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

include/my_base.h

@@ -445,7 +445,12 @@ enum ha_base_keytype {
 #define HA_ERR_FILE_TOO_SHORT	  175	 /* File too short */
 #define HA_ERR_WRONG_CRC	  176	 /* Wrong CRC on page */
 #define HA_ERR_TOO_MANY_CONCURRENT_TRXS 177 /*Too many active concurrent transactions */
-#define HA_ERR_LAST               177    /* Copy of last error nr */
+
+/* The error codes from 178 to 180 is not used, because we need to
+maintain forward compatibility with higher versions. */
+
+#define HA_ERR_TABLE_IN_FK_CHECK  181	 /* Table being used in foreign key check */
+#define HA_ERR_LAST               181    /* Copy of last error nr */
 
 /* Number of different errors */
 #define HA_ERR_ERRORS            (HA_ERR_LAST - HA_ERR_FIRST + 1)