mgmt: mcumgr: transport: Add zcbor buffer reset function

Adds a common zcbor reset buffer function, this allows for the
buffer to be reset after data has been added already to add a
different response, e.g. an error response

Signed-off-by: Jamie McCrae <[email protected]>

Author: thedjnK

subsys/mgmt/mcumgr/transport/include/mgmt/mcumgr/transport/smp_internal.h

@@ -12,6 +12,7 @@
 #include <zephyr/kernel.h>
 #include <zephyr/net_buf.h>
 #include <zephyr/mgmt/mcumgr/transport/smp.h>
+#include <zcbor_encode.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -79,6 +80,30 @@ void *smp_alloc_rsp(const void *req, void *arg);
  */
 void smp_free_buf(void *buf, void *arg);
 
+/**
+ * @brief	Reeset a zcbor encoder to allow a new response.
+ *
+ * If a response has already been (partially) generated than this will allow resetting back to
+ * the default state so that new response can be used (e.g. an error).
+ *
+ * @param streamer	The streamer providing the required SMP callbacks.
+ *
+ * @return	true on success, false on failure (memory error).
+ */
+static inline bool smp_mgmt_reset_zse(struct smp_streamer *streamer)
+{
+	zcbor_state_t *zse = streamer->writer->zs;
+
+	/* Because there is already data in the buffer, it must be cleared first */
+	net_buf_reset(streamer->writer->nb);
+	streamer->writer->nb->len = sizeof(struct smp_hdr);
+	zcbor_new_encode_state(zse, ARRAY_SIZE(streamer->writer->zs),
+			       streamer->writer->nb->data + sizeof(struct smp_hdr),
+			       net_buf_tailroom(streamer->writer->nb), 0);
+
+	return zcbor_map_start_encode(zse, CONFIG_MCUMGR_SMP_CBOR_MAX_MAIN_MAP_ENTRIES);
+}
+
 #ifdef __cplusplus
 }
 #endif