use Perl

Here's some behaviour that surprised me. I would expect this program to loop forever:

#!/usr/bin/perl

use strict;
use warnings;

my $file1 = './test_file_1';
my $file2 = './test_file_2';

my $count = 1;
while(1) {
    unlink($file1);                      # create first file
    open(FILE, ">$file1");
    close(FILE);

    my $t0 = (stat($file1))[9];          # wait
    while(time() <= $t0) {
        sleep 1;
    }
    my $t1 = time();

    unlink($file2);                      # create second file
    open(FILE, ">$file2");
    close(FILE);

    if((stat($file2))[9] <= (stat($file1))[9]) {
        die sprintf(
            "Timestamps: %u %u  t0: %u t1: %u\n",
            (stat($file2))[9], (stat($file1))[9], $t0, $t1
        );
    }
    printf("Loop count = %u\n", $count++);
}

Given that test_file_2 is not created until the system clock is greater than the modification time on test_file_1, I would expect that the die statement would never be reached. In fact, it fails for me on multiple systems after running for a small number of minutes.

My test systems are all running Linux, and using local filesystems. I have seen this sort of funky effect when using network filesystems, presumably due to clock drift between client and fileserver. I've also run into problems where updating a file on a local NTFS partition under Windows would either not update the time or would update it to have an earlier value.

This came up because Andreas Koenig did some extensive testing to highlight random failures in XML::Simple's text suite. Now that I know it's the test itself that's at fault I just need to make it a bit more robust.