Re: Stable library ABI compatibility checking

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/02/2021 11:05, Jan Beulich wrote:
> On 11.02.2021 02:08, Andrew Cooper wrote:
>> Hello,
>>
>> Last things first, some examples:
>>
>> http://xenbits.xen.org/people/andrewcoop/abi/libxenevtchn-1.1_to_1.2.html
>> http://xenbits.xen.org/people/andrewcoop/abi/libxenforeignmemory-1.3_to_1.4.html
>>
>> These are an ABI compatibility report between RELEASE-4.14.0 and staging.
>>
>> They're performed with abi-dumper (takes a shared object and header
>> file(s) and write out a text "dump" file which happens to be a perl
>> hash) and abi-compliance-checker checker, which takes two dumps and
>> produces the HTML reports linked above.  They're both debian tools
>> originally, but have found their way across the ecosystem.  They have no
>> impact on build time (when invoked correctly).
>>
>> I'm encouraged to see that the foreignmem analysis has even spotted that
>> we deliberately renamed one parameter to clarify its purpose.
>>
>>
>> For the stable libraries, the RELEASE-$X.$Y.0 tag is the formal point
>> when accumulated changes in staging become fixed.  What we ought to be
>> doing is taking a "dump" of libraries at this point, and publishing
>> them, probably on xenbits.
>>
>> Subsequent builds on all the staging branches should be performing an
>> ABI check against the appropriate lib version.  This will let us catch
>> breakages in staging (c/s e8af54084586f4) as well as breakages if we
>> ever need to backport changes to the libs.
>>
>> For libraries wrapped by Juergen's tools/libs/ common-makefile changes,
>> adding ABI dumping is easy.  The only complicating is needing to build
>> everything with "-Og -g", but this is easy to arrange, and frankly ought
>> to be the default for debug builds anyway (the current use of -O0 is
>> silly and interferes with common distro hardening settings).
>>
>> What I propose is tweaking the library build to write out
>> lib$FOO.so.$X.$Y-$(ARCH).abi.dump files.  A pristine set of these should
>> be put somewhere on xenbits, and a task put on the release technicians
>> checklist for future releases.
>>
>> That way, subsequent builds which have these tools available can include
>> a call to abi-compliance-checker between the authoritative copy and the
>> one from the local build, which will make the report available, and exit
>> nonzero on breaking problems.
>>
>>
>> To make the pristine set, I need to retrofit the tooling to 4.14 and
>> ideally 4.13.  This is in contravention to our normal policy of not
>> backporting features, but I think, being optional build-time-only
>> tooling, it is worthy of an exception considering the gains we get
>> (specifically - to be able to check for ABI breakages on these branches
>> in OSSTest).  Backporting to 4.12 and older is far more invasive, due to
>> it being before the library build systems were common'd.
>>
>>
>> Anyway, thoughts?
> +1
>
> Not sure about the backporting effects - tools/libs/ had quite a bit
> less content in 4.14 and older, so the coverage would be smaller.

tools/libs/ has been the stable libraries, since IanC split them years
ago.  The only odd-one-out is libxenstored IIRC, which moved during the
4.15 window.

~Andrew