fix(notifier): take alertmanagerSet.mtx before checking alertmanagerSet.ams in sendAll

Signed-off-by: machine424 <ayoubmrini424@gmail.com>

notifier/notifier.go

@@ -475,10 +475,6 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 		numSuccess atomic.Uint64
 	)
 	for _, ams := range amSets {
-		if len(ams.ams) == 0 {
-			continue
-		}
-
 		var (
 			payload  []byte
 			err      error
@@ -487,6 +483,10 @@ func (n *Manager) sendAll(alerts ...*Alert) bool {
 
 		ams.mtx.RLock()
 
+		if len(ams.ams) == 0 {
+			ams.mtx.RUnlock()
+			continue
+		}
 
 		if len(ams.cfg.AlertRelabelConfigs) > 0 {
 			amAlerts = relabelAlerts(ams.cfg.AlertRelabelConfigs, labels.Labels{}, alerts)