Description:
------------
There appears to be a concurrency issue when mkdir is called on two separate threads with the following parameters:

Thread 1:
mkdir('/tmp/foo/bar/1', 0777, true);

Thread 2:
mkdir('/tmp/foo/bar/2', 0777, true);

Sometimes one of the threads (e.g. thread 2) two fails with "Warning: mkdir(): File exists" and the directory /tmp/foo/bar/2 does not exist afterwards.

What I think is happening in thread 2 is is this:
PHP looks for /tmp/foo/bar/2 and sees that it does not exist. Neither does /tmp/foo/bar and /tmp/foo. So it tries to create /tmp/foo, but that directory was just created a millisecond second ago by another thread, so mkdir gives instead of just moving on to create /tmp/foo/bar and /tmp/foo/bar/2.

Reproduce code:
---------------
This code reproduces the problem in about every second attempt. It consists of two files, index.php and iframe.php.


index.php:

<?php
$fire = time() + 2;
?>
<h1>Nr. 1</h1>
<iframe src="iframe.php?fire=<?= $fire ?>&amp;no=1"></iframe>
<h1>Nr. 2</h1>
<iframe src="iframe.php?fire=<?= $fire ?>&amp;no=2"></iframe>


iframe.php:

<?php
$fire = $_GET['fire'];
$no = $_GET['no'];
// synchronize the two threads
while ($fire > time());
$dir = '/tmp/' . $fire . '/foo/bar/foo/bar/' . $no;
//sleep($no);
var_dump(is_dir($dir));
echo "Making $dir ";
mkdir($dir, 0777, true);
var_dump(is_dir($dir));
?>


Expected result:
----------------
Thread 2 should output this:
bool(false) Making /tmp/1132653651/foo/bar/foo/bar/2 bool(true)


Actual result:
--------------
Thread 2 sometimes outputs this:
bool(false) Making /tmp/1132655181/foo/bar/foo/bar/2 Warning: mkdir(): File exists in /home/chsc/public_html/mkdir/iframe.php on line 10 bool(false) 

Notice that the directory does not exist afterwards.

If I uncomment the sleep() call above, there are no problems.


A workaround:
$ok = mkdir($dir, 0777, true);
if (!$ok && !is_dir($dir)) {
    sleep(1);
    mkdir($dir, 0777, true);
}


In other words, it works if I try again. I hope that mkdir() can be changed so that it only returns false if the directory cannot be created at all (e.g. because of lack of permissions or because it already exists).