Bug#13635833: MULTIPLE CRASHES IN FOREIGN KEY CODE WITH CONCURRENT DDL/DML

There are two threads.  In one thread, dml operation is going on 
involving cascaded update operation.  In another thread, alter 
table add foreign key constraint is happening.  Under these 
circumstances, it is possible for the dml thread to access a 
dict_foreign_t object that has been freed by the ddl thread.  
The debug sync test case provides the sequence of operations.  
Without fix, the test case will crash the server (because of 
newly added assert).  With fix, the alter table stmt will return 
an error message.  
      
Backporting the fix from MySQL 5.5 to 5.1

rb:961
rb:947

storage/innodb_plugin/include/db0err.h

@@ -97,6 +97,8 @@ enum db_err {
 	DB_FOREIGN_EXCEED_MAX_CASCADE,	/* Foreign key constraint related
 					cascading delete/update exceeds
 					maximum allowed depth */
+	DB_TABLE_IN_FK_CHECK,		/* table is being used in foreign
+					key check */
 
 	/* The following are partial failure codes */
 	DB_FAIL = 1000,