Bug#21381806: JSON: ASSERTION FAILED: ARG->NULL_VALUE

The failing assertion asserts that null_value is true if the field
type of an item is MYSQL_TYPE_NULL. This happens before a val_*
function has been called on the item, and null_value is not
necessarily updated until one of the val_* functions is called.

This patch calls update_null_value() on the item to make sure that
null_value is up to date before we try to use it.

Author: kahatlen

mysql-test/r/json.result

@@ -12728,3 +12728,15 @@ SELECT JSON_VALID(REPEAT('{"a":[', 100000));
 ERROR 22032: The JSON document exceeds the maximum depth.
 SELECT JSON_VALID(REPEAT('[{"a":', 100000));
 ERROR 22032: The JSON document exceeds the maximum depth.
+#
+# Bug#21381806 JSON: ASSERTION FAILED: ARG->NULL_VALUE
+#
+SELECT JSON_SET(CASE WHEN 1 THEN NULL ELSE NULL END, '{}', '{}');
+JSON_SET(CASE WHEN 1 THEN NULL ELSE NULL END, '{}', '{}')
+NULL
+SELECT JSON_VALID(CASE WHEN 1 THEN NULL ELSE NULL END);
+JSON_VALID(CASE WHEN 1 THEN NULL ELSE NULL END)
+NULL
+SELECT JSON_ARRAY(CASE WHEN 1 THEN NULL ELSE NULL END);
+JSON_ARRAY(CASE WHEN 1 THEN NULL ELSE NULL END)
+[null]

mysql-test/t/json.test

@@ -5714,6 +5714,13 @@ SELECT JSON_VALID(REPEAT('{"a":[', 100000));
 --error ER_JSON_DOCUMENT_TOO_DEEP
 SELECT JSON_VALID(REPEAT('[{"a":', 100000));
 
+--echo #
+--echo # Bug#21381806 JSON: ASSERTION FAILED: ARG->NULL_VALUE
+--echo #
+SELECT JSON_SET(CASE WHEN 1 THEN NULL ELSE NULL END, '{}', '{}');
+SELECT JSON_VALID(CASE WHEN 1 THEN NULL ELSE NULL END);
+SELECT JSON_ARRAY(CASE WHEN 1 THEN NULL ELSE NULL END);
+
 # Local Variables:
 # mode: sql
 # sql-product: mysql

sql/item_json_func.cc

@@ -260,6 +260,7 @@ static bool json_is_valid(Item **args,
   switch (get_normalized_field_type(arg_item))
   {
   case MYSQL_TYPE_NULL:
+    arg_item->update_null_value();
     DBUG_ASSERT(arg_item->null_value);
     return true;
   case MYSQL_TYPE_JSON:
@@ -1007,6 +1008,7 @@ bool json_value(Item **args, uint arg_idx, Json_wrapper *result)
 
   if (arg->field_type() == MYSQL_TYPE_NULL)
   {
+    arg->update_null_value();
     DBUG_ASSERT(arg->null_value);
     return false;
   }
@@ -1533,9 +1535,16 @@ bool val_json_func_field_subselect(Item* arg,
     my_error(ER_NOT_SUPPORTED_YET, MYF(0), "old decimal type");
     return true;
 
-  case MYSQL_TYPE_NULL:                         // possible?
+  case MYSQL_TYPE_NULL:
+    /*
+      This shouldn't happen, since the only caller of this function
+      returns earlier if it sees that the type is MYSQL_TYPE_NULL.
+    */
+    /* purecov: begin inspected */
+    arg->update_null_value();
     DBUG_ASSERT(arg->null_value);
     return false;
+    /* purecov: end */
 
   case MYSQL_TYPE_JSON:
     DBUG_ABORT();                               /* purecov: inspected */