Bug#51650: crash with user variables and triggers

The problem was that bits of the destructive equality propagation
optimization weren't being undone after the execution of a stored
program. Modifications to the parse tree that are based on transient
properties must be undone to enable the re-execution of stored
programs.

The solution is to cleanup any references to predicates generated
by the equality propagation during the execution of a stored program.

mysql-test/r/trigger.result

@@ -2087,4 +2087,28 @@ ERROR 42S02: Table 'test.a_nonextisting_table' doesn't exist
 SELECT * FROM t2;
 a	b
 DROP TABLE t1, t2;
+#
+# Bug#51650 crash with user variables and triggers
+#
+DROP TRIGGER IF EXISTS trg1;
+DROP TABLE IF EXISTS t1, t2;
+CREATE TABLE t1 (b VARCHAR(50) NOT NULL);
+CREATE TABLE t2 (a VARCHAR(10) NOT NULL DEFAULT '');
+CREATE TRIGGER trg1 AFTER INSERT ON t2
+FOR EACH ROW BEGIN
+SELECT 1 FROM t1 c WHERE
+(@bug51650 IS NULL OR @bug51650 != c.b) AND c.b = NEW.a LIMIT 1 INTO @foo;
+END//
+SET @bug51650 = 1;
+INSERT IGNORE INTO t2 VALUES();
+Warnings:
+Warning	1329	No data - zero rows fetched, selected, or processed
+INSERT IGNORE INTO t1 SET b = '777';
+INSERT IGNORE INTO t2 SET a = '111';
+Warnings:
+Warning	1329	No data - zero rows fetched, selected, or processed
+SET @bug51650 = 1;
+INSERT IGNORE INTO t2 SET a = '777';
+DROP TRIGGER trg1;
+DROP TABLE t1, t2;
 End of 5.1 tests.

mysql-test/t/trigger.test

@@ -2396,4 +2396,34 @@ SELECT * FROM t2;
 
 DROP TABLE t1, t2;
 
+--echo #
+--echo # Bug#51650 crash with user variables and triggers
+--echo #
+
+--disable_warnings
+DROP TRIGGER IF EXISTS trg1;
+DROP TABLE IF EXISTS t1, t2;
+--enable_warnings
+
+CREATE TABLE t1 (b VARCHAR(50) NOT NULL);
+CREATE TABLE t2 (a VARCHAR(10) NOT NULL DEFAULT '');
+
+delimiter //;
+CREATE TRIGGER trg1 AFTER INSERT ON t2
+FOR EACH ROW BEGIN
+  SELECT 1 FROM t1 c WHERE
+    (@bug51650 IS NULL OR @bug51650 != c.b) AND c.b = NEW.a LIMIT 1 INTO @foo;
+END//
+delimiter ;//
+
+SET @bug51650 = 1;
+INSERT IGNORE INTO t2 VALUES();
+INSERT IGNORE INTO t1 SET b = '777';
+INSERT IGNORE INTO t2 SET a = '111';
+SET @bug51650 = 1;
+INSERT IGNORE INTO t2 SET a = '777';
+
+DROP TRIGGER trg1;
+DROP TABLE t1, t2;
+
 --echo End of 5.1 tests.

sql/item.cc

@@ -4475,6 +4475,7 @@ void Item_field::cleanup()
     I.e. we can drop 'field'.
    */
   field= result_field= 0;
+  item_equal= NULL;
   null_value= FALSE;
   DBUG_VOID_RETURN;
 }