MDEV-25178 JSON_TABLE: ASAN use-after-poison in my_fill_8bit...

MDEV-25178 JSON_TABLE: ASAN use-after-poison in my_fill_8bit Json_table_column::On_response::respond.

table record buffer size was smaller than required when the charset
changes.

mysql-test/suite/json/r/json_table.result

@@ -516,5 +516,11 @@ a	b
 NULL	NULL
 DROP VIEW v;
 #
+# MDEV-25178 JSON_TABLE: ASAN use-after-poison in my_fill_8bit / Json_table_column::On_response::respond
+#
+SELECT * FROM JSON_TABLE('{}', '$' COLUMNS(a CHAR(100) PATH '$' DEFAULT "0" ON ERROR)) AS jt;
+a
+0
+#
 # End of 10.6 tests
 #

mysql-test/suite/json/t/json_table.test

@@ -395,6 +395,11 @@ SHOW CREATE VIEW v;
 SELECT * FROM v;
 DROP VIEW v;
 
+--echo #
+--echo # MDEV-25178 JSON_TABLE: ASAN use-after-poison in my_fill_8bit / Json_table_column::On_response::respond
+--echo #
+SELECT * FROM JSON_TABLE('{}', '$' COLUMNS(a CHAR(100) PATH '$' DEFAULT "0" ON ERROR)) AS jt;
+
 --echo #
 --echo # End of 10.6 tests
 --echo #

sql/field.h

@@ -1611,7 +1611,8 @@ class Field: public Value_source
   virtual longlong val_time_packed(THD *thd);
   virtual const TYPELIB *get_typelib() const { return NULL; }
   virtual CHARSET_INFO *charset() const= 0;
-  virtual void change_charset(const DTCollation &new_cs) {}
+  /* returns TRUE if the new charset differs. */
+  virtual bool change_charset(const DTCollation &new_cs) { return FALSE; }
   virtual const DTCollation &dtcollation() const= 0;
   virtual CHARSET_INFO *charset_for_protocol(void) const
   { return binary() ? &my_charset_bin : charset(); }
@@ -2110,11 +2111,16 @@ class Field_str :public Field {
   {
     return m_collation;
   }
-  void change_charset(const DTCollation &new_cs) override
+  bool change_charset(const DTCollation &new_cs) override
   {
-    field_length= (field_length * new_cs.collation->mbmaxlen) /
-                  m_collation.collation->mbmaxlen;
-    m_collation= new_cs;
+    if (m_collation.collation != new_cs.collation)
+    {
+      field_length= (field_length * new_cs.collation->mbmaxlen) /
+        m_collation.collation->mbmaxlen;
+      m_collation= new_cs;
+      return TRUE;
+    }
+    return FALSE;
   }
   bool binary() const override { return field_charset() == &my_charset_bin; }
   uint32 max_display_length() const override { return field_length; }

sql/json_table.cc

@@ -811,7 +811,7 @@ bool Create_json_table::add_json_table_fields(THD *thd, TABLE *table,
     */
     sql_f->length= sql_f->char_length;
     if (!(jc->m_explicit_cs= sql_f->charset))
-      sql_f->charset= thd->variables.collation_server;
+      sql_f->charset= &my_charset_utf8mb4_bin;
 
     if (sql_f->prepare_stage1(thd, thd->mem_root, table->file,
                               table->file->ha_table_flags()))
@@ -1148,12 +1148,12 @@ int Table_function_json_table::setup(THD *thd, TABLE_LIST *sql_table,
       Json_table_column *jc= jc_i++;
       uint32 old_pack_length= f->pack_length();
 
-      f->change_charset(
-           jc->m_explicit_cs ? jc->m_explicit_cs : m_json->collation);
-
-      if (field_offset)
+      if (f->change_charset(
+                jc->m_explicit_cs ? jc->m_explicit_cs : m_json->collation) ||
+          field_offset)
       {
-        f->move_field(f->ptr + field_offset, f->null_ptr, f->null_bit);
+        if (field_offset)
+          f->move_field(f->ptr + field_offset, f->null_ptr, f->null_bit);
         f->reset();
       }