Fixed bug #29611.

If a primary key is defined over column c of enum type then 
the EXPLAIN command for a look-up query of the form
  SELECT * FROM t WHERE c=0
said that the query was with an impossible where condition though the
query correctly returned non-empty result set when the table indeed 
contained rows with error empty strings for column c. 

This kind of misbehavior was due to a bug in the function 
Field_enum::store(longlong,bool) that erroneously returned 1 if
the the value to be stored was equal to 0. 
Note that the method 
Field_enum::store(const char *from,uint length,CHARSET_INFO *cs)
correctly returned 0 if a value of the error empty string 
was stored. 

mysql-test/r/type_enum.result

@@ -1829,3 +1829,27 @@ c1 + 0
 0
 2
 DROP TABLE t1,t2;
+CREATE TABLE t1(a enum('a','b','c','d'));
+INSERT INTO t1 VALUES (4),(1),(0),(3);
+Warnings:
+Warning	1265	Data truncated for column 'a' at row 3
+SELECT a FROM t1;
+a
+d
+a
+
+c
+EXPLAIN SELECT a FROM t1 WHERE a=0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using where
+SELECT a FROM t1 WHERE a=0;
+a
+
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+EXPLAIN SELECT a FROM t1 WHERE a=0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	const	PRIMARY	PRIMARY	1	const	1	Using index
+SELECT a FROM t1 WHERE a=0;
+a
+
+DROP TABLE t1;

mysql-test/t/type_enum.test

@@ -200,3 +200,27 @@ CREATE TABLE t2 SELECT * FROM t1;
 SELECT c1 + 0 FROM t2;
 
 DROP TABLE t1,t2;
+
+#
+# Bug#29661: Lookup by 0 for a primary index over a enum type  
+#
+
+CREATE TABLE t1(a enum('a','b','c','d'));
+INSERT INTO t1 VALUES (4),(1),(0),(3);
+
+SELECT a FROM t1;
+
+EXPLAIN SELECT a FROM t1 WHERE a=0;
+SELECT a FROM t1 WHERE a=0;
+
+ALTER TABLE t1 ADD PRIMARY KEY (a);
+
+EXPLAIN SELECT a FROM t1 WHERE a=0;
+SELECT a FROM t1 WHERE a=0;
+
+DROP TABLE t1;
+
+
+
+
+

sql/field.cc

@@ -7640,8 +7640,11 @@ int Field_enum::store(longlong nr, bool unsigned_val)
   if ((ulonglong) nr > typelib->count || nr == 0)
   {
     set_warning(MYSQL_ERROR::WARN_LEVEL_WARN, WARN_DATA_TRUNCATED, 1);
-    nr=0;
-    error=1;
+    if (nr != 0 || table->in_use->count_cuted_fields)
+    {
+      nr= 0;
+      error= 1;
+    }
   }
   store_type((ulonglong) (uint) nr);
   return error;