all: Add custom JSON marshaller for sx1301 global config file

pkg/pfconfig/semtechudp/semtechudp.go

@@ -16,6 +16,8 @@
 package semtechudp
 
 import (
+	"encoding/json"
+
 	"go.thethings.network/lorawan-stack/v3/pkg/frequencyplans"
 	"go.thethings.network/lorawan-stack/v3/pkg/pfconfig/shared"
 	"go.thethings.network/lorawan-stack/v3/pkg/ttnpb"
@@ -28,6 +30,24 @@ type Config struct {
 	GatewayConf GatewayConf            `json:"gateway_conf"`
 }
 
+// MarshalJSON implements json.Marshaler.
+// Serializes the SX1301Conf field as an object if it contains a single element and as an array otherwise.
+func (c Config) MarshalJSON() ([]byte, error) {
+	if len(c.SX1301Conf) == 1 {
+		return json.Marshal(
+			struct {
+				SX1301Conf  *shared.SX1301Config `json:"SX1301_conf"`
+				GatewayConf GatewayConf          `json:"gateway_conf"`
+			}{
+				SX1301Conf:  c.SX1301Conf[0],
+				GatewayConf: c.GatewayConf,
+			},
+		)
+	}
+	type Alias Config
+	return json.Marshal((Alias)(c))
+}
+
 // GatewayConf contains the configuration for the gateway's server connection.
 type GatewayConf struct {
 	GatewayID      string        `json:"gateway_ID,omitempty"`