MDEV-17001 JSON_MERGE returns nullwhen merging empty array.

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

1 files changed, 13 insertions, 16 deletions

diff --git a/sql/item_jsonfunc.cc b/sql/item_jsonfunc.cc
index 4cb79749903..60726050aca 100644
--- a/sql/item_jsonfunc.cc
+++ b/sql/item_jsonfunc.cc
@@ -2020,23 +2020,14 @@ continue_j2:
   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 @@ continue_j2:
         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 @@ continue_j2:
     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 &&