feat(health): change timeout mechanism

- Healthcheck timeout is no longer fixed to 3 seconds
- Healthcheck timeout increases from 2s to 4s, 6s, 8s, 10s
- No 1 second wait time between check retries after failure
- VPN internal restart may be delayed by a maximum of 10 seconds

internal/healthcheck/health.go

@@ -11,55 +11,58 @@ import (
 func (s *Server) runHealthcheckLoop(ctx context.Context, done chan<- struct{}) {
 	defer close(done)
 
+	timeoutIndex := 0
+	healthcheckTimeouts := []time.Duration{
+		2 * time.Second,
+		4 * time.Second,
+		6 * time.Second,
+		8 * time.Second,
+		// This can be useful when the connection is under stress
+		// See https://github.com/qdm12/gluetun/issues/2270
+		10 * time.Second,
+	}
 	s.vpn.healthyTimer = time.NewTimer(s.vpn.healthyWait)
 
 	for {
 		previousErr := s.handler.getErr()
 
-		const healthcheckTimeout = 3 * time.Second
+		timeout := healthcheckTimeouts[timeoutIndex]
 		healthcheckCtx, healthcheckCancel := context.WithTimeout(
-			ctx, healthcheckTimeout)
+			ctx, timeout)
 		err := s.healthCheck(healthcheckCtx)
 		healthcheckCancel()
 
 		s.handler.setErr(err)
 
-		if previousErr != nil && err == nil {
+		switch {
+		case previousErr != nil && err == nil: // First success
 			s.logger.Info("healthy!")
+			timeoutIndex = 0
 			s.vpn.healthyTimer.Stop()
 			s.vpn.healthyWait = *s.config.VPN.Initial
-		} else if previousErr == nil && err != nil {
+		case previousErr == nil && err != nil: // First failure
 			s.logger.Debug("unhealthy: " + err.Error())
 			s.vpn.healthyTimer.Stop()
 			s.vpn.healthyTimer = time.NewTimer(s.vpn.healthyWait)
+		case previousErr != nil && err != nil: // Nth failure
+			if timeoutIndex < len(healthcheckTimeouts)-1 {
+				timeoutIndex++
 			}
-
-		if err != nil { // try again after 1 second
-			timer := time.NewTimer(time.Second)
 			select {
-			case <-ctx.Done():
-				if !timer.Stop() {
-					<-timer.C
-				}
-				return
-			case <-timer.C:
 			case <-s.vpn.healthyTimer.C:
+				timeoutIndex = 0 // retry next with the smallest timeout
 				s.onUnhealthyVPN(ctx)
+			default:
 			}
-			continue
-		}
-
-		// Success, check again after the success wait duration
+		case previousErr == nil && err == nil: // Nth success
 			timer := time.NewTimer(s.config.SuccessWait)
 			select {
 			case <-ctx.Done():
-			if !timer.Stop() {
-				<-timer.C
-			}
 				return
 			case <-timer.C:
 			}
 		}
+	}
 }
 
 func (s *Server) healthCheck(ctx context.Context) (err error) {