MDEV-17001 JSON_MERGE returns nullwhen merging empty array.

Don't add the comma if nothing appended to the array.

mysql-test/r/func_json.result

@@ -804,3 +804,9 @@ SELECT JSON_SEARCH(@`json`, 'one', @`value`);
 JSON_SEARCH(@`json`, 'one', @`value`)
 "$[2].C"
 SET @`json` := NULL, @`value` := NULL;
+#
+# MDEV-17001 JSON_MERGE returns nullwhen merging empty array.
+#
+SELECT JSON_MERGE('[1]', '[]');
+JSON_MERGE('[1]', '[]')
+[1]

mysql-test/t/func_json.test

@@ -463,3 +463,8 @@ SET @`json` := '["A", [{"B": "1"}], {"C": "AB"}, {"D": "BC"}]', @`value` := 'AB'
 SELECT JSON_SEARCH(@`json`, 'one', @`value`);
 SET @`json` := NULL, @`value` := NULL;
 
+--echo #
+--echo # MDEV-17001 JSON_MERGE returns nullwhen merging empty array.
+--echo #
+
+SELECT JSON_MERGE('[1]', '[]');

sql/item_jsonfunc.cc

@@ -2020,23 +2020,14 @@ static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
   else
   {
     const uchar *end1, *beg1, *end2, *beg2;
-    int empty_array= 0;
+    int n_items1=1, n_items2= 1;
 
     beg1= je1->value_begin;
 
     /* Merge as a single array. */
     if (je1->value_type == JSON_VALUE_ARRAY)
     {
-      int cur_level= je1->stack_p;
-      empty_array= 1;
-      while (json_scan_next(je1) == 0)
-      {
-        if (je1->stack_p < cur_level)
-          break;
-        empty_array= 0;
-      }
-
-      if (je1->s.error)
+      if (json_skip_level_and_count(je1, &n_items1))
         return 1;
 
       end1= je1->s.c_str - je1->sav_c_len;
@@ -2055,8 +2046,7 @@ static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
         end1= je1->value_end;
     }
 
-    if (str->append((const char*) beg1, end1 - beg1) ||
-        (!empty_array && str->append(", ", 2)))
+    if (str->append((const char*) beg1, end1 - beg1))
       return 3;
 
     if (json_value_scalar(je2))
@@ -2067,15 +2057,22 @@ static int do_merge(String *str, json_engine_t *je1, json_engine_t *je2)
     else
     {
       if (je2->value_type == JSON_VALUE_OBJECT)
+      {
         beg2= je2->value_begin;
+        if (json_skip_level(je2))
+          return 2;
+      }
       else
+      {
         beg2= je2->s.c_str;
-      if (json_skip_level(je2))
-        return 2;
+        if (json_skip_level_and_count(je2, &n_items2))
+          return 2;
+      }
       end2= je2->s.c_str;
     }
 
-    if (str->append((const char*) beg2, end2 - beg2))
+    if ((n_items1 && n_items2 && str->append(", ", 2)) ||
+        str->append((const char*) beg2, end2 - beg2))
       return 3;
 
     if (je2->value_type != JSON_VALUE_ARRAY &&

strings/json_lib.c

@@ -1197,10 +1197,6 @@ int json_skip_to_level(json_engine_t *j, int level)
 }
 
 
-#define json_skip_level(json_engine) \
-  json_skip_to_level((json_engine), (json_engine)->stack_p)
-
-
 /*
   works as json_skip_level() but also counts items on the current
   level skipped.