fix: normalize wildcard hosts in CheckHealth, isolate config test env

Co-authored-by: tma <4719+tma@users.noreply.github.com>

Author: Copilot

internal/config/config_test.go

@@ -198,12 +198,19 @@ func TestLoad_InvalidDuration(t *testing.T) {
 }
 
 func TestLoad_HealthListenCustom(t *testing.T) {
-	os.Setenv("MODBUS_UPSTREAM", "localhost:502")
-	os.Setenv("HEALTH_LISTEN", ":9090")
-	defer func() {
-		os.Unsetenv("MODBUS_UPSTREAM")
-		os.Unsetenv("HEALTH_LISTEN")
-	}()
+	// Ensure optional env vars that Load() may read do not inherit
+	// potentially invalid values from the surrounding environment.
+	t.Setenv("MODBUS_LISTEN", "")
+	t.Setenv("MODBUS_READONLY", "")
+	t.Setenv("MODBUS_CACHE_TTL", "")
+	t.Setenv("MODBUS_TIMEOUT", "")
+	t.Setenv("MODBUS_REQUEST_DELAY", "")
+	t.Setenv("MODBUS_CONNECT_DELAY", "")
+	t.Setenv("MODBUS_SHUTDOWN_TIMEOUT", "")
+
+	// Set required and explicitly tested env vars.
+	t.Setenv("MODBUS_UPSTREAM", "localhost:502")
+	t.Setenv("HEALTH_LISTEN", ":9090")
 
 	cfg, err := Load()
 	if err != nil {

internal/health/health.go

@@ -107,17 +107,21 @@ func (s *Server) Shutdown(ctx context.Context) error {
 
 // CheckHealth performs an HTTP health check against the given address.
 // It returns nil if the endpoint responds with 200 OK.
+// Wildcard listen addresses (e.g. ":8080", "0.0.0.0:8080", "[::]:8080") are
+// normalized to localhost so they can be used as dial targets. IPv6 addresses
+// are handled correctly via net.JoinHostPort.
 func CheckHealth(addr string) error {
 	// Resolve the address so we can build a proper URL.
 	host, port, err := net.SplitHostPort(addr)
 	if err != nil {
 		return fmt.Errorf("invalid address %q: %w", addr, err)
 	}
-	if host == "" {
+	// Normalize wildcard and empty hosts to localhost.
+	if host == "" || host == "0.0.0.0" || host == "::" {
 		host = "localhost"
 	}
 
-	url := fmt.Sprintf("http://%s:%s/health", host, port)
+	url := fmt.Sprintf("http://%s/health", net.JoinHostPort(host, port))
 
 	client := &http.Client{Timeout: 3 * time.Second}
 	resp, err := client.Get(url)

internal/health/health_test.go

@@ -149,3 +149,49 @@ func TestCheckHealth_ConnectionRefused(t *testing.T) {
 		t.Error("expected error for connection refused")
 	}
 }
+
+func TestCheckHealth_WildcardAddresses(t *testing.T) {
+	// Start a test server bound to localhost so wildcard addresses can reach it.
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode(Response{Status: "ok"})
+	}))
+	defer ts.Close()
+
+	_, port, err := net.SplitHostPort(ts.Listener.Addr().String())
+	if err != nil {
+		t.Fatalf("failed to parse test server address: %v", err)
+	}
+
+	// Each of these listen-style addresses should be normalized to localhost.
+	wildcards := []string{
+		":" + port,        // empty host (":8080")
+		"0.0.0.0:" + port, // IPv4 wildcard
+		"[::]:" + port,    // IPv6 wildcard
+	}
+	for _, addr := range wildcards {
+		if err := CheckHealth(addr); err != nil {
+			t.Errorf("CheckHealth(%q) expected success, got: %v", addr, err)
+		}
+	}
+}
+
+func TestCheckHealth_IPv6Loopback(t *testing.T) {
+	// Start a test server bound to the IPv6 loopback address.
+	ln, err := net.Listen("tcp6", "[::1]:0")
+	if err != nil {
+		t.Skip("IPv6 loopback not available:", err)
+	}
+	ts := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		json.NewEncoder(w).Encode(Response{Status: "ok"})
+	}))
+	ts.Listener = ln
+	ts.Start()
+	defer ts.Close()
+
+	addr := ts.Listener.Addr().String() // "[::1]:PORT"
+	if err := CheckHealth(addr); err != nil {
+		t.Errorf("CheckHealth(%q) expected success for IPv6 loopback, got: %v", addr, err)
+	}
+}