commit edb5a1efd22eb9918574d962640cd2ae3bb45ad3
Author: Christopher Faulet <cfaulet@haproxy.com>
Date:   Wed May 2 12:12:45 2018 +0200

    BUG/MINOR: checks: Fix check->health computation for flapping servers
    
    This patch fixes an old bug introduced in the commit 7b1d47ce ("MAJOR: checks:
    move health checks changes to set_server_check_status()"). When a DOWN server is
    flapping, everytime a check succeds, check->health is incremented. But when a
    check fails, it is decremented only when it is higher than the rise value. So if
    only one check succeds for a DOWN server, check->health will remain set to 1 for
    all subsequent failing checks.
    
    So, at first glance, it seems not that terrible because the server remains
    DOWN. But it is reported in the transitional state "DOWN server, going up". And
    it will remain in this state until it is UP again. And there is also an
    insidious side effect. If a DOWN server is flapping time to time, It will end to
    be considered UP after a uniq successful check, , regardless the rise threshold,
    because check->health will be increased slowly and never decreased.
    
    To fix the bug, we just need to reset check->health to 0 when a check fails for
    a DOWN server. To do so, we just need to relax the condition to handle a failure
    in the function set_server_check_status.
    
    This patch must be backported to haproxy 1.5 and newer.
    
    (cherry picked from commit b119a79fc336f2b6074de1c3113b1682c717985c)
    Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/src/checks.c b/src/checks.c
index 80a9c70d..d07a82f8 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -243,7 +243,7 @@ static void set_server_check_status(struct check *check, short status, const cha
 		 */
 		if ((!(check->state & CHK_ST_AGENT) ||
 		    (check->status >= HCHK_STATUS_L57DATA)) &&
-		    (check->health >= check->rise)) {
+		    (check->health > 0)) {
 			HA_ATOMIC_ADD(&s->counters.failed_checks, 1);
 			report = 1;
 			check->health--;