Finish the merge of the patch for bug #15195 from 4.1 -> 5.0

- Adapt it to work with the handlerton class

sql/handler.cc

@@ -302,7 +302,9 @@ handler *get_new_handler(TABLE *table, MEM_ROOT *alloc, enum db_type db_type)
 #endif
   case DB_TYPE_MRG_MYISAM:
   case DB_TYPE_MRG_ISAM:
-    return new (alloc) ha_myisammrg(table);
+    if (have_merge_db == SHOW_OPTION_YES)
+      return new (alloc) ha_myisammrg(table);
+    return NULL;
 #ifdef HAVE_BERKELEY_DB
   case DB_TYPE_BERKELEY_DB:
     if (have_berkeley_db == SHOW_OPTION_YES)

sql/mysql_priv.h

@@ -1323,8 +1323,11 @@ extern handlerton ndbcluster_hton;
 extern SHOW_COMP_OPTION have_ndbcluster;
 #endif
 
+/* MRG_MYISAM handler is always built, but may be skipped */
+extern handlerton myisammrg_hton;
+#define have_merge_db myisammrg_hton.state
+
 extern SHOW_COMP_OPTION have_isam;
-extern SHOW_COMP_OPTION have_merge_db;
 extern SHOW_COMP_OPTION have_raid, have_openssl, have_symlink, have_dlopen;
 extern SHOW_COMP_OPTION have_query_cache;
 extern SHOW_COMP_OPTION have_geometry, have_rtree_keys;

sql/mysqld.cc

@@ -6887,6 +6887,11 @@ get_one_option(int optid, const struct my_option *opt __attribute__((unused)),
     global_system_variables.tx_isolation= (type-1);
     break;
   }
+  case OPT_MERGE:
+    if (opt_merge)
+      have_merge_db= SHOW_OPTION_YES;
+    else
+      have_merge_db= SHOW_OPTION_DISABLED;
 #ifdef HAVE_BERKELEY_DB
   case OPT_BDB_NOSYNC:
     /* Deprecated option */