BUG/MEDIUM: master: force the thread count earlier

Christopher bisected that recent commit d0b73bca71 ("MEDIUM: listener:
switch bind_thread from global to group-local") broke the master socket
in that only the first out of the Nth initial connections would work,
where N is the number of threads, after which they all work.

The cause is that the master socket was bound to multiple threads,
despite global.nbthread being 1 there, so the incoming connection load
balancing would try to send incoming connections to non-existing threads,
however the bind_thread mask would nonetheless include multiple threads.

What happened is that in 1.9 we forced "nbthread" to 1 in the master's poll
loop with commit b3f2be338b ("MEDIUM: mworker: use the haproxy poll loop").

In 2.0, nbthread detection was enabled by default in commit 149ab779cc
("MAJOR: threads: enable one thread per CPU by default"). From this point
on, the operation above is unsafe because everything during startup is
performed with nbthread corresponding to the default value, then it
changes to one when starting the polling loop. But by then we weren't
using the wait mode except for reload errors, so even if it would have
happened nobody would have noticed.

In 2.5 with commit fab0fdce9 ("MEDIUM: mworker: reexec in waitpid mode
after successful loading") we started to rexecute all the time, not just
for errors, so as to release precious resources and to possibly spot bugs
that were rarely exposed in this mode. By then the incoming connection LB
was enforcing all_threads_mask on the listener's thread mask so that the
incorrect value was being corrected while using it.

Finally in 2.7 commit d0b73bca71 ("MEDIUM: listener: switch bind_thread
from global to group-local") replaces the all_threads_mask there with
the listener's bind_thread, but that one was never adjusted by the
starting master, whose thread group was filled to N threads by the
automatic detection during early setup.

The best approach here is to set nbthread to 1 very early in init()
when we're in the master in wait mode, so that we don't try to guess
the best value and don't end up with incorrect bindings anymore. This
patch does this and also sets nbtgroups to 1 in preparation for a
possible future where this will also be automatically calculated.

There is no need to backport this patch since no other versions were
affected, but if it were to be discovered that the incorrect bind mask
on some of the master's FDs could be responsible for any trouble in
older versions, then the backport should be safe (provided that
nbtgroups is dropped of course).

src/haproxy.c

@@ -833,8 +833,6 @@ static void mworker_loop()
 	mworker_catch_sigchld(NULL); /* ensure we clean the children in case
 				     some SIGCHLD were lost */
 
-	global.nbthread = 1;
-
 	jobs++; /* this is the "master" job, we want to take care of the
 		signals even if there is no listener so the poll loop don't
 		leave */
@@ -2076,6 +2074,16 @@ static void init(int argc, char **argv)
 
 		LIST_APPEND(&proc_list, &tmproc->list);
 	}
+
+	if (global.mode & MODE_MWORKER_WAIT) {
+		/* in exec mode, there's always exactly one thread. Failure to
+		 * set these ones now will result in nbthread being detected
+		 * automatically.
+		 */
+		global.nbtgroups = 1;
+		global.nbthread = 1;
+	}
+
 	if (global.mode & (MODE_MWORKER|MODE_MWORKER_WAIT)) {
 		struct wordlist *it, *c;