net/gcoap: Make references to coap_resource_t const in gcoap

    A CoAP resource is a primary object between the application
    and CoAP library.  The Library needs the paths, methods,
    and handlers from it, so that it can call the right handler.
    However, it never needs to change any of them.

    The application also needs the resources.  The application
    may want to declare the resources as const, since it may
    want to store them in flash.

sys/include/net/gcoap.h

@@ -433,10 +433,10 @@ extern "C" {
  * @brief   A modular collection of resources for a server
  */
 typedef struct gcoap_listener {
-    coap_resource_t *resources;     /**< First element in the array of
-                                     *   resources; must order alphabetically */
-    size_t resources_len;           /**< Length of array */
-    struct gcoap_listener *next;    /**< Next listener in list */
+    const coap_resource_t *resources;   /**< First element in the array of
+                                         *   resources; must order alphabetically */
+    size_t resources_len;               /**< Length of array */
+    struct gcoap_listener *next;        /**< Next listener in list */
 } gcoap_listener_t;
 
 /**
@@ -480,7 +480,7 @@ typedef struct {
  */
 typedef struct {
     sock_udp_ep_t *observer;            /**< Client endpoint; unused if null */
-    coap_resource_t *resource;          /**< Entity being observed */
+    const coap_resource_t *resource;    /**< Entity being observed */
     uint8_t token[GCOAP_TOKENLEN_MAX];  /**< Client token for notifications */
     unsigned token_len;                 /**< Actual length of token attribute */
 } gcoap_observe_memo_t;

sys/net/application_layer/gcoap/gcoap.c

@@ -48,7 +48,7 @@ static void _expire_request(gcoap_request_memo_t *memo);
 static bool _endpoints_equal(const sock_udp_ep_t *ep1, const sock_udp_ep_t *ep2);
 static void _find_req_memo(gcoap_request_memo_t **memo_ptr, coap_pkt_t *pdu,
                            const sock_udp_ep_t *remote);
-static int _find_resource(coap_pkt_t *pdu, coap_resource_t **resource_ptr,
+static int _find_resource(coap_pkt_t *pdu, const coap_resource_t **resource_ptr,
                                             gcoap_listener_t **listener_ptr);
 static int _find_observer(sock_udp_ep_t **observer, sock_udp_ep_t *remote);
 static int _find_obs_memo(gcoap_observe_memo_t **memo, sock_udp_ep_t *remote,
@@ -62,7 +62,7 @@ const coap_resource_t _default_resources[] = {
 };
 
 static gcoap_listener_t _default_listener = {
-    (coap_resource_t *)&_default_resources[0],
+    &_default_resources[0],
     sizeof(_default_resources) / sizeof(_default_resources[0]),
     NULL
 };
@@ -267,10 +267,10 @@ static void _listen(sock_udp_t *sock)
 static size_t _handle_req(coap_pkt_t *pdu, uint8_t *buf, size_t len,
                                                          sock_udp_ep_t *remote)
 {
-    coap_resource_t *resource  = NULL;
-    gcoap_listener_t *listener = NULL;
-    sock_udp_ep_t *observer    = NULL;
-    gcoap_observe_memo_t *memo = NULL;
+    const coap_resource_t *resource     = NULL;
+    gcoap_listener_t *listener          = NULL;
+    sock_udp_ep_t *observer             = NULL;
+    gcoap_observe_memo_t *memo          = NULL;
     gcoap_observe_memo_t *resource_memo = NULL;
 
     switch (_find_resource(pdu, &resource, &listener)) {
@@ -382,7 +382,7 @@ static size_t _handle_req(coap_pkt_t *pdu, uint8_t *buf, size_t len,
  *        code didn't match and `GCOAP_RESOURCE_NO_PATH` if no matching
  *        resource was found.
  */
-static int _find_resource(coap_pkt_t *pdu, coap_resource_t **resource_ptr,
+static int _find_resource(coap_pkt_t *pdu, const coap_resource_t **resource_ptr,
                                             gcoap_listener_t **listener_ptr)
 {
     int ret = GCOAP_RESOURCE_NO_PATH;
@@ -391,7 +391,7 @@ static int _find_resource(coap_pkt_t *pdu, coap_resource_t **resource_ptr,
     /* Find path for CoAP msg among listener resources and execute callback. */
     gcoap_listener_t *listener = _coap_state.listeners;
     while (listener) {
-        coap_resource_t *resource = listener->resources;
+        const coap_resource_t *resource = listener->resources;
         for (size_t i = 0; i < listener->resources_len; i++) {
             if (i) {
                 resource++;
@@ -1008,7 +1008,7 @@ int gcoap_get_resource_list(void *buf, size_t maxlen, uint8_t cf)
 
     /* write payload */
     while (listener) {
-        coap_resource_t *resource = listener->resources;
+        const coap_resource_t *resource = listener->resources;
 
         for (unsigned i = 0; i < listener->resources_len; i++) {
             size_t path_len = strlen(resource->path);