lib/56148: lib/libc/stdio/t_printf:snprintf_float test randomly fails

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         56148
>Category:       lib
>Synopsis:       lib/libc/stdio/t_printf:snprintf_float test randomly fails
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu May 06 07:30:00 +0000 2021
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current, also -9
>Organization:

>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The snprintf_float test case of the lib/libc/stdio/t_printf test
program randomly fails with a SIGSEGV once in a few thousand runs.

One such failure was recorded by the new NetBSD-9/amd64 testbed:

  http://releng.netbsd.org/b5reports/amd64-9/2021/2021.04.30.13.54.00/test.html#lib_libc_stdio_t_printf_snprintf_float

I have reproduced this in -current and on real amd64 hardware.

>How-To-Repeat:

cd /usr/tests/lib/libc/stdio
while ./t_printf snprintf_float; do true; done >log 2>&1
tail log

This typically takes more than a minute but less than an hour to fail.

>Fix: