MEDIUM: poller: disable thread-groups for poll() and select()

These old legacy pollers are not designed for this. They're still
using a shared list of events for all threads, this will not scale at
all, so there's no point in enabling thread-groups there. Modern
systems have epoll, kqueue or event ports and do not need these ones.

We arrange for failing at boot time, only when thread-groups > 1 so
that existing setups will remain unaffected.

If there's a compelling reason for supporting thread groups with these
pollers in the future, the rework should not be too hard, it would just
consume a lot of memory to have an fd_evts[] array per thread, but that
is doable.

src/ev_poll.c

@@ -264,6 +264,13 @@ static int _do_init(struct poller *p)
 	int fd_evts_bytes;
 
 	p->private = NULL;
+
+	/* this old poller uses a process-wide FD list that cannot work with
+	 * groups.
+	 */
+	if (global.nbtgroups > 1)
+		goto fail_srevt;
+
 	fd_evts_bytes = (global.maxsock + sizeof(**fd_evts) * 8 - 1) / (sizeof(**fd_evts) * 8) * sizeof(**fd_evts);
 
 	if ((fd_evts[DIR_RD] = calloc(1, fd_evts_bytes)) == NULL)

src/ev_select.c

@@ -247,6 +247,12 @@ static int _do_init(struct poller *p)
 
 	p->private = NULL;
 
+	/* this old poller uses a process-wide FD list that cannot work with
+	 * groups.
+	 */
+	if (global.nbtgroups > 1)
+		goto fail_srevt;
+
 	if (global.maxsock > FD_SETSIZE)
 		goto fail_srevt;
 

src/haproxy.c

@@ -2533,7 +2533,8 @@ static void init(int argc, char **argv)
 
 	if (!init_pollers()) {
 		ha_alert("No polling mechanism available.\n"
-			 "  It is likely that haproxy was built with TARGET=generic and that FD_SETSIZE\n"
+			 "  This may happen when using thread-groups with old pollers (poll/select), or\n"
+			 "  it is possible that haproxy was built with TARGET=generic and that FD_SETSIZE\n"
 			 "  is too low on this platform to support maxconn and the number of listeners\n"
 			 "  and servers. You should rebuild haproxy specifying your system using TARGET=\n"
 			 "  in order to support other polling systems (poll, epoll, kqueue) or reduce the\n"