"set -m" stalls shell when run in a new process group

[Julio Merino <jmmv%julipedia.org@localhost>]

Hello,

I am not sure what the right behavior is here, but I am seeing this with 
/bin/sh and /bin/ksh but not bash.

I have a program (atf) that spawns a child process and sets the child 
process to run in its own process group.  This child process spawns a 
shell and the shell script that it runs does "set -m".

At this point, the shell blocks.  In the case of /bin/sh, the block 
happens at the "killpg(0, SIGTTIN);" line.

What is the expected behavior of this?  Are sh and ksh correct in 
stalling, or is bash?  Is this a bug?  (If the latter, I'll be glad to 
file a PR and add a test case.)

Thanks.