[libvirt] [PATCH] libvirt_public.syms: Fix virDomainSetBlockThreshold placement

[Michal Privoznik]

On 04/04/2017 11:56 AM, Daniel P. Berrange wrote:
> On Tue, Apr 04, 2017 at 09:42:47AM +0200, Michal Privoznik wrote:
>> On 04/04/2017 09:27 AM, Jiri Denemark wrote:
>>> On Tue, Apr 04, 2017 at 09:24:41 +0200, Michal Privoznik wrote:
>>>> The symbol was introduced in the 3.2.0 release (bb09798fbeb5f)
>>>> and not 3.1.0 release as its current placement in the file
>>>> suggests.
>>>>
>>>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>>>> ---
>>>>  src/libvirt_public.syms | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/libvirt_public.syms b/src/libvirt_public.syms
>>>> index 428cf2e..37a8a18 100644
>>>> --- a/src/libvirt_public.syms
>>>> +++ b/src/libvirt_public.syms
>>>> @@ -755,8 +755,12 @@ LIBVIRT_3.0.0 {
>>>>
>>>>  LIBVIRT_3.1.0 {
>>>>      global:
>>>> -        virDomainSetBlockThreshold;
>>>>          virDomainSetVcpu;
>>>>  } LIBVIRT_3.0.0;
>>>>
>>>> +LIBVIRT_3.2.0 {
>>>> +     global:
>>>> +        virDomainSetBlockThreshold;
>>>> +} LIBVIRT_3.1.0;
>>>> +
>>>>  # .... define new API here using predicted next version number ....
>>>
>>> Oh, this is unfortunate. But doesn't this change break ABI?
>>
>>
>> Well, the change I'm referring to in the commit message broke ABI in the
>> first place. This will break ABI only for apps linked with 3.2.0 release and
>> un-break apps linked with every other release.
>
> No, the original change doesn't break ABI at all. It merely added a new
> symbol to the existing ABI. This means that applications which used the
> new API would build, link & run just fine - they'll simply not get an
> automatic dependancy on libvirt 3.2.0 added to the RPM deps. By contrast,
> changing the version now, will definitely break ABI, since any apps which
> linked to libvirt 3.2.0 using this API will have unresolveable symbols at
> link load time.

The apps linked with 3.2.0 and using the SetBlockThreshold API would get 
a false dependency on 3.1.0 so downgrading libvirt from 3.2.0 to 3.1.0 
would break for them: rpm would allow it but in the end the app fails to 
load. But this is much narrower use case. I agree. So lets keep the code 
as is.

Michal