PHP locking not working. Can't figure out why - php

Question

Hi guys I'm following up from my question Acquire_lock() not working. Bot still sending requests quickly. PHP + AJAX which I haven't been able to get an answer for.
I've simplified everything and have three files
abc.txt
file1.php
file2.php
All in the same directory and the contents of both php files are the same:
<?php
$x = fopen("/var/www/abc.txt", "w");
if (flock($x, LOCK_EX|LOCK_NB)) {
print "No problems, I got the lock, now I'm going to sit on it.";
while (true)
sleep(5);
} else {
print "Didn't quite get the lock. Quitting now. Good night.";
}
fclose($x);
?>
Yet when I load either of them, I get the second print message: "Didn't quite get the lock. Quitting now. Good night.".
Anyone have any idea, either to this question or the former as to what's going on? Literally at my wits end with this.
Thank you as always.

Answer 1

If you want the php script to not exit then you would want a blocking lock.
flock documentation says you can do it by specifying third parameter. Also removing LOCK_NB might help.
<?php
$x = fopen("/var/www/abc.txt", "w");
if (flock($x, LOCK_EX, 1)) {
print "No problems, I got the lock, now I'm going to sit on it.";
// wait for 5 seconds
sleep(5);
// Release the lock now so that next script is executed
flock($x , LOCK_UN);
} else {
print "Didn't quite get the lock. Quitting now. Good night.";
}
fclose($x);
?>

Answer 2

Here is what's going on in your script:
1) You code is going into an infinite loop while(true) and it is never reaching the fclose() statement at the end.
2) I tested both files on my local server: File1.php kept looping while the second file2.php gave the file is locked messaged immediately (which means the first file locked correctly). I tried refreshing both files afterwards and they both failed the lock test.
If you are using PHP> 5.3.2 then you have to unlock the file manually:
The automatic unlocking when the file's resource handle is closed was
removed. Unlocking now always has to be done manually.
Source
If you are using an older version of PHP, then the script will unlock the file once the script is done executing, since you are going into an infinite loop, the script is never done and therefore the file is never unlocked.
Also, even if you stop the script from running in your browser window, the process php-cgi.exe related to that script remains running and has to be terminated manually from the task manager (I verified that myself)
Solution:
1)
To fix this issue and make the script wait for the lock, you need to make sure first that the script actually stops gracefully by removing the infinite loop:
Here is a script that will lock the file for 30 seconds (loop removed):
<?php
$x = fopen("/var/www/abc.txt", "w");
if (flock($x, LOCK_EX|LOCK_NB)) {
print "No problems, I got the lock, now I'm going to sit on it.";
sleep(30);
fclose($x); // it is good practice to always close even your PHP <5.3.2
}
else {
print "Didn't quite get the lock. Quitting now. Good night.";
}
?>
2) If you are on a linux machine, you can use the LOCK_NB flag to determine whether the file is locked or not. The usage of LOCK_NB should be like this:
while ( ! flock($f, LOCK_NB) )
{
sleep(1);
}
this will force the script to check every second for the lock and wait for the other script to finish.
3) use flock($fp, LOCK_UN) to explicitly remove the lock when you are done instead of fclose();
In summary, this is how your code should look like:
<?php
$x = fopen("/var/www/abc.txt", "w");
while(!flock($x,LOCK_NB)
sleep(1);
if (flock($x, LOCK_EX,true)) {
print "No problems, I got the lock, now I'm going to sit on it.";
sleep(30);
fflush($fp); // flush output before releasing the lock
flock($fp, LOCK_UN); // release the lock
} else {
print "Didn't quite get the lock. Quitting now. Good night.";
}
fclose($x);
?>