Re: signal() behavior

[Mark Valence <kurash@sassafras.com>]

> I've noticed an annoying behavior in the alpha slapd on Linux: If the daemon
> is running in the background, it appears to ignore SIGTERM. What has
> actually happened is that the daemon is in select(), and the select call has
> been restarted after the signal handler was invoked. I.e., the default
> behavior on Linux for signal() is to restart system calls - it uses the
> default BSD semantics. Anyone testing on Solaris or other SVR4-derived OSs
> will not see this problem, because SysV signal() does not restart system
> calls by default.
>
> The solution seems to be to check for sigaction() in configure, and use it
> instead of signal() if it's found. I'll make this fix in a little while.

I've been fighting with this for a while, too.  However, it's not as 
simple as using sigaction (or sigvec) in place of signal where 
possible.  Some systems don't return EINTR from select even when a 
signal is set with the SA_RESTART (or SA_INTERRUPT) flag set in these 
two calls.  The issue is that some systems set the signal, and the 
first thread to notice the signal will service it.  If the thread 
that has called select is not the thread to pick up the signal, the 
select call will not return with EINTR.

There has to be some way to make this work "correctly" on all 
systems, and that's what I've been searching for.  For now, I've been 
using a kludge solution that never passes NULL as the time value to 
select.  Instead, tvp is set to point to a tv that specifies some 
small number of seconds.  This uses a bit of CPU time, but solves the 
problem of killing slapd (without using kill -9).

Here are diffs from daemon.c.  I have not submitted them because they 
are not the ideal fix for this problem.  However, if there is no good 
fix, I'll submit them.

--- daemon.c	1999/08/19 09:44:44	1.106
+++ daemon.c	1999/08/24 23:46:36
@@ -517,5 +517,5 @@
         struct hostent		*hp;
 #endif
-		struct timeval		zero;
+		struct timeval		zero , five;
 		struct timeval		*tvp;

@@ -532,4 +532,6 @@
 		zero.tv_sec = 0;
 		zero.tv_usec = 0;
+		five.tv_sec = 5;
+		five.tv_usec = 0;

 		ldap_pvt_thread_mutex_lock( &slap_daemon.sd_mutex );
@@ -573,4 +575,7 @@
 #endif

+		if (!tvp)
+			tvp = &five;
+
 		for ( l = 0; slap_listeners[l] != NULL; l++ ) {
 			if ( slap_listeners[l]->sl_sd == AC_SOCKET_INVALID )