Undefined use of weak symbols in gnulib

[Paul Eggert]

On 4/26/21 10:53 PM, Florian Weimer via Libc-alpha wrote:
> This will become an urgent issue with glibc 2.34, which defines
> pthread_mutexattr_gettype unconditionally.  Certain gnulib modules will
> stop working until the binaries are relinked.

Thanks for mentioning this. But in what sense will the modules stop 
working? I'm a little lost.

glibc 2.34 also defines pthread_once unconditionally, right? So why 
doesn't code like this:

   if (pthread_mutexattr_gettype != NULL)
     pthread_once (control, callback);

continue to work in 2.34?