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Re: [Qemu-block] [PATCH 2/2] qapi: deprecate implicit filters

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 2/2] qapi: deprecate implicit filters
Date: Wed, 28 Aug 2019 09:20:51 +0000

27.08.2019 23:12, John Snow wrote:
> On 8/23/19 5:22 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 14.08.2019 13:07, Vladimir Sementsov-Ogievskiy wrote:
>>> To get rid of implicit filters related workarounds in future let's
>>> deprecate them now.
>> Interesting, could we deprecate implicit filter without deprecation of 
>> unnecessity of
>> parameter? As actually, it's good when this parameter is not necessary, in 
>> most cases
>> user is not interested in node-name.
> https://en.wiktionary.org/wiki/unnecessity -- I am surprised to learn
> that this a real word in the language I speak. :)
> I assume you're referring to making the optional argument mandatory.

exactly, it's my a bit "google-translate-driven" English)

>> Obviously we can do the following:
>> 1. In 4.2 we deprecate unnecessity, which implies deprecation of implicit 
>> filters
>> 2. After some releases in 4.x we can drop deprecated functionality, so we 
>> drop it together with
>> implicit filters. And, in same release 4.x we return it back (as it's 
>> compatible change :)
>> but without implicit filters (so, if filter-node-name not specified, we just 
>> create
>> explicit filter with autogenerated node-name)
>> So, effectively we just drop "deprecation mark" together with implicit 
>> filters, which is nice
>> but actually confusing.
>> Instead, we may do
>> 1. In 4.2 deprecate
>> 2. In 4.x drop optionality together with implicit filters
>> 3. In 4.y (y > x of course) return optionality back
> Ah, I see what you're digging at here now...
>> It's a bit safer, but for users who miss releases [4.x, 4.y) it's no 
>> difference..
>> Or we just write in spec, that implicit filters are deprecated? But we have 
>> nothing about implicit
>> filters in spec. More over, we directly write that we have filter, and if 
>> parameter is omitted
>> it's node-name is autogenerated. So actually, the fact the filter is hidden 
>> when filter-node-name is
>> unspecified is _undocumented_.
>> So, finally, it looks like nothing to deprecated in specification, we can 
>> just drop implicit filters :)
>> What do you think?
> What exactly _IS_ an implicit filter? How does it differ today from an
> explicit filter? I assumed the only difference was if it was named or
> not; but I think I must be mistaken now if you're proposing leaving the
> interface alone entirely.
> Are they instantiated differently?

As I understand, the only difference is their BlockDriverState.impicit field, 
and several places in code
where we skip implicit filter when trying to find something in a chain starting 
from a device.

Hmm, OK, let's see:

1. the only implicit filters are commit_top and mirror_top if user don't 
specify filter-node-name.

Where it make sense, i.e., where implicit field used?

2. bdrv_query_info, bdrv_query_bds_stats, bdrv_block_device_info(only when 
called from bdrv_query_info), they'll
report filter as top node if we don't mark it implicit.

3. bdrv_refresh_filename, bdrv_reopen_parse_backing, bdrv_drop_intermediate:
   I think it's not a problem, just drop special case for implicit fitlers

So, seems the only real change is query-block and query-blockstats output when 
mirror or commit is started
without specifying filter-node-name (filter would be on top)

So, how should we deprecate this, or can we just change it?

Best regards,

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