Fixed bug #52394 / LP bug #623209.

When an incremental join cache is used to join a table whose
fields are not referenced anywhere in the query the association
pointer to the last record in the such cache can be the same
as the pointer to the end of the buffer. 
The function JOIN_CACHE_BKA::get_next_key must take into 
consideration this when iterating over the keys of the records
from the join buffer. 
The assertion in JOIN_TAB_SCAN_MRR::next also must take this
into consideration.
Borrowed a slightly changed test case from a patch attached to the
bug #52394.

mysql-test/r/join_cache.result

@@ -5174,3 +5174,31 @@ a	b
 set join_cache_level=default;
 drop table t1,t2,t3;
 set @@optimizer_switch=@save_optimizer_switch;
+#
+# Bug #52394:  using join buffer for 3 table join with ref access 
+# LP #623209: and no references to the columns of the middle table
+#
+set join_cache_level=6;
+CREATE TABLE t1 (a int(11), b varchar(1));
+INSERT INTO t1 VALUES (6,'r'),(27,'o');
+CREATE TABLE t2(a int);
+INSERT INTO t2 VALUES(1),(2),(3),(4),(5);
+CREATE TABLE t3 (a int(11) primary key, b varchar(1));
+INSERT INTO t3 VALUES
+(14,'d'),(15,'z'),(16,'e'),(17,'h'),(18,'b'),(19,'s'),(20,'e'),
+(21,'j'),(22,'e'),(23,'f'),(24,'v'),(25,'x'),(26,'m'),(27,'o');
+EXPLAIN
+SELECT t3.a FROM t1,t2,t3 WHERE t1.a = t3.a AND t1.b = t3.b;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+1	SIMPLE	t2	ALL	NULL	NULL	NULL	NULL	5	Using join buffer
+1	SIMPLE	t3	eq_ref	PRIMARY	PRIMARY	4	test.t1.a	1	Using where; Using join buffer
+SELECT t3.a FROM t1,t2,t3 WHERE t1.a = t3.a AND t1.b = t3.b;
+a
+27
+27
+27
+27
+27
+DROP TABLE t1,t2,t3;
+set join_cache_level=default;

mysql-test/t/join_cache.test

@@ -1956,3 +1956,31 @@ set join_cache_level=default;
 drop table t1,t2,t3;
 
 set @@optimizer_switch=@save_optimizer_switch;
+
+--echo #
+--echo # Bug #52394:  using join buffer for 3 table join with ref access 
+--echo # LP #623209: and no references to the columns of the middle table
+--echo #
+
+
+set join_cache_level=6;
+
+CREATE TABLE t1 (a int(11), b varchar(1));
+INSERT INTO t1 VALUES (6,'r'),(27,'o');
+
+CREATE TABLE t2(a int);
+INSERT INTO t2 VALUES(1),(2),(3),(4),(5);
+
+CREATE TABLE t3 (a int(11) primary key, b varchar(1));
+INSERT INTO t3 VALUES
+(14,'d'),(15,'z'),(16,'e'),(17,'h'),(18,'b'),(19,'s'),(20,'e'),
+(21,'j'),(22,'e'),(23,'f'),(24,'v'),(25,'x'),(26,'m'),(27,'o');
+
+EXPLAIN
+SELECT t3.a FROM t1,t2,t3 WHERE t1.a = t3.a AND t1.b = t3.b;
+SELECT t3.a FROM t1,t2,t3 WHERE t1.a = t3.a AND t1.b = t3.b;
+
+DROP TABLE t1,t2,t3;
+
+set join_cache_level=default;
+

sql/sql_join_cache.cc

@@ -3391,8 +3391,12 @@ int JOIN_TAB_SCAN_MRR::next()
   uint rc= join_tab->table->file->multi_range_read_next(ptr) ? -1 : 0;
   if (!rc)
   {
+    /* 
+      If a record in in an incremental cache contains no fields then the
+      association for the last record in cache will be equal to cache->end_pos
+    */ 
     DBUG_ASSERT(cache->buff <= (uchar *) (*ptr) &&
-                (uchar *) (*ptr) < cache->end_pos);
+                (uchar *) (*ptr) <= cache->end_pos);
     update_virtual_fields(join_tab->table);
   }
   return rc;
@@ -3728,12 +3732,12 @@ uint JOIN_CACHE_BKA::get_next_key(uchar ** key)
   uchar *init_pos;
   JOIN_CACHE *cache;
   
-  if (pos > last_rec_pos || !records)
-    return 0;
-
   /* Any record in a BKA cache is prepended with its length */
   DBUG_ASSERT(with_length);
    
+  if ((pos+size_of_rec_len) > last_rec_pos || !records)
+    return 0;
+
   /* Read the length of the record */
   rec_len= get_rec_length(pos);
   pos+= size_of_rec_len;