coap_pdu.c: Support incrementing reference count on const coap_pdu_t parameters

const coap_pdu_t * parameters are passed into application (call-back handler)
code.  If the application code then calls a libcoap API with a
const coap_pdu_t * parameter, it is not possible to increment the reference
counter so that multiple entities can reference the same PDU and an entity's
call to coap_delete_pdu() does not prematurely remove the PDU until all the
entities have finished.

This code allows the (const) PDU's reference counter to be incremented.

include/coap3/coap_pdu_internal.h

@@ -426,6 +426,24 @@ coap_pdu_t *coap_pdu_duplicate_lkd(const coap_pdu_t *old_pdu,
  * @return same as PDU
  */
 coap_pdu_t *coap_pdu_reference_lkd(coap_pdu_t *pdu);
+
+/**
+ * Increment reference counter on a const pdu to stop it prematurely getting freed
+ * off when coap_delete_pdu() is called.
+ *
+ * In other words, if coap_const_pdu_reference_lkd() is called once, then the first
+ * call to coap_delete_pdu() will decrement the reference counter, but not delete
+ * the PDU and the second call to coap_delete_pdu() then actually deletes the PDU.
+ *
+ * Needed when const coap_pdu_t* is passed from application handlers.
+ *
+ * Note: This function must be called in the locked state.
+ *
+ * @param pdu The CoAP PDU.
+ * @return non const version of PDU.
+ */
+coap_pdu_t *coap_const_pdu_reference_lkd(const coap_pdu_t *pdu);
+
 /** @} */
 
 #endif /* COAP_COAP_PDU_INTERNAL_H_ */

src/coap_pdu.c

@@ -1626,3 +1626,16 @@ coap_pdu_reference_lkd(coap_pdu_t *pdu) {
   }
   return pdu;
 }
+
+coap_pdu_t *
+coap_const_pdu_reference_lkd(const coap_pdu_t *pdu) {
+  coap_pdu_t *pdu_rw = NULL;
+
+  if (pdu != NULL) {
+
+    /* Need to do this to not get a compiler warning about const parameters */
+    memcpy(&pdu_rw, &pdu, sizeof(pdu_rw));
+    ++pdu_rw->ref;
+  }
+  return pdu_rw;
+}