Re: [Qemu-devel] [PATCH 08/23] block: Eliminate BlockDriverState member device_name[]

[Markus Armbruster]

Benoît Canet <address@hidden> writes:

> The Thursday 11 Sep 2014 à 07:00:41 (-0600), Eric Blake wrote :
>> On 09/11/2014 05:34 AM, Benoît Canet wrote:
>> > The Wednesday 10 Sep 2014 à 10:13:37 (+0200), Markus Armbruster wrote :
>> >> device_name[] is can become non-empty only in bdrv_new_named() and
>> >> bdrv_move_feature_fields().  The latter is used only to undo damage
>> >> done by bdrv_swap().  The former is called only by blk_new_with_bs().
>> >> Therefore, when a BlockDriverState's device_name[] is non-empty, then
>> >> it's owned by a BlockBackend.
>> 
>> [lots of lines trimmed - it's not only okay, but desirable to trim out
>> portions of a patch that you are okay with, in order to call attention
>> to the problem spots that you are commenting on without making the
>> reader have to scroll through pages of quoted context]
>> 
>> >>  
>> >> -const char *bdrv_get_device_name(BlockDriverState *bs)
>> >> +const char *bdrv_get_device_name(const BlockDriverState *bs)
>> >>  {
>> >> -    return bs->device_name;
>> >> +    const char *name = bs->blk ? blk_name(bs->blk) : NULL;
>> >> +    return name ?: "";
>> >>  }
>> > 
>> > Why not ?
>> > 
>> >     return bs->blk ? blk_name(bs->blk) : "";
>> 
>> If I understand right, it was because blk_name(bs->blk) may return NULL,
>
> It think it can't: see patch 2 extract:
>
>> +BlockBackend *blk_new(const char *name, Error **errp)
>> +{
>> +    BlockBackend *blk = g_new0(BlockBackend, 1);
>> +     
>> +    assert(name && name[0]);
>
>> but this function is guaranteed to return non-NULL.

You're right, blk_new() always returns a non-null, non-empty string.

The condition to check here is "bs is not owned by a BB".  Benoît's

    bs->blk ? blk_name(bs->blk) : ""

does that nicely.

Of course, Eric is right in that returning NULL on "not owned by a BB"
would be cleaner than returning "".  However, doing that in the middle
of a series with a four-digit diffstat doesn't strike me as a good
idea.  Better do it on top.