Bug#32096341: ISNULL continuously returns the same value for LOAD_FILE

The LOAD_FILE() function must be evaluated per execution, but was
classified as a function returning a const value.

Fixed by adding the proper get_initial_pseudo_tables()
implementation for the class implementing LOAD_FILE().

Reviewed by: Chaithra Gopalareddy <chaithra.gopalareddy@oracle.com>

mysql-test/r/func_str.result

@@ -5203,3 +5203,14 @@ DROP TABLE t1;
 SELECT INSERT(1, 1, 2, _utf8mb4 x'E4B8ADE69687');
 INSERT(1, 1, 2, _utf8mb4 x'E4B8ADE69687')
 中文
+# Bug#32096341: ISNULL continuously returns the same value for LOAD_FILE
+CREATE TABLE t1(id INTEGER NOT NULL PRIMARY KEY);
+INSERT INTO t1 VALUES(0);
+CALL p1();
+is null
+1
+CALL p1();
+is null
+0
+DROP PROCEDURE p1;
+DROP TABLE t1;

mysql-test/t/func_str.test

@@ -2118,3 +2118,22 @@ DROP TABLE t1;
 
 # E4B8ADE69687 == Zhōngwén
 SELECT INSERT(1, 1, 2, _utf8mb4 x'E4B8ADE69687');
+
+--echo # Bug#32096341: ISNULL continuously returns the same value for LOAD_FILE
+
+--let $file=$MYSQLTEST_VARDIR/tmp/bug32096341.txt
+CREATE TABLE t1(id INTEGER NOT NULL PRIMARY KEY);
+INSERT INTO t1 VALUES(0);
+--disable_query_log
+eval CREATE PROCEDURE p1()
+SELECT ISNULL(LOAD_FILE("$file")) AS "is null";
+--enable_query_log
+CALL p1();
+--disable_query_log
+eval SELECT * INTO OUTFILE "$file" from t1;
+--enable_query_log
+CALL p1();
+DROP PROCEDURE p1;
+DROP TABLE t1;
+#isable_query_log;
+remove_file $file;

sql/item_strfunc.h

@@ -959,6 +959,9 @@ class Item_load_file final : public Item_str_func {
   bool itemize(Parse_context *pc, Item **res) override;
   String *val_str(String *) override;
   const char *func_name() const override { return "load_file"; }
+  table_map get_initial_pseudo_tables() const override {
+    return INNER_TABLE_BIT;
+  }
   bool resolve_type(THD *thd) override {
     if (param_type_is_default(thd, 0, 1)) return true;
     collation.set(&my_charset_bin, DERIVATION_COERCIBLE);