mi_fix_rec_buff_for_blob() generalized a bit

myisam/mi_extra.c:
  make use of new mi_fix_rec_buff_for_blob(), and fix a (possible) bug
myisam/mi_open.c:
  make use of new mi_fix_rec_buff_for_blob()

myisam/mi_dynrec.c

@@ -1094,16 +1094,23 @@ int _mi_read_dynamic_record(MI_INFO *info, my_off_t filepos, byte *buf)
 byte *mi_fix_rec_buff_for_blob(MI_INFO *info, ulong length)
 {
   uint extra;
+
+  /* to simplify initial init of info->rec_buf in mi_open and mi_extra */
+  if (!length)
+      length=max(info->s->base.pack_reclength,info->s->base.max_key_length);
+
   if (! info->rec_buff || length > info->alloced_rec_buff_length)
   {
     byte *newptr;
-    extra=ALIGN_SIZE(MI_MAX_DYN_BLOCK_HEADER)+MI_SPLIT_LENGTH+
-      MI_DYN_DELETE_BLOCK_HEADER;
-    if (!(newptr=(byte*) my_realloc((gptr) info->rec_alloc,length+extra,
+    extra= ((info->s->options & HA_OPTION_PACK_RECORD) ?
+        ALIGN_SIZE(MI_MAX_DYN_BLOCK_HEADER)+MI_SPLIT_LENGTH+
+        MI_DYN_DELETE_BLOCK_HEADER : 0);
+    if (!(newptr=(byte*) my_realloc((gptr) info->rec_alloc,length+extra+8,
 				    MYF(MY_ALLOW_ZERO_PTR))))
-      return newptr;
+      return (byte *)0;
     info->rec_alloc=newptr;
-    info->rec_buff=newptr+ALIGN_SIZE(MI_DYN_DELETE_BLOCK_HEADER);
+    info->rec_buff=newptr+(extra ?
+                   ALIGN_SIZE(MI_DYN_DELETE_BLOCK_HEADER) : 0);
     info->alloced_rec_buff_length=length;
   }
   return info->rec_buff;

myisam/mi_extra.c

@@ -331,7 +331,7 @@ int mi_extra(MI_INFO *info, enum ha_extra_function function)
     {
       my_free(info->rec_alloc,MYF(MY_ALLOW_ZERO_PTR));
       info->rec_alloc=info->rec_buff=0;
-      mi_fix_rec_buff_for_blob(info,info->s->base.pack_reclength);
+      mi_fix_rec_buff_for_blob(info, 0);
     }
     break;
   case HA_EXTRA_NORMAL:				/* Theese isn't in use */

myisam/mi_open.c

@@ -506,20 +506,10 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
 
   /* Allocate buffer for one record */
 
-  extra=0;
-  if (share->options & HA_OPTION_PACK_RECORD)
-    extra=ALIGN_SIZE(MI_MAX_DYN_BLOCK_HEADER)+MI_SPLIT_LENGTH+
-      MI_DYN_DELETE_BLOCK_HEADER;
-
-  tmp_length=max(share->base.pack_reclength,share->base.max_key_length);
-  info.alloced_rec_buff_length=tmp_length;
-  if (!(info.rec_alloc=(byte*) my_malloc(tmp_length+extra+8,
-					 MYF(MY_WME | MY_ZEROFILL))))
+  /* prerequisites: bzero(info) && info->s=share; are met. */
+  if (!mi_fix_rec_buff_for_blob(&info, 0))
     goto err;
-  if (extra)
-    info.rec_buff=info.rec_alloc+ALIGN_SIZE(MI_MAX_DYN_BLOCK_HEADER);
-  else
-    info.rec_buff=info.rec_alloc;
+  bzero(info.rec_alloc, info.alloced_rec_buff_length);
 
   *m_info=info;
 #ifdef THREAD