Edit report at https://bugs.php.net/bug.php?id=63709&edit=1
ID: 63709 Updated by: ahar...@php.net Reported by: eric dot saintetienne at gmail dot com Summary: flock() doesn't trigger mandatory locks on linux Status: Analyzed Type: Bug Package: Filesystem function related Operating System: Linux PHP Version: 5.3.19 Block user comment: N Private report: N New Comment: My worry there is that dio resources are (as I recall, anyway) completely distinct from normal file resources, so you couldn't fopen() a file and then dio_fcntl() it: it's all or nothing. Previous Comments: ------------------------------------------------------------------------ [2012-12-07 08:42:12] eric dot saintetienne at gmail dot com A fifth option is to pull the "Direct IO" extension into the mainline. http://pecl.php.net/package/dio This extension already expose fcntl() as well as a few other low-level POSIX routines, and have some amount of testing as it's in its fourth version already (though it's said to be in beta state) ------------------------------------------------------------------------ [2012-12-07 03:24:16] larue...@php.net I like 3 :) change the behavior of flock will intruduce a visible bc break ------------------------------------------------------------------------ [2012-12-07 01:48:08] ahar...@php.net The key difference between Python and PHP here is that Python always uses fcntl() internally, whereas PHP will use flock() if it's available (which it obviously is on Linux) and will only fall back to fcntl() if it's not. flock() will never create a mandatory lock, so the manual page is wrong, which I'm pretty sure is my fault. Mea culpa. We can probably fix this by switching to preferring fcntl() within our flock() function as Python does, since that's actually the more useful behaviour, but that would be a (mild) BC break in how flock() behaves in practice â although it would actually bring it into line with what's documented. The options I see are: 1. Change the behaviour of flock() as described above to prefer fcntl(). 2. Add a new lockf() function, as suggested. 3. Just bite the bullet and expose fcntl() as a PHP function on POSIX platforms. 4. Do nothing and update the manual. :) Does anyone have any thoughts? I'm happy to do the donkey work, but am not really sure on the best way to proceed. ------------------------------------------------------------------------ [2012-12-06 14:19:38] eric dot saintetienne at gmail dot com Note that dio_fcntl() of the "Direct IO" extension can successfully exclusively lock the file, but this shouldn't be considered as a workaround as it's not always possible to install extensions. If flock() couldn't be modified for backward compatibility reasons, options could be added to alter its behaviour, or a new call lockf() would be welcome too. ------------------------------------------------------------------------ [2012-12-06 13:12:43] eric dot saintetienne at gmail dot com 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 this bug report at https://bugs.php.net/bug.php?id=63709&edit=1
