From @jonathanstowe

This came up on IRC regarding what is "thread-safe" and what isn't in NativeCall​:

[jonathan@​cannibal ~]$ perl6 -e ' use NativeCall; await start { my $a = "dubba"; explicitly-manage($a); } xx 15'

Incompatible MROs in P6opaque rebless
  in block <unit> at -e line 1

I would speculate that the change that explicitly-manage makes to the string is somehow shared among the threads.

It could present a major problem to people trying to use a native sub with a "const char *" argument in a concurrent application.

This is with a straight up​:

This is Rakudo version 2015.12 built on MoarVM version 2015.12
implementing Perl 6.c

From @dogbert17

This seems to have been fixed as far as I can tell. Unfortunately the bisectbot was unable to show which commit might have solved the issue.
Tried it several times on my 32 bit Linux VM with 'Rakudo version 2017.09-348-g830084430 built on MoarVM version 2017.09.1-575-gd4e230a69' without any problems.

The RT System itself - Status changed from 'new' to 'open'