random: Fix multithread-safety bug in general

[Bruno Haible]

Since CheriBSD is derived from FreeBSD, it sounds likely that FreeBSD's random()
function is equally not multithread-safe.

To verify this hypothesis, I wrote a MT-safety test for random(). The result
(with a REPEAT_COUNT of 1000000):

glibc         OK
musl libc     OK
FreeBSD 11.0  Expected value #932913 not found in multithreaded results. (18 
failures among 100 runs)
FreeBSD 13.2  Expected value #712890 not found in multithreaded results. (2 
failures among 100 runs)
NetBSD        OK
OpenBSD       Expected value #0 not found in multithreaded results. => SKIP
macOS         Expected value #16 not found in multithreaded results.
AIX           OK
Solaris10     Expected value #4 not found in multithreaded results.
Solaris11     Expected value #46 not found in multithreaded results.
Cygwin 2.9.0  Expected value #2499 not found in multithreaded results.
Cygwin 3.4.6  Expected value #367 not found in multithreaded results.
mingw         OK (uses gnulib implementation)
MSVC          OK (uses gnulib implementation)
Haiku         Expected value #188587 not found in multithreaded results. (1 
failure among 100 runs)
Minix         SKIP
Android       OK

So, in general, it requires 100'000'000 repetitions to determine
whether the function is MT-safe. But that's too long for a unit test.
I'm therefore reducing it to 100'000 repetitions and hardcoding the
known platforms in the configure test. Even these 100'000 repetitions
take 3 seconds on Haiku or 1 second on mingw or MSVC.


2023-11-10  Bruno Haible  <bruno@clisp.org>

        doc: Mention an srandom limitation on OpenBSD.
        * doc/posix-functions/srandom.texi: Mention the OpenBSD limitation.

2023-11-10  Bruno Haible  <bruno@clisp.org>

        random tests: Add multithread-safety test.
        * tests/test-random-mt.c: New file.
        * modules/random-tests (Files): Add it.
        (Depends-on): Add xalloc, thread, yield.
        (Makefile.am): Also build and test test-random-mt.

        random: Fix multithread-safety bug in general.
        * m4/random.m4 (gl_FUNC_RANDOM): Override also macOS, FreeBSD, Solaris,
        Cygwin, Haiku.
        * doc/posix-functions/random.texi: Mention the wider scope of the
        multithread-safety bug.

0001-random-Fix-multithread-safety-bug-in-general.patch
Description: Text Data

0002-random-tests-Add-multithread-safety-test.patch
Description: Text Data

0003-doc-Mention-an-srandom-limitation-on-OpenBSD.patch
Description: Text Data