net: mqtt_sn: document address size and add checks for UDP

Add guidelines in Kconfig explaining the minimum required
MQTT_SN_LIB_MAX_ADDR_SIZE for UDP transport and IPv4 and or IPv6. Also
explain when this value might need to be changed.

In addition, introduce BUILD_ASSERT checks in the UDP transport
implementation to ensure the configured maximum address size is large
enough for the enabled IP families.

Signed-off-by: Aurelien Jarno <[email protected]>

Author: aurel32

subsys/net/lib/mqtt_sn/Kconfig

@@ -45,6 +45,11 @@ config MQTT_SN_LIB_MAX_ADDR_SIZE
 	  needs to know how long your addresses are. Set this to the maximum
 	  length in bytes of the address data structure for your implemented transport.
 
+	  A UDP transport requires at least 8 bytes on IPv4 and 21 bytes on
+	  IPv6, corresponding to the sizes of the sockaddr_in and sockaddr_in6
+	  structs. For an out-of-tree transport, you may need to adjust this
+	  value.
+
 config MQTT_SN_LIB_BROADCAST_RADIUS
 	int "Radius for broadcast messages"
 	default 1

subsys/net/lib/mqtt_sn/mqtt_sn_transport_udp.c

@@ -19,6 +19,12 @@
 #include <zephyr/logging/log.h>
 LOG_MODULE_DECLARE(net_mqtt_sn, CONFIG_MQTT_SN_LOG_LEVEL);
 
+BUILD_ASSERT(CONFIG_MQTT_SN_LIB_MAX_ADDR_SIZE >= sizeof(struct net_sockaddr_in) ||
+	     !IS_ENABLED(CONFIG_NET_IPV4));
+
+BUILD_ASSERT(CONFIG_MQTT_SN_LIB_MAX_ADDR_SIZE >= sizeof(struct net_sockaddr_in6) ||
+	     !IS_ENABLED(CONFIG_NET_IPV6));
+
 static char *get_ip_str(const struct net_sockaddr *sa, char *s, size_t maxlen)
 {
 	switch (sa->sa_family) {