MDEV-21034 GREATEST() and LEAST() malfunction for NULL

There is a convention that Item::val_int() and Item::val_real() return
SQL NULL doing effectively what this code does:
  null_value= true;
  return 0; // Always return 0 for SQL NULL

This is done to optimize boolean value evaluation:
if Item::val_int() or Item::val_real() returned 1 -
that always means TRUE and never can means SQL NULL.
This convention helps to avoid unnecessary testing
Item::null_value after getting a non-zero return value.

Item_func_min_max did not follow this convention.
It could return a non-zero value together with null_value==true.
This made evaluate_join_record() erroneously misinterpret
SQL NULL as TRUE in this call:

  select_cond_result= MY_TEST(select_cond->val_int());

Fixing Item_func_min_max to follow the convention.

mysql-test/main/func_hybrid_type.result

@@ -4313,5 +4313,30 @@ ERROR HY000: Illegal parameter data types bigint unsigned and row for operation
 SELECT @@max_allowed_packet=ROW(1,1);
 ERROR HY000: Illegal parameter data types bigint unsigned and row for operation '='
 #
+# MDEV-21034 GREATEST() and LEAST() malfunction for NULL
+#
+SELECT 5 AS c1 FROM dual WHERE GREATEST(1, NULL);
+c1
+SELECT 5 AS c1 FROM dual WHERE LEAST(1, NULL);
+c1
+CREATE TABLE t0 (c0 INT);
+INSERT INTO t0 VALUES (1);
+SELECT * FROM t0 WHERE GREATEST(c0, NULL);
+c0
+SELECT * FROM t0 WHERE LEAST(c0, NULL);
+c0
+DROP TABLE t0;
+SELECT 5 AS c1 FROM dual WHERE GREATEST(1e0, NULL);
+c1
+SELECT 5 AS c1 FROM dual WHERE LEAST(1e0, NULL);
+c1
+CREATE TABLE t0 (c0 DOUBLE);
+INSERT INTO t0 VALUES (1);
+SELECT * FROM t0 WHERE GREATEST(c0, NULL);
+c0
+SELECT * FROM t0 WHERE LEAST(c0, NULL);
+c0
+DROP TABLE t0;
+#
 # End of 10.5 tests
 #

mysql-test/main/func_hybrid_type.test

@@ -1115,6 +1115,29 @@ SELECT 0x20+ROW(1,1);
 --error ER_ILLEGAL_PARAMETER_DATA_TYPES2_FOR_OPERATION
 SELECT @@max_allowed_packet=ROW(1,1);
 
+--echo #
+--echo # MDEV-21034 GREATEST() and LEAST() malfunction for NULL
+--echo #
+
+SELECT 5 AS c1 FROM dual WHERE GREATEST(1, NULL);
+SELECT 5 AS c1 FROM dual WHERE LEAST(1, NULL);
+
+CREATE TABLE t0 (c0 INT);
+INSERT INTO t0 VALUES (1);
+SELECT * FROM t0 WHERE GREATEST(c0, NULL);
+SELECT * FROM t0 WHERE LEAST(c0, NULL);
+DROP TABLE t0;
+
+SELECT 5 AS c1 FROM dual WHERE GREATEST(1e0, NULL);
+SELECT 5 AS c1 FROM dual WHERE LEAST(1e0, NULL);
+
+CREATE TABLE t0 (c0 DOUBLE);
+INSERT INTO t0 VALUES (1);
+SELECT * FROM t0 WHERE GREATEST(c0, NULL);
+SELECT * FROM t0 WHERE LEAST(c0, NULL);
+DROP TABLE t0;
+
+
 --echo #
 --echo # End of 10.5 tests
 --echo #

sql/item_func.cc

@@ -3022,7 +3022,7 @@ double Item_func_min_max::val_real_native()
 	value=tmp;
     }
     if ((null_value= args[i]->null_value))
-      break;
+      return 0;
   }
   return value;
 }
@@ -3043,7 +3043,7 @@ longlong Item_func_min_max::val_int_native()
 	value=tmp;
     }
     if ((null_value= args[i]->null_value))
-      break;
+      return 0;
   }
   return value;
 }