tests: Fix test FormatDateTime with invalid formats on macOS/*BSD

[yhabteab]

strftime(3) seems to return the provided invalid format specifiers on all Platforms, i.e even on macOS, but the macOS/*BSD libc implementations of strftime(3) appears to behave differently and causes the % character not to be populated into the results buffer if invalid format specifiers such as "x %! y" are given. If such specifiers are provided, the output will contain x ! y instead of the given invalid format specifiers.

See https://opensource.apple.com/source/Libc/Libc-1439.40.11/stdtime/FreeBSD/strftime.c.auto.html

[oxzi]

First, thanks for fixing the tests on certain platforms.

However, this is not a macOS bug, but a difference in the strftime implementation between different libcs. I created a small test program for x %! y and got the same output on a Linux with glibc and the "shorter output" x ! y on an OpenBSD.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
  char *buf;
  time_t t;
  struct tm *tmp;

  t = time(NULL);
  if (! (tmp = localtime(&t))) {
    perror("localtime");
    return EXIT_FAILURE;
  }

  if (! (buf = malloc(BUFSIZ))) {
    perror("malloc");
    return EXIT_FAILURE;
  }

  if (! (strftime(buf, BUFSIZ, "x %! y", tmp))) {
    perror("strftime");
    return EXIT_FAILURE;
  }

  printf("out: \"%s\"\n", buf);
  free(buf);
}

$ uname -sr
OpenBSD 7.6
$ cc -Wall -Werror -pedantic a.c
$ ./a.out
out: "x ! y"

$ uname -sr
Linux 6.10.8
$ cc -Wall -pedantic a.c
a.c: In function ‘main’:
a.c:21:36: warning: unknown conversion type character ‘!’ in format [-Wformat=]
   21 |   if (! (strftime(buf, BUFSIZ, "x %! y", tmp))) {
      |                                    ^
$ ./a.out
out: "x %! y"

Based on strftime from POSIX.1-2017, "[i]f a conversion specification does not correspond to any of the above, the behavior is undefined."

Thus, don't put the blame on 🍎 and please reword this PR and allow the other implementation chosen by macOS and OpenBSD.

[oxzi]

Please rename the variable - s/darwinBug/percentLessOutput/g or the like - and change the commit message.