@proski I never saw any /sbin/sln issues during my upgrade, and no scriptlets in glibc use sln. We removed /sbin/sln on Jan 2018 (commit d8e1573f9c8c28f6390791c329512045f0c52868). Which means that no F29 release (branched 2018-02-20) has ever has /sbin/sln. I believe you have a unique configuration issue with your install that is triggering the use of sln and is not related to this update.
Update installs, and reboot succeeds without problem. Bug 1657395 is fixed: [carlos@localhost ~]$ ./test free(): double free detected in tcache 2 Aborted (core dumped) The new glibc detects the double free.
Tested glibc-2.28-23.fc29, installs fine, system is functional.
Test for #1647395 works now:
./test Pass: malloc success Info: ptr2 = ptr1 free(): double free detected in tcache 2 Aborted (core dumped)
New glibc detects double free in tcache.