sys/net/nanocoap: add and use coap_get_response_hdr_len()

Before, handlers writing blockwise transfer assumed that the response
header length will match the request header length. This is true for
UDP, but not for TCP: The CoAP over TCP header contains a Len field,
that gets extended for larger messages. Since the reply often is indeed
larger than the request, this is indeed often the case for CoAP over
TCP.

Note: Right now, no CoAP over TCP implementation is upstream. However,
      getting rid of incorrect assumptions now will make life easier
      later on.

examples/nanocoap_server/coap_handler.c

@@ -53,7 +53,7 @@ static ssize_t _riot_block2_handler(coap_pkt_t *pkt, uint8_t *buf, size_t len, c
     (void)context;
     coap_block_slicer_t slicer;
     coap_block2_init(pkt, &slicer);
-    uint8_t *payload = buf + coap_get_total_hdr_len(pkt);
+    uint8_t *payload = buf + coap_get_response_hdr_len(pkt);
 
     uint8_t *bufpos = payload;
 

sys/include/net/nanocoap.h

@@ -677,6 +677,22 @@ static inline unsigned coap_get_total_hdr_len(const coap_pkt_t *pkt)
            coap_get_token_len(pkt);
 }
 
+/**
+ * @brief   Get the header length a response to the given packet will have
+ *
+ * @param[in]   pkt     CoAP packet to reply to
+ * @return      Length of the response header including token excluding
+ *              CoAP options and any payload marker
+ *
+ * @note    The main use case is the use of @ref coap_block2_build_reply, which
+ *          is building the CoAP header of the response after options and
+ *          payload have been added.
+ */
+static inline unsigned coap_get_response_hdr_len(const coap_pkt_t *pkt)
+{
+    return coap_get_total_hdr_len(pkt);
+}
+
 /**
  * @brief   Write the given raw message code to given CoAP header
  *
@@ -1939,6 +1955,9 @@ static inline size_t coap_put_option_ct(uint8_t *buf, uint16_t lastonum,
  * @param[in]   payload_len length of payload
  * @param[in]   slicer      slicer to use
  *
+ * @warning Use @ref coap_get_response_hdr_len to determine the size of the
+ *          header this will write.
+ *
  * @returns     size of reply packet on success
  * @returns     <0 on error
  */

sys/include/net/nanocoap_sock.h

@@ -110,10 +110,13 @@
  * Start with coap_block2_init() to read the client request and initialize a
  * coap_slicer_t struct with the size and location for this slice of the
  * overall payload. Then write the block2 option in the response with
- * coap_opt_put_block2(). The option includes an indicator ("more") that a
- * slice completes the overall payload transfer. You may not know the value for
- * _more_ at this point, but you must initialize the space in the packet for
- * the option before writing the payload. The option is rewritten later.
+ * coap_opt_put_block2(). Use @ref coap_get_response_hdr_len to get the length
+ * of the response header: This will be the offset in the buffer where you
+ * should start adding options. The Block2 option includes an indicator
+ * ("more") that a slice completes the overall payload transfer. You may not
+ * know the value for _more_ at this point, but you must initialize the space
+ * in the packet for the option before writing the payload. The option is
+ * rewritten later.
  *
  * Next, use the coap_blockwise_put_xxx() functions to write the payload
  * content. These functions use the coap_block_slicer_t to enable or disable

sys/net/application_layer/nanocoap/nanocoap.c

@@ -1415,7 +1415,7 @@ ssize_t coap_well_known_core_default_handler(coap_pkt_t *pkt, uint8_t *buf, \
     (void)context;
     coap_block_slicer_t slicer;
     coap_block2_init(pkt, &slicer);
-    uint8_t *payload = buf + coap_get_total_hdr_len(pkt);
+    uint8_t *payload = buf + coap_get_response_hdr_len(pkt);
     uint8_t *bufpos = payload;
     bufpos += coap_put_option_ct(bufpos, 0, COAP_FORMAT_LINK);
     bufpos += coap_opt_put_block2(bufpos, COAP_OPT_CONTENT_FORMAT, &slicer, 1);