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

[Bruce Richardson <bruce.richardson@intel.com>]

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.