bulk_insert feature can be turned off

Docs/manual.texi

@@ -18370,7 +18370,8 @@ After this many write locks, allow some read locks to run in between.
 MySQL uses special tree-like cache to make bulk inserts (that is,
 @code{INSERT ... SELECT}, @code{INSERT ... VALUES (...), (...), ...}, and
 @code{LOAD DATA INFILE}) faster. This variable limits
-the size of the cache tree in bytes per thread.
+the size of the cache tree in bytes per thread. Setting it to 0
+will disable this optimization.
 @strong{Note:} This cache is only used when adding data to non-empty table.
 Default value is 8 MB.
 

myisam/mi_write.c

@@ -809,7 +809,7 @@ int _mi_init_bulk_insert(MI_INFO *info)
     }
   }
 
-  if (!num_keys)
+  if (num_keys==0 || num_keys>myisam_bulk_insert_tree_size)
     return 0;
 
   info->bulk_insert=(TREE *)

sql/mysqld.cc

@@ -2930,7 +2930,7 @@ CHANGEABLE_VAR changeable_vars[] = {
   { "max_write_lock_count",    (long*) &max_write_lock_count,
       ~0L, 1, ~0L, 0, 1 },
   { "myisam_bulk_insert_tree_size", (long*) &myisam_bulk_insert_tree_size,
-      8192*1024, 4, ~0L, 0, 1 },
+      8192*1024, 0, ~0L, 0, 1 },
   { "myisam_block_size", 	(long*) &opt_myisam_block_size,
       MI_KEY_BLOCK_LENGTH, MI_MIN_KEY_BLOCK_LENGTH, MI_MAX_KEY_BLOCK_LENGTH,
     0, MI_MIN_KEY_BLOCK_LENGTH },