Add a flag to maintain backward compatibility

By default CoAP will create a copy of the whole data to be passed to application and it keeps the backward compatibility.

If enabled, application must NOT free the payload when it gets the COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED status.
And application must call sn_coap_protocol_block_remove() instead.

features/frameworks/mbed-coap/CHANGELOG.md

@@ -1,24 +1,18 @@
 # Change Log
 
+## [v5.1.0](https://github.com/ARMmbed/mbed-coap/releases/tag/v5.1.0)
+
+- Introduce SN_COAP_REDUCE_BLOCKWISE_HEAP_FOOTPRINT configuration flag. 
+  Flag is disabled by default to keep the backward compatibility in place.
+  If flag is enabled, application must NOT free the payload when it gets the COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED status.
+  And application must call sn_coap_protocol_block_remove() instead.
+
+-[Full Changelog](https://github.com/ARMmbed/mbed-coap/compare/v5.0.0...v5.1.0)
+
 ## [v5.0.0](https://github.com/ARMmbed/mbed-coap/releases/tag/v5.0.0)
- **NOTE! Blockwise functionality has changed and it is not backward compatible. User is now responsible of freeing the data by calling sn_coap_protocol_block_remove() and must NOT free the payload anymore separately.**
-
-Here is the change needed on application side:
-
-```
-if (received_coap_message->coap_status == COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED) {
-    free(received_coap_header->payload_ptr);
-}
-
--->
-
-if (received_coap_message->coap_status == COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED) {
-    // Free the block message from the CoAP list
-    sn_nsdl_remove_coap_block(_nsdl_handle, address, received_coap_header->payload_len, received_coap_header->payload_ptr);
-}
-```
 
 - Reduce heap footprint by storing only single block when receiving a blockwise message.
+    * User is now responsible of freeing the data by calling sn_coap_protocol_block_remove() and must not free the payload separately.
 - Bug fix: Request blockwise transfer if incoming payload length is too large and when it comes without block indication. 
 
 -[Full Changelog](https://github.com/ARMmbed/mbed-coap/compare/v4.8.1...v5.0.0)
@@ -250,4 +244,3 @@ Extend blockwise message transfer status to have states for sending as well.
 **New feature**
 
  - Initial release of mbed-coap separated from mbed-client-c
-

features/frameworks/mbed-coap/mbed-coap/sn_config.h

@@ -260,4 +260,14 @@
 #define SN_COAP_BLOCKWISE_INTERNAL_BLOCK_2_HANDLING_ENABLED  1
 #endif
 
+/**
+ * \def SN_COAP_REDUCE_BLOCKWISE_HEAP_FOOTPRINT
+ * \brief A heap optimization switch, which removes unnecessary copy of the blockwise data.
+ * If enabled, application must NOT free the payload when it gets the COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED status.
+ * Application must call sn_coap_protocol_block_remove() instead.
+ */
+#ifndef SN_COAP_REDUCE_BLOCKWISE_HEAP_FOOTPRINT
+#define SN_COAP_REDUCE_BLOCKWISE_HEAP_FOOTPRINT              0   /**< Disabled by default */
+#endif
+
 #endif // SN_CONFIG_H

features/frameworks/mbed-coap/source/sn_coap_protocol.c

@@ -2293,8 +2293,18 @@ static bool sn_coap_handle_last_blockwise(struct coap_s *handle, const sn_nsdl_a
     if (!whole_payload_len) {
         return false;
     }
+
+#if SN_COAP_REDUCE_BLOCKWISE_HEAP_FOOTPRINT
     received_coap_msg_ptr->payload_ptr = payload_ptr;
     received_coap_msg_ptr->payload_len = whole_payload_len;
+#else
+    received_coap_msg_ptr->payload_ptr = sn_coap_protocol_malloc_copy(handle, payload_ptr, whole_payload_len);
+    if (received_coap_msg_ptr->payload_ptr == NULL) {
+        tr_error("sn_coap_handle_last_blockwise - failed to allocate whole package!");
+        return false;
+    }
+    received_coap_msg_ptr->payload_len = whole_payload_len;
+#endif
     received_coap_msg_ptr->coap_status = COAP_STATUS_PARSER_BLOCKWISE_MSG_RECEIVED;
 
     return true;