use strict;
use warnings;
use Carp;
use File::Copy;
use Fcntl qw/LOCK_SH LOCK_EX LOCK_NB LOCK_UN/;

sub x_copy # NOT USED
{
  my ($src, $dst) = @_;
  open (my $f, "<", $src) or confess;
  binmode $f;
  read $f, my $buf, -s $f or confess ;
  close $f;
  open (my $o, ">", $dst) or confess;
  binmode $o;
  print $o $buf;
  close $o;
}


sub getlock
{
  open my $f, ">", "lock.tmp" or confess;
  flock $f, LOCK_EX or confess;
  shift->();
  flock $f, LOCK_UN or confess;
  close $f;
}


for (1..5) {
 unless (fork()) {
 for (1..5) {
  my $filename = "somefile$_.tmp";
  my $f=undef;
  getlock sub {
   unless (-e $filename) {
    open ($f, ">", $filename) or confess;
    binmode $f;
   }
  };
  if ($f) {
  print ($f "x") for (1..40_000_000);
  close $f;
  }
  my $newfilename = "c_${$}_$filename";
  unless (-e $newfilename) {
    copy($filename, $newfilename) or confess;
    die if -s $filename != -s $newfilename;
  }
 }
 exit;
 }
}

while (wait != -1) { };
[download]

outputs the following (tested on Linux, perl 5.10 and 5.18):

-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24171_somefile1.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24171_somefile2.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24171_somefile3.tmp
-rw-r--r-- 1 vse vse 32288768 2013-09-28 19:10 c_24171_somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24171_somefile5.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24172_somefile1.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24172_somefile2.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24172_somefile3.tmp
-rw-r--r-- 1 vse vse 28540928 2013-09-28 19:10 c_24172_somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24172_somefile5.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24173_somefile1.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24173_somefile2.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24173_somefile3.tmp
-rw-r--r-- 1 vse vse 21893120 2013-09-28 19:10 c_24173_somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24173_somefile5.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24174_somefile1.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24174_somefile2.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24174_somefile3.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24174_somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24174_somefile5.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24175_somefile1.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24175_somefile2.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24175_somefile3.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 c_24175_somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 c_24175_somefile5.tmp
-rw-r--r-- 1 vse vse        0 2013-09-28 19:10 lock.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 somefile1.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 somefile2.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 somefile3.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 somefile4.tmp
-rw-r--r-- 1 vse vse 40000000 2013-09-28 19:10 somefile5.tmp

So:
1. file size of some c_* files is less than 40000000, however this should not happen, because of line:

die if -s $filename != -s $newfilename;
[download]

2. If I replace copy() with x_copy() call, the following line reports error (or eof):

read $f, my $buf, -s $f or confess ;
[download]

3. there is locking via getlock() where it should be.
4. all files are closed, so buffering is not an issue?

5. this code works, with 1 child process, but fails on concurrent processes.

Question: I assume, after file is closed, it's written to filesystem Am I right? If yes, then I have to suspect some bug in perl, please help me find where I am wrong.
Initially I had this issue in a bigger program, but now I simplified it to this proof-of-concept code which reproduce issue.

UPD: Solved: Re^2: Strange IO + concurrency issue

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