tl;dr we should force things through :)
There are at the time of writing two autopkgtest failures preventing glibc from migrating: r-cran-rgenoud and python-cffi
The r-cran-genoud failure is mystifying in that I have no idea how the glibc version is affecting the package (it doesn't call many libc or libm functions!). I've spent far too long trying on this but as the package has no rdeps we should probably just remove it and whoever brings it back can try to understand why it changes behaviour with new libc. (The test also looks a bit fragile, it is asserting that optimizing a particular function with the rng seeded a particular way finds a different maxima to seeding the rng a different way, afaict).
The python-cffi is exposing a bug that exists already. A minimal failing example is this:
root@cosmic32:~# cat n.py
from cffi.backend_ctypes import CTypesBackend
from cffi import FFI
ffi = FFI(backend=CTypesBackend())
ffi.cdef(""" void fabs(double x); """)
root@cosmic32:~# python3 n.py
Traceback (most recent call last):
File "n.py", line 7, in <module>
ValueError: math domain error
The bug is that when you lie to cffi about the return type of a variable, in particular when you say a function returning a float does not return a float, the x87 stack is not wiped to the extent required by the calling convention. glibc 2.28 rewrote the cos function in a way that apparently now cares about this (or I guess gcc now compiles the function in a way that cares about this) but make no mistake, it only ever worked by chance before. So I think the test_sin_no_return_value test should be skipped on i386. (I'll file an upstream issue I guess).