[collectd] collectd 5.10 threading

[Brandon Hume]

  On 09/14/12 12:37 PM, Cyril Feraudet wrote:
> This is mine ;-) Try the last version on my branch.
> This plugin work fine in a large environment (5300 server and more than 19000 update by second in MySQL) but never tested on Solaris.

I think I found the issue.  If you look at:

     
https://github.com/feraudet/collectd/blob/cf/perfwatcher/src/write_mysql.c#L532-576

On line 536 the mutex is unlocked.  And then, on line 576 it's unlocked 
again.  On a sufficiently fast/multithreaded machine another thread can 
grab the lock in between those two lines, and then probably get hung up 
on I/O talking to the mysql server right afterward.  Then the lock gets 
undone, and then another thread will lock it again and try to talk to 
Mysql on its own.

So the locks/unlocks get out of sync and there's no longer exclusive 
access to the database handle.

Since there's an unlock on line 572, the one on line 576 can probably be 
removed.