Re: What does rpc.mountd dlopen() libnfsjunct.so rather than libnfsjunct.so.0

[Steve Dickson <SteveD@redhat.com>]

On 02/26/2014 12:10 PM, Simo Sorce wrote:
> On Wed, 2014-02-26 at 08:54 -0800, Chuck Lever wrote:
>> On Feb 26, 2014, at 8:25 AM, Simo Sorce <simo@redhat.com> wrote:
>>
>>> On Wed, 2014-02-26 at 08:02 -0800, Chuck Lever wrote:
>>>> On Feb 26, 2014, at 6:39 AM, Simo Sorce <simo@redhat.com> wrote:
>>>>
>>>>> On Wed, 2014-02-26 at 16:16 +1100, NeilBrown wrote:
>>>>>> See $SUBJ
>>>>>>
>>>>>> Shared libraries are usually versioned so you can release a new version with
>>>>>> an incompatible API and gradually transition to it.
>>>>>>
>>>>>> A rpc.mountd dlopens libnfsjunct.so with no version it is effectively
>>>>>> prohibited from ever changing the API in an incompatible way.
>>>>>>
>>>>>> Both Fedora and openSUSE get upset about packaging a libFOO.so in a non
>>>>>> "-devel" package and so trip over this library which clearly needs to be
>>>>>> installed even if you aren't doing 'devel'opment.
>>>>>
>>>>> Keep in mind this rule is there only for real shared libraries that are
>>>>> loaded by the the system loader.
>>>>>
>>>>> however it is waived for 'modules' that are opened dynamically but are
>>>>> private to the application.
>>>>>
>>>>>> I would like to change mountd as per the patch below to use the ".0" file.
>>>>>> I believe this will not break any installation as the ".so" is installed as a
>>>>>> symlink to the ".0" (or maybe ".0.0.0").
>>>>>>
>>>>>> Would this be acceptable?
>>>>>
>>>>> It looks to me like this is an internal module for mountd that is not
>>>>> for use by other apps (which is why it is not versioned and can be
>>>>> changed at will as it is deployed at the same time mountd is ?
>>>>
>>>> The plug-in API is versioned internally, but maybe I got that wrong,
>>>> and should remove the API version field in favor of having consumers
>>>> load via a specific .so number.
>>>
>>> Either way works the same, it just changes what component makes the
>>> determination (app code vs linker)
>>>
>>>>> Or am I wrong here ?
>>>>>
>>>>> If I am not wrong I would be against this change personally and would
>>>>> rather move the .so file in a private library dir (if it is not already
>>>>> there) to make it clear it is a private module.
>>>>
>>>> rpc.mountd is the only user currently, but it’s not necessarily
>>>> private to mountd.  A generic storage manager tool might use it to
>>>> resolve NFS and FedFS referrals for display, for example.  We could
>>>> add plug-in API functions for creating and removing referrals to
>>>> enable generic tools to perform these operations.
>>>
>>> If it is a generic library why is it dlopened() instead of being simply
>>> linked in at build time ?
>>
>> Handling NFS and FedFS junctions requires support for sqlite3, LDAP,
>> and XML, among others.  The maintainer of nfs-utils preferred to add
>> zero new build dependencies when we introduced this functionality.
>> The design we came up with was to dlopen() a library that would pull
>> in everything that was needed at run time.
>>
>> If the plug-in is not installed, mountd simply skips trying to resolve
>> junctions.  This would be the case for embedded NFS servers, for
>> example.
>>
> Therefore this is an intimate library and the separation is a mere
> exercise in keeping the ability to not drag in dependencies in some
> install scenarios.
> 
>>>> A separate directory makes sense if there’s more than one thing to put
>>>> in it.  Right now we just have the plug-in library, and no plans to
>>>> add more.
>>>
>>> directories are cheap, don't fear them :)
>>>
>>>> I took an expedient approach when implementing the plug-in, and could
>>>> have gotten it wrong.  I’m open to make this mechanism fit packaging
>>>> guidelines and requirements.
>>>
>>> Packaging guidelines vary depending on whether the library is public or
>>> private and therefore you need to guarantee ABI compatibility or not.
>>>
>>> I think you need to make that determination first.
> 
> I think based on the above that we are looking at a library that is
> currently just a private plugin. The best course of action IMHO is to
> move it to /usr/lib[64]/nfs-mountd or something so that it is clear that
> it is a private plugin. At least until mountd is the only user.
> 
>> I attempted to guarantee API compatibility using the API version field
>> and by publishing the API definition in a header under /usr/include.
>> By that definition it is a public API that happens to have only one
>> current user.
> 
> API compatibility is all you need for a private plugin indeed, and
> perhaps not even that.
> 
> However for a public library what would matter is ABI compatibility, not
> API compatibility.
> 
> Given it is a lot of effort to guarantee a public API and that there
> really is no other user on the horizon I would recommend to consider
> this code a private plugin and treat it as such.
> 
> That is:
> 1. consider it tightly integrated with rpc.mountd and to be installed in
> lockstep
> 2. consider it's API/ABI not stable and a private contract within
> rpc.mountd
> 3. package it accordingly
No... I do not want to make rpc.mount and the libnfsjunct.so tightly
coupled... Again, the use of it... if it exists... paradigm is good...

steved.

> 
> Simo.
>