[Bug 252579] fork() causes process to hang in rare circumstances.

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252579

            Bug ID: 252579
           Summary: fork() causes process to hang in rare circumstances.
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: threads
          Assignee: threads at FreeBSD.org
          Reporter: shamaz.mazum at gmail.com

Created attachment 221461
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=221461&action=edit
NSS module and test program

This bug is discovered on FreeBSD 13-CURRENT but can also be reproduced on
12.2-RELEASE. It causes a process to hung when fork(2) is called and specific
NSS (Name Switch Service) module is used.

How to reproduce:

1) Download archive in the attachment.
2) Compile NSS stub module (do not forget .1 at the end of compiled module):
   cc -shared -fPIC -pthread -o nss_stub.so.1 nss_stub.c
3) Copy nss_stub.so.1 to /usr/local/lib
4) Edit /etc/nsswitch.conf and replace 'hosts: files dns' witch 'hosts: files
dns stub'
5) Compile test program: cc -o bug bug.c
6) Run it, it will hang, so even killall -9 bug won't kill it.

There is a small and unpleasant discussion on freebsd-net mailing list with
Konstantin Belousov who wanted me to reproduce this bug without editing
/etc/nsswitch.conf I think it's either impossible, because NSS system is
somehow messing with fork, or it's beyond my competence. So the provided way to
reproduce the bug is as minimal as I can get.

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