Bug #63709 [NEW]: flock() doesn't trigger mandatory locks on linux

From: Date: Thu, 06 Dec 2012 13:12:43 +0000
Subject: Bug #63709 [NEW]: flock() doesn't trigger mandatory locks on linux
Groups: php.bugs 
Request: Send a blank email to php-bugs+get-175165@lists.php.net to get a copy of this message 
From:             eric dot saintetienne at gmail dot com
Operating system: Linux
PHP version:      5.3.19
Package:          Filesystem function related
Bug Type:         Bug
Bug description:flock() doesn't trigger mandatory locks on linux

Description:
------------
Locking exclusively via flock a file opened for writing doesn't trigger a 
mandatory lock.
The python script below could trigger the mandatory lock.

Maybe it's because PHP flock() is built on the C function call flock():
"When a program attempts to lock a file with lockf or fcntl that has
mandatory 
locking set, the kernel will prevent all other programs from accessing the
file. 
Processes which use flock will not trigger a mandatory lock."
source: http://www.hackinglinuxexposed.com/articles/20030623.html

Python script:
#!/usr/bin/python

import os, fcntl

fd = os.open('mandlock-file', os.O_RDWR, 0755)
print 'fd=', fd
fcntl.lockf(fd, fcntl.LOCK_EX)

a = raw_input() # during that time, any attempt to open the file will hang
os.write(fd, a+'\n')

fcntl.lockf(fd, fcntl.LOCK_UN)
# now any attempt to open the file will succeed

os.close(fd)

# eof

Test script:
---------------
<?php
define('FILENAME', 'mandlock-file');

/* first create a file with a mandatory lock:
 *   $ touch mandlock-file
 *   $ chmod g+s,g-x mandlock-file
 *   $ sudo mount -o remount,mand / # my file is inside the '/' mountpoint
*/

function openlockedfile($filename) {
  $fp = fopen($filename, 'w+');

  while (($fp === false) || (flock($fp, LOCK_EX) !== true)) {
    sleep(1);
    echo "spin lock\n";

    if ($fp === false)
      $fp = fopen($filename, 'w+');
  }
  return $fp;
}

$fd = openlockedfile(FILENAME); // open + lock EX

sleep(10);

/* During this sleep time, accessing the file should hang until the
 * php script releases the exclusive lock:
 *   $ cat mandlock-file # should hang until php script termination
 * Practically, cat doesn't hang (and works) proving the file isn't
 * exclusively locked by the PHP script. */

closefile($fd); // unlock + close

/* eof */
?>

Expected result:
----------------
once the file is created and the mandatory lock setup for it:
during the 10 sec timer, opening the file (with or without explicit
locking) 
should hand until the php script terminates.

Actual result:
--------------
it's possible to opening the locked file during the 10s timer for reading
and 
writing.

-- 
Edit bug report at https://bugs.php.net/bug.php?id=63709&edit=1
-- 
Try a snapshot (PHP 5.4):   https://bugs.php.net/fix.php?id=63709&r=trysnapshot54
Try a snapshot (PHP 5.3):   https://bugs.php.net/fix.php?id=63709&r=trysnapshot53
Try a snapshot (trunk):     https://bugs.php.net/fix.php?id=63709&r=trysnapshottrunk
Fixed in SVN:               https://bugs.php.net/fix.php?id=63709&r=fixed
Fixed in release:           https://bugs.php.net/fix.php?id=63709&r=alreadyfixed
Need backtrace:             https://bugs.php.net/fix.php?id=63709&r=needtrace
Need Reproduce Script:      https://bugs.php.net/fix.php?id=63709&r=needscript
Try newer version:          https://bugs.php.net/fix.php?id=63709&r=oldversion
Not developer issue:        https://bugs.php.net/fix.php?id=63709&r=support
Expected behavior:          https://bugs.php.net/fix.php?id=63709&r=notwrong
Not enough info:            https://bugs.php.net/fix.php?id=63709&r=notenoughinfo
Submitted twice:            https://bugs.php.net/fix.php?id=63709&r=submittedtwice
register_globals:           https://bugs.php.net/fix.php?id=63709&r=globals
PHP 4 support discontinued: https://bugs.php.net/fix.php?id=63709&r=php4
Daylight Savings:           https://bugs.php.net/fix.php?id=63709&r=dst
IIS Stability:              https://bugs.php.net/fix.php?id=63709&r=isapi
Install GNU Sed:            https://bugs.php.net/fix.php?id=63709&r=gnused
Floating point limitations: https://bugs.php.net/fix.php?id=63709&r=float
No Zend Extensions:         https://bugs.php.net/fix.php?id=63709&r=nozend
MySQL Configuration Error:  https://bugs.php.net/fix.php?id=63709&r=mysqlcfg

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