[dpdk-dev] How to manage new APIs added after major ABI release?

[Bruce Richardson]

On Tue, Dec 10, 2019 at 03:03:51PM +0000, Luca Boccassi wrote:
> On Tue, 2019-12-10 at 14:36 +0000, Bruce Richardson wrote:
> > On Tue, Dec 10, 2019 at 12:40:53PM +0000, Ferruh Yigit wrote:
> > > On 12/10/2019 12:04 PM, Bruce Richardson wrote:
> > > > On Tue, Dec 10, 2019 at 11:56:28AM +0000, Ferruh Yigit wrote:
> > > > > Hi,
> > > > > 
> > > > > With new process, the major ABI releases will be compatible
> > > > > until it is
> > > > > deprecated (until next LTS for now),
> > > > > like current ABI version is 20 in DPDK_19.11 and DPDK versions
> > > > > until DPDK_20.11
> > > > > will be ABI compatible with this version.
> > > > > 
> > > > > But if we introduce a new API after major ABI, say in 20.02
> > > > > release, are we
> > > > > allowed to break the ABI for that API before DPDK_20.11?
> > > > > 
> > > > > If we allow it break, following problem will be observed:
> > > > > Assume an application using .so.20.1 library, and using the new
> > > > > API introduced
> > > > > in 20.02, lets say foo(),
> > > > > but when application switches to .so.20.2 (released via
> > > > > DPDK_20.05), application
> > > > > will fail because of ABI breakage in foo().
> > > > > 
> > > > > I think it is fair that application expects forward
> > > > > compatibility in minor
> > > > > versions of a shared library.
> > > > > Like if application linked against .so.20.2, fair to expect
> > > > > .so.20.3, .so.20.4
> > > > > etc will work fine. I think currently only .so.20.0 is fully
> > > > > forward compatible.
> > > > > 
> > > > > If we all agree on this, we may need to tweak the process a
> > > > > little, but before
> > > > > diving into implementation details, I would like to be sure we
> > > > > are in same page.
> > > > > 
> > > > 
> > > > Well, any new API's generally come in as experimental, in which
> > > > case
> > > > changes are allowed, and breakage can be expected. If they are
> > > > not
> > > > experiemental, then the ABI policy applies to them in that they
> > > > cannot
> > > > change since they are part of the .21 ABI, even if that ABI is
> > > > not fully
> > > > complete yet. For any application only using stable, non-
> > > > experimental
> > > > functions, forward compatibility must be maintained IMHO.
> > > > 
> > > 
> > > Talking about not experimental APIs, experimental ones free from
> > > the process.
> > > 
> > > And when and API added in 20.02 (ABI_20.1) it is kind of still
> > > ABI_20, because
> > > it should be supported for following ABI_20.x, instead of calling
> > > it ABI_21, and
> > > this minor tweak (and mind shift) in .map files can be our
> > > solution.
> > 
> > Related at what to do with adding versions between major ABI
> > versions, when
> > investigating with Kevin the ABI checking we have made an unpleasant
> > discovery:
> > 
> > This minor version bumping from 20.0 to 20.1 has apparently already
> > broken
> > our ABI according to libabigail.
> > 
> > The Gory Details [skip to the end for suggestions to fix]
> > ------------------------------------------------------------
> > 
> > The reason for this is that the soversion encoded in each library -
> > whether
> > built with meson or make - is the full 20.0 version, not just the
> > major ABI
> > .20 part. Then when apps link against DPDK, they actually encode the
> > 20.0.
> > 
> > So what this means is that currently - using a make build as an
> > example
> > here - ldd on the latest head build gives:
> > 
> >  LD_LIBRARY_PATH=$(pwd)/x86_64-native-linux-gcc/lib ldd x86_64-
> > native-linux-gcc/app/testpmd | head
> >         linux-vdso.so.1 (0x00007fff6813d000)
> >         librte_pmd_bond.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_bond.so.20.1 (0x00007f36d723c000)
> >         librte_pmd_dpaa.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_dpaa.so.20.1 (0x00007f36d7229000)
> >         librte_mempool_dpaa.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_mempool_dpaa.so.20.1 (0x00007f36d7224000)
> >         librte_pmd_ixgbe.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_ixgbe.so.20.1 (0x00007f36d71ba000)
> >         librte_pmd_i40e.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_i40e.so.20.1 (0x00007f36d7126000)
> >         librte_pmd_bnxt.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_bnxt.so.20.1 (0x00007f36d70e5000)
> >         librte_pmd_softnic.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pmd_softnic.so.20.1 (0x00007f36d70b7000)
> >         librte_flow_classify.so.0.201 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_flow_classify.so.0.201
> > (0x00007f36d70b1000)
> >         librte_pipeline.so.20.1 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc/lib/librte_pipeline.so.20.1 (0x00007f36d7088000)
> > ...
> > 
> > Similarly ldd on a 19.11 checkout gives:
> > 
> > LD_LIBRARY_PATH=$(pwd)/x86_64-native-linux-gcc_v19.11/lib ldd x86_64-
> > native-linux-gcc_v19.11/app/testpmd | head
> >         linux-vdso.so.1 (0x00007ffc2a964000)
> >         librte_pmd_bond.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_bond.so.20.0
> > (0x00007fd4dc6b6000)
> >         librte_pmd_dpaa.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_dpaa.so.20.0
> > (0x00007fd4dc6a3000)
> >         librte_mempool_dpaa.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_mempool_dpaa.so.20.0
> > (0x00007fd4dc69e000)
> >         librte_pmd_ixgbe.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_ixgbe.so.20.0
> > (0x00007fd4dc634000)
> >         librte_pmd_i40e.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_i40e.so.20.0
> > (0x00007fd4dc5a0000)
> >         librte_pmd_bnxt.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_bnxt.so.20.0
> > (0x00007fd4dc55d000)
> >         librte_pmd_softnic.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pmd_softnic.so.20.0
> > (0x00007fd4dc531000)
> >         librte_flow_classify.so.0.200 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_flow_classify.so.0.200
> > (0x00007fd4dc52b000)
> >         librte_pipeline.so.20.0 => /home/bruce/dpdk.org/x86_64-
> > native-linux-gcc_v19.11/lib/librte_pipeline.so.20.0
> > (0x00007fd4dc502000)
> > 
> > The final check - using the 19.11 compiled testpmd with the library
> > path
> > set to 20.02 versionned libs:
> > 
> > LD_LIBRARY_PATH=$(pwd)/x86_64-native-linux-gcc/lib ldd x86_64-native-
> > linux-gcc_v19.11/app/testpmd | head
> >         linux-vdso.so.1 (0x00007ffc711fc000)
> >         librte_pmd_bond.so.20.0 => not found
> >         librte_pmd_dpaa.so.20.0 => not found
> >         librte_mempool_dpaa.so.20.0 => not found
> >         librte_pmd_ixgbe.so.20.0 => not found
> >         librte_pmd_i40e.so.20.0 => not found
> >         librte_pmd_bnxt.so.20.0 => not found
> >         librte_pmd_softnic.so.20.0 => not found
> >         librte_flow_classify.so.0.200 => not found
> >         librte_pipeline.so.20.0 => not found
> > 
> > Fixing This
> > -----------
> > 
> > To fix this, we need to ensure that the SONAME remains constant
> > across the
> > releases. Therefore, I currently see two options:
> > 
> > 1. keep 20.0 as the version and soname across all releases in 2020,
> > i.e.
> >   just revert the ABIVERSION change patch. Trouble there is how to
> > track
> >   20.02 vs 20.05 etc. etc.
> > 
> > 2. remove the .0, .1 from the SONAMES stored in the libraries. This
> > has the
> >   advantage of keeping the existing planned schemes, but has the
> > really big
> >   downside of breaking ABI compatibility with anyone who has already
> >   compiled with 19.11.
> > 
> > Personally, of the two options - unless someone can come up with a
> > third
> > option - I'd tend towards the second, fixing the builds to remove the
> > .0 in
> > the soname, and releasing that ASAP as 19.11.1 before 19.11 gets
> > widespread
> > adoption. Since this ABI stability is new, teething problems may be
> > expected.
> > 
> > Thoughts and comments?
> > /Bruce
> > 
> > BTW: For meson, the patch for option 2 is just to remove the
> > so_version
> > variable and all references to it from lib/meson.build and
> > drivers/meson.build. Haven't looked into a "make" fix yet.
> 
> Hi,
> 
> With libtool and its (arguably arcane) format, only the first digit is
> the ABI current version and gets encoded in the elf header. The other
> digits can be used to track compatible minor increments, and are mostly
> ignored. On the system a symlink libfoo.so.major ->
> libfoo.so.major.minor is added.
> 
> Eg:
> 
> $ readelf -d /usr/lib/x86_64-linux-gnu/libzmq.so.5.2.3 | grep SONAME
>  0x000000000000000e (SONAME)             Library soname: [libzmq.so.5]
> $ ls -l /usr/lib/x86_64-linux-gnu/libzmq.so.5
> lrwxrwxrwx 1 root root 15 Dec 31  2014 /usr/lib/x86_64-linux-
> gnu/libzmq.so.5 -> libzmq.so.5.2.3
> 
> Can we do the same? Not sure what the right incantation is for Meson,
> but it should be possibly.
>

That's essentially option 2, and it's still an ABI break because existing
builds of 19.11 have the soname will the full version number in it. The
default behaviour for meson is exactly how you described it, except that
previously we needed more exact control over the version info (for your
dpdk-specific versions in the sonames) and so overrode the soversion
explicitly. The fix for meson is to remove this overriding i.e. remove
"soversion:" parameter for each shared_library() call.

 
> Also, we should leave the current at 20.0 - let's not break
> compatibility already, please :-)
> 

If we do this, maybe we can use 20.0.1 and 20.0.2 version numbers?