In-depth understanding of PHP principles: a small probability of Session Gc Notice

In-depth understanding of PHP principles: a small probability of Session Gc Notice_PHP Tutorial

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Release： 2016-07-21 15:30:45

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If you use PHP installed with apt under ubuntu/Debian, you may encounter this prompt with a small probability when using Session.

Copy code The code is as follows:

 
PHP Notice: session_start(): ps_files_cleanup_dir: 
opendir(/var/lib/php5) failed: Permission denied (13) 
in /home/laruence/ www/htdocs/index.php on line 22 

This is because, in PHP, if you use file_handler as the save handler of the Session, then there is a probability that the Gc of the Session will be run every time session_start Process.

Copy code The code is as follows:

 
//Omitted 
int nrdels = -1; 
 nrand = (int) ((float) PS(gc_divisor) * php_combined_lcg(TSRMLS_C)); 
if (nrand < PS(gc_probability)) { 
PS(mod)->s_gc(&PS(mod_data) , PS(gc_maxlifetime), &nrdels TSRMLS_CC); 
} 
//Omitted 

The reason for this warning is because in apt's PHP, the default directory of session /var/lib The permissions of /php5 are 733 with sticky bit, that is,

Copy the code The code is as follows:

drwx-wx-wt root roo

Generally, PHP workers run under non-root identities, so they do not have permission to open this folder (but because it can write, it does not affect normal Session file reading). So the following code in s_gc will trigger the Notice mentioned at the beginning:

Copy the code The code is as follows:

 
/ /For the file handler, s_gc indirectly calls ps_files_cleanup_dir: 
dir = opendir(dirname); 
if (!dir) { 
php_error_docref(NULL TSRMLS_CC, E_NOTICE, 
"ps_files_cleanup_dir: opendir(% s) failed: %s (%d)", 
dirname, strerror(errno), errno); 
return (0); 
} 

Of course, in ubuntu Under /Debian, there is still gc recycling, but it is done by an external cron process. The default is /etc/cron.d/php5:,

Copy code The code is as follows:

 
09,39 * * * * root [ -x /usr/lib/php5/maxlifetime ] 
&& [ -d /var/lib/php5 ] && find /var/lib/php5/ 
-type f -cmin +$(/usr/lib/php5/maxlifetime) -print0 
| xargs -n 200 -r -0 r 

In addition, you can see that when determining whether s_gc is running, there are two key variables: PS (gc_divisor) and PS (gc_probability). These two variables correspond to the two same-named runtime configuration items of the session. Configuration items:
session.gc_probability and session.gc_divisor, they default to 1 and 100 respectively.
And php_combined_lcg is a random number generator that generates random numbers in the range of 0 to 1, so the above discrimination is equivalent to:

Copy code The code is as follows:

rand < probability / gc_diviso

That is to say , by default, the gc process can be called almost once every 100 times. Therefore, there is a small probability that you can see this Notice.
To close this Notice, you only need to set:
session.gc_probability = 0, let s_gc There is no possibility of running at all.
Of course, you can also change the permissions of this folder...
Finally, thank CFC4N for providing this question.
Author: Laruence( )
This article address: http: //www.laruence.com/2011/03/29/1949.html