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Date: Wed, 03 Dec 2003 18:06:13 +0100
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    internals@lists.php.net
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Subject: browscap and nesting level too deep (bug #25916)
From: uwe@thetaphi.de (Uwe Schindler)

Today I got the error from bug #25916 several times on our webserver. 
Looking through the code I found out the following:
* It depends NOT on the fact if there is a parameter to get_browser() or not
* It happens sometimes when server is very heavy loaded, the homepage of 
the domain uses the get_browser() function and is the most visited page.

So it must be a multithreading issue (NSAPI is a multithreading webserver). 
And I have an idea:
Line 257 uses:
         zend_hash_apply_with_arguments(&browser_hash, (apply_func_args_t) 
browser_reg_compare, 2, lookup_browser_name, &found_browser_entry);

This is the only function in this context in zend_hash.c which uses the 
Recursion protection with
#define 
HASH_PROTECT_RECURSION(ht) 
\
         if ((ht)->bApplyProtection) 
{ 
\
                 if ((ht)->nApplyCount++ >= 3) 
{ 
\
                         zend_error(E_ERROR, "Nesting level too deep - 
recursive dependency?");          \
                 } 
\
         }

The browser hashtable is a global variable in browscap.c and can be used by 
more than one call to get_browser() even at the same time. So if one 
zend_hash_apply_with_arguments() locks the hashtable and a second and third 
thread tries to do that you will get the error, because (ht)->nApplyCount++ 
raises and raises...

This evening I will try to put a mutex at the beginning of get_browser to 
prevent more threads running at the same time there. But as I see this, 
this zend_hash_apply function is used very often could there be other 
effects if a global variable is a hashtable?

Only one question: Is there a special PHP way to use mutexes? I am not 
familar in Zend programming (I do only SAPI...)

-----
Uwe Schindler
thetaphi@php.net - http://www.php.net
NSAPI SAPI developer
Erlangen, Germany