define QAPI_LIST_ADD(list, element) do { \
>>     typeof(list) _tmp = g_new(typeof(*(list)), 1); \
>>     _tmp->value = (element); \
>>     _tmp->next = (list); \
>>     (list) = _tmp; \
>> } while (0)
>>
>>
>> Markus, thoughts on if we should publish this macro,
> 
> If it's widely useful.
> 
> "git-grep -- '->value ='" matches ~200 times.  A patch converting these
> to the macro where possible would make a strong case for having the
> macro.
> 
>>                                                      and if so, which
>> header would be best?
> 
> The macro is generic: @list's type may be any of the struct TYPEList we
> generate for the QAPI type ['TYPE'].
> 
> We don't want to generate this macro next to each of these struct
> definitions.  A non-generic macro would go there, but let's not generate
> almost a hundred non-generic macros where a single generic one can do
> the job.

Agreed.

> 
> The closest we have to a common base is GenericList.  It's in in
> visitor.h because it's only used by visitors so far.  Adding the macro
> next it is not so smart, because we don't want non-visitor code to
> include visitor.h just for this macro.

Also agreed.

> 
> Perhaps the macro should go into qapi/util.h, and perhaps GenericList
> and GenericAlternate should move there, too.

GenericList is easy, but GenericAlternate is harder: it would introduce
a cyclic declaration dependency (generated qapi-builtin-types.h includes
qapi/util.h for the definition of QEnumLookup, but qapi/util.h declaring
GenericAlternate would depend on including qapi-builtin-types.h for the
definition of QType).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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