RFC/PATCH: Handle external service check results in seperate thread

[Stefan Rompf]

Hi,

like other people on this list, we've been bitten by the problem that nagios 
fork()s subprocesses when service check results arrive via the external 
command pipe. When nagios lags for example due to hostchecks, in most cases 
enough forked processes pile up to bring nagios over its resource limits. 
Even if this doesn't happen, results will be fed in the wrong order.

I've developed the following solution that is quite different to the spool 
directory approach:

-passive service check results are added to passive_check_result_list as 
before. However, for our use case it does not make sense to keep multiple 
results for one service as soon as nagios starts lagging. So we have a 
duplicate detection that keeps only the newest check result per service.
-Instead of forking subprocesses, a permanently running thread feeds the 
results on passive_check_result_list back via write_svc_message(). So two 
threads of the process talk to each other via a pipe, but I didn't want to 
make my changes too invasive ;-)
-Instead of polling the command pipe every 0.5 seconds, select() on the file 
descriptor is used now if there are enough external_command_buffer_slots. 
Problem here was that with no writer on the pipe, select() endlessly signaled 
an EOF. Fixed by opening the command pipe R/W.

The patch has been developed on nagios 2.6 and linux, afterwards forward 
ported to current CVS. It seems to work, but needs further testing. Even 
compilation tests on different architectures would be interesting, I'm not 
sure how widespread the tsearch()-API is.

Thoughts?

Stefan
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