Add function to export subsets in chunks

This is useful if large amounts of data should be exported using small
buffer sizes.

include/thingset.h

@@ -1693,6 +1693,27 @@ void thingset_init_global(struct thingset_context *ts);
 int thingset_process_message(struct thingset_context *ts, const uint8_t *msg, size_t msg_len,
                              uint8_t *rsp, size_t rsp_size);
 
+/**
+ * Retrieve data for given subset(s) in chunks.
+ *
+ * Exports object data for the given subset to the supplied buffer in the specified format starting
+ * at the given object index. At present, only binary formats are supported.
+ *
+ * @param ts Pointer to ThingSet context.
+ * @param buf Pointer to the buffer where the data should be stored
+ * @param buf_size Size of the buffer, i.e. maximum allowed length of the data
+ * @param subsets Flags to select which subset(s) of data items should be exported
+ * @param format Protocol data format to be used (text, binary with IDs or binary with names)
+ * @param index Pointer to an integer which tracks the current object being exported
+ *
+ * @retval len Length of the written chunk.
+ * @retval 0 When export is complete.
+ * @retval err Negative ThingSet response code in case of error.
+ */
+int thingset_export_subsets_chunks(struct thingset_context *ts, uint8_t *buf, size_t buf_size,
+                                  uint16_t subsets, enum thingset_data_format format,
+                                  uint16_t *index);
+
 /**
  * Retrieve data for given subset(s).
  *
@@ -1710,8 +1731,12 @@ int thingset_process_message(struct thingset_context *ts, const uint8_t *msg, si
  *
  * @return Actual length of the data or negative ThingSet response code in case of error.
  */
-int thingset_export_subsets(struct thingset_context *ts, uint8_t *buf, size_t buf_size,
-                            uint16_t subsets, enum thingset_data_format format);
+static inline int thingset_export_subsets(struct thingset_context *ts, uint8_t *buf,
+                                          size_t buf_size, uint16_t subsets,
+                                          enum thingset_data_format format)
+{
+    return thingset_export_subsets_chunks(ts, buf, buf_size, subsets, format, NULL);
+}
 
 /**
  * Export id, value and/or name of a single data item.

src/thingset.c

@@ -130,8 +130,9 @@ int thingset_process_message(struct thingset_context *ts, const uint8_t *msg, si
     return ret;
 }
 
-int thingset_export_subsets(struct thingset_context *ts, uint8_t *buf, size_t buf_size,
-                            uint16_t subsets, enum thingset_data_format format)
+int thingset_export_subsets_chunks(struct thingset_context *ts, uint8_t *buf, size_t buf_size,
+                                   uint16_t subsets, enum thingset_data_format format,
+                                   uint16_t *index)
 {
     int ret;
 
@@ -156,11 +157,11 @@ int thingset_export_subsets(struct thingset_context *ts, uint8_t *buf, size_t bu
             return -THINGSET_ERR_NOT_IMPLEMENTED;
     }
 
-    ret = ts->api->serialize_subsets(ts, subsets, NULL);
+    ret = ts->api->serialize_subsets(ts, subsets, index);
 
     ts->api->serialize_finish(ts);
 
-    if (ret == 0) {
+    if (ret == 0 || ret == 1) {
         ret = ts->rsp_pos;
     }
 

tests/protocol/src/bin.c

@@ -876,6 +876,40 @@ ZTEST(thingset_bin, test_export_item)
     zassert_mem_equal(buf_exp, buf_act, 2);
 }
 
+ZTEST(thingset_bin, test_export_subsets_chunks)
+{
+    uint8_t buf_chunk[16];
+    uint16_t index = 0;
+    int ret;
+
+    const char chunk1_exp_hex[] =
+        "A3 "
+        "10 19 03 E8 "             /* t_s */
+        "19 02 01 F5 "             /* Types/wBool */
+        "19 06 00 02";             /* Records: 2 */
+    uint8_t chunk1_exp[THINGSET_TEST_BUF_SIZE];
+    int chunk1_exp_len = hex2bin_spaced(chunk1_exp_hex, chunk1_exp, sizeof(chunk1_exp));
+
+    const char chunk2_exp_hex[] =
+        "A2 "
+        "19 07 01 01 "             /* Nested/rBeginning */
+        "19 07 08 FA 40 0C CC CD"; /* Nested/Obj2/rItem2_V */
+    uint8_t chunk2_exp[THINGSET_TEST_BUF_SIZE];
+    int chunk2_exp_len = hex2bin_spaced(chunk2_exp_hex, chunk2_exp, sizeof(chunk2_exp));
+
+    ret = thingset_export_subsets_chunks(&ts, buf_chunk, sizeof(buf_chunk), SUBSET_LIVE,
+                                                THINGSET_BIN_IDS_VALUES, &index);
+    zassert_equal(ret, chunk1_exp_len);
+    zassert_not_equal(index, 0);
+    zassert_mem_equal(chunk1_exp, buf_chunk, chunk1_exp_len);
+
+    ret = thingset_export_subsets_chunks(&ts, buf_chunk, sizeof(buf_chunk), SUBSET_LIVE,
+                                                THINGSET_BIN_IDS_VALUES, &index);
+    zassert_equal(ret, chunk2_exp_len);
+    zassert_equal(index, 0);
+    zassert_mem_equal(chunk2_exp, buf_chunk, chunk2_exp_len);
+}
+
 ZTEST(thingset_bin, test_iterate_subsets)
 {
     struct thingset_data_object *obj = NULL;