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

From: Markus Armbruster
Subject: Re: [PATCH v4 4/7] nbd: Update qapi to support exporting multiple bitmaps
Date: Tue, 20 Oct 2020 10:51:37 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux)

Eric Blake <eblake@redhat.com> writes:

> On 10/14/20 7:15 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 10.10.2020 00:55, Eric Blake wrote:
>>> Since 'nbd-server-add' is deprecated, and 'block-export-add' is new to
>>> 5.2, we can still tweak the interface.  Allowing 'bitmaps':['str'] is
>>> nicer than 'bitmap':'str'.  This wires up the qapi and qemu-nbd
>>> changes to permit passing multiple bitmaps as distinct metadata
>>> contexts that the NBD client may request, but the actual support for
>>> more than one will require a further patch to the server.
>>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>> ---
>> [..]
>>>               break;
>>>           case 'B':
>>> -            bitmap = optarg;
>>> +            tmp = g_new(strList, 1);
>>> +            tmp->value = g_strdup(optarg);
>>> +            tmp->next = bitmaps;
>>> +            bitmaps = tmp;
>> If publish QAPI_LIST_ADD, defined in block.c, it would look like:
>>      QAPI_LIST_ADD(bitmaps, g_strdup(optarg));
> #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

>                                                      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.

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.

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


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