fix(healthcheck): guard periodic checks against divide-by-zero when config is cleared

internal/healthcheck/checker.go

@@ -159,6 +159,8 @@ func (c *Checker) Start(ctx context.Context) (runError <-chan error, err error)
 func (c *Checker) Stop() error {
 	c.stop()
 	<-c.done
+	c.configMutex.Lock()
+	defer c.configMutex.Unlock()
 	c.tlsDialAddrs = nil
 	c.icmpTargetIPs = nil
 	c.smallCheckType = ""
@@ -170,6 +172,9 @@ func (c *Checker) smallPeriodicCheck(ctx context.Context) error {
 	icmpTargetIPs := make([]netip.Addr, len(c.icmpTargetIPs))
 	copy(icmpTargetIPs, c.icmpTargetIPs)
 	c.configMutex.Unlock()
+	if c.smallCheckType != smallCheckDNS && len(icmpTargetIPs) == 0 {
+		return nil
+	}
 	tryTimeouts := []time.Duration{
 		5 * time.Second,
 		5 * time.Second,
@@ -202,11 +207,18 @@ func (c *Checker) smallPeriodicCheck(ctx context.Context) error {
 }
 
 func (c *Checker) fullPeriodicCheck(ctx context.Context) error {
+	c.configMutex.Lock()
+	tlsDialAddrs := make([]string, len(c.tlsDialAddrs))
+	copy(tlsDialAddrs, c.tlsDialAddrs)
+	c.configMutex.Unlock()
+	if len(tlsDialAddrs) == 0 {
+		return nil
+	}
 	// 20s timeout in case the connection is under stress
 	// See https://github.com/qdm12/gluetun/issues/2270
 	tryTimeouts := []time.Duration{10 * time.Second, 15 * time.Second, 30 * time.Second}
 	check := func(ctx context.Context, try int) error {
-		tlsDialAddr := c.tlsDialAddrs[try%len(c.tlsDialAddrs)]
+		tlsDialAddr := tlsDialAddrs[try%len(tlsDialAddrs)]
 		return tcpTLSCheck(ctx, c.dialer, tlsDialAddr)
 	}
 	return withRetries(ctx, tryTimeouts, c.logger, "TCP+TLS dial", check)

internal/healthcheck/checker_test.go

@@ -61,6 +61,20 @@ func Test_Checker_fullcheck(t *testing.T) {
 	})
 }
 
+func Test_Checker_periodicCheckEmptyTargets(t *testing.T) {
+	t.Parallel()
+
+	checker := &Checker{smallCheckType: smallCheckICMP}
+
+	err := checker.fullPeriodicCheck(context.Background())
+
+	assert.NoError(t, err)
+
+	err = checker.smallPeriodicCheck(context.Background())
+
+	assert.NoError(t, err)
+}
+
 func Test_makeAddressToDial(t *testing.T) {
 	t.Parallel()