nrf_rpc: add function for decoding string without copying

This commit adds new helper function nrf_rpc_decode_str_ptr_and_len()
that decodes CBOR string without copying it to a buffer. It allows to
get string length before buffer allocation.

Signed-off-by: Konrad Derda <[email protected]>

include/nrf_rpc/nrf_rpc_serialize.h

@@ -229,6 +229,15 @@ int64_t nrf_rpc_decode_int64(struct nrf_rpc_cbor_ctx *ctx);
  */
 char *nrf_rpc_decode_str(struct nrf_rpc_cbor_ctx *ctx, char *buffer, size_t buffer_size);
 
+/** @brief Decode a string pointer and length. Moves CBOR pointer past string on success.
+ *
+ * @param[in,out] ctx CBOR decoding context.
+ * @param[out]  size String length.
+ *
+ * @retval Pointer to a string within CBOR stream or NULL on error.
+ */
+const void *nrf_rpc_decode_str_ptr_and_len(struct nrf_rpc_cbor_ctx *ctx, size_t *len);
+
 /** Decode a string value into a scratchpad.
  *
  * @param[in] scratchpad Pointer to the scratchpad.

subsys/nrf_rpc/nrf_rpc_serialize.c

@@ -318,6 +318,32 @@ char *nrf_rpc_decode_str(struct nrf_rpc_cbor_ctx *ctx, char *buffer, size_t buff
 	return buffer;
 }
 
+const void *nrf_rpc_decode_str_ptr_and_len(struct nrf_rpc_cbor_ctx *ctx, size_t *size)
+{
+	struct zcbor_string zst = {0};
+
+	if (is_decoder_invalid(ctx)) {
+		return NULL;
+	}
+
+	if (zcbor_nil_expect(ctx->zs, NULL)) {
+		return NULL;
+	}
+
+	if (ctx->zs->constant_state->error != ZCBOR_ERR_WRONG_TYPE) {
+		return NULL;
+	}
+
+	zcbor_pop_error(ctx->zs);
+
+	if (!zcbor_tstr_decode(ctx->zs, &zst)) {
+		return NULL;
+	}
+
+	*size = zst.len;
+	return zst.value;
+}
+
 char *nrf_rpc_decode_str_into_scratchpad(struct nrf_rpc_scratchpad *scratchpad, size_t *len)
 {
 	struct nrf_rpc_cbor_ctx *ctx = scratchpad->ctx;