sys/net/application_layer/nanocoap: Add path prefix opt

This adds a prefix option for the methods field of a coap resource and
modifies the way the path is matched on a request to accept prefix
matching.

sys/include/net/nanocoap.h

@@ -26,6 +26,25 @@
  * capital precede lower case). nanocoap provides the
  * COAP_WELL_KNOWN_CORE_DEFAULT_HANDLER entry for `/.well-known/core`.
  *
+ * ### Path matching ###
+ * By default the URI-path of an incoming request should match exactly one of
+ * the registered resources. But also, a resource can be configured to
+ * match just a prefix of the URI-path of the request by adding the
+ * @ref COAP_MATCH_SUBTREE option to coap_resource_t::methods.
+ *
+ * For example, if a resource is configured with a
+ * @ref coap_resource_t::path "path" `/resource01` and the
+ * @ref COAP_MATCH_SUBTREE option is used it would match any of `/resource01/`,
+ * `/resource01/sub/path`, `/resource01alt`.
+ *
+ * If the behavior of matching `/resource01alt` is not wanted and only subtrees
+ * are wanted to match, the path should be `/resource01/`.
+ *
+ * If in addition just `/resource01` is wanted to match, together with any
+ * subtrees of `/resource01/`, then a first resource with the path `/resource01`
+ * and exact matching should be register, and then a second one with the path
+ * `/resource01/` and subtree matching.
+ *
  * ### Handler functions ###
  *
  * For each resource, you must implement a ::coap_handler_t handler function.
@@ -170,6 +189,8 @@ extern "C" {
 #define COAP_POST               (0x2)
 #define COAP_PUT                (0x4)
 #define COAP_DELETE             (0x8)
+#define COAP_MATCH_SUBTREE      (0x8000) /**< Path is considered as a prefix
+                                              when matching */
 /** @} */
 
 /**
@@ -1065,6 +1086,23 @@ static inline unsigned coap_method2flag(unsigned code)
     return (1 << (code - 1));
 }
 
+/**
+ * @brief   Checks if a CoAP resource path matches a given URI
+ *
+ * Builds on strcmp() with rules specific to URI path matching
+ *
+ * @note This function is not intended for application use.
+ * @internal
+ *
+ * @param[in] resource CoAP resource to check
+ * @param[in] uri Null-terminated string URI to compare
+ *
+ * @return 0  if the resource path matches the URI
+ * @return <0 if the resource path sorts before the URI
+ * @return >0 if the resource path sorts after the URI
+ */
+int coap_match_path(const coap_resource_t *resource, uint8_t *uri);
+
 #if defined(MODULE_GCOAP) || defined(DOXYGEN)
 /**
  * @brief   Identifies a packet containing an observe option

sys/net/application_layer/gcoap/gcoap.c

@@ -399,7 +399,7 @@ static int _find_resource(coap_pkt_t *pdu, const coap_resource_t **resource_ptr,
                 resource++;
             }
 
-            int res = strcmp((char *)&uri[0], resource->path);
+            int res = coap_match_path(resource, uri);
             if (res > 0) {
                 continue;
             }

sys/net/application_layer/nanocoap/nanocoap.c

@@ -146,6 +146,21 @@ int coap_parse(coap_pkt_t *pkt, uint8_t *buf, size_t len)
     return 0;
 }
 
+int coap_match_path(const coap_resource_t *resource, uint8_t *uri)
+{
+    assert(resource && uri);
+    int res;
+
+    if (resource->methods & COAP_MATCH_SUBTREE) {
+        int len = strlen(resource->path);
+        res = strncmp((char *)uri, resource->path, len);
+    }
+    else {
+        res = strcmp((char *)uri, resource->path);
+    }
+    return res;
+}
+
 uint8_t *coap_find_option(const coap_pkt_t *pkt, unsigned opt_num)
 {
     const coap_optpos_t *optpos = pkt->options;
@@ -323,7 +338,7 @@ ssize_t coap_handle_req(coap_pkt_t *pkt, uint8_t *resp_buf, unsigned resp_buf_le
             continue;
         }
 
-        int res = strcmp((char *)uri, resource->path);
+        int res = coap_match_path(resource, uri);
         if (res > 0) {
             continue;
         }