Bug#858529: libc6: fgets repeats content after fork on stretch only

[Florian Weimer <fw@deneb.enyo.de>]

tags 858529 upstream
forwarded 858529 https://sourceware.org/bugzilla/show_bug.cgi?id=20598
thanks

* Neil Spring:

>   if(fork() == 0) { exit(1); }

exit flushes the stdio buffers in the child.  Upstream concluded that
this leads to undefined behavior:

| Yes, this is about the exit actually.  But reading "2.5.1
| Interaction of File Descriptors and Standard I/O Streams", I think
| this is really undefined, because the required action is not
| performed before the call to fork, and the correct fix is to use
| _exit in the forked child.

<https://sourceware.org/bugzilla/show_bug.cgi?id=20598#c2>