Bug#797989: fastjet: ABI transition needed for libstdc++ v5

[Simon McVittie]

Source: fastjet
Version: 3.0.6+dfsg-1
Severity: serious
Justification: causes rivet to FTBFS
Tags: sid stretch confirmed
User: debian-gcc at lists.debian.org
Usertags: libstdc++-cxx11
Control: block -1 by 791283

Background[1]: libstdc++6 introduces a new ABI to conform to the
C++11 standard, but keeps the old ABI to not break existing binaries.
Packages which are built with g++-5 from experimental (not the one
from testing/unstable) are using the new ABI.  Libraries built from
this source package export some of the new __cxx11 or B5cxx11 symbols,
dropping other symbols.  If these symbols are part of the API of
the library, then this rebuild with g++-5 will trigger a transition
for the library.

In the case of fastjet, the rivet source package FTBFS when built
against a version of yaml-cpp that starts *its* transition,
so I can confirm that a transition is definitely needed.
The transition normally consists of renaming the
affected library packages, adding a v5 suffix (libfastjet0v5, etc.).
The SONAME should not be changed when doing this.

If an upgrade to a new upstream SONAME is already planned, and that
SONAME has never been available in Debian compiled with g++-4, then an
alternative way to carry out the transition would be to bump the
SONAME. However, the libstdc++ transition has been going on for a
month already, and anything that makes it take longer is bad for Debian,
so introducing new upstream code is not desired at this stage.

These follow-up transitions for libstdc++ are not going through exactly
the normal transition procedure, because many entangled transitions are
going on at the same time, and the usual ordered transition procedure
does not scale that far. When all the C++ libraries on which this library
depends have started their transitions in unstable if required, this
library should do the same, closing this bug; the release team will deal
with binNMUs as needed.

Looking at the build-dependencies of fastjet, siscone needs a
library transition first; in fastjet, please add versioned Build-Depends
on the version of siscone where #791283 has been fixed.

The package might be NMU'd if there is no maintainer response. The
release team have declared a 2 day NMU delay[2] for packages involved
in the libstdc++ transition, in order to get unstable back to a usable
state in a finite time.

Regards,
    S

[1] https://wiki.debian.org/GCC5#libstdc.2B-.2B-_ABI_transition
[2] https://lists.debian.org/debian-devel-announce/2015/08/msg00000.html