RHEL5 x64, skiplist statuscache SIGV

[Bron Gondwana]

On Sun, 23 Sep 2007 21:58:57 +0200, "Michael Glad" <glad at daimi.au.dk> said:
> 
> Hello Bron, I've tested Cyrus 2.3.9 + your FM patches as of September 20
> on a RHEL5 x64 Opteron virtual machine. 

We don't have any experience (yet) with 64 bit.  I believe it's less
well tested.

> As I'm a BDB skepticist  I use skiplists where ever possible including 
> the statuscache.

>From memory it's not a good match because of the various strengths and
weaknesses of the the database methods used.  Skiplist is good for heavy
read loads, not so good for heavy write loads.

> But Imapd SIGV's in the skiplist mydelete routine (invoked from
> statuscache_invalidate) when the first 'select INBOX' command is
> received.
> 
> I've looked into the cause and would to hear your opinion before I
> post the info below to the devel list.
> 
> It seems to me that the skiplist routine is to blame. The SIGV
> arises from line 1305:
> 
> 	newoffset = htonl(FORWARD(ptr, i));
> 
> But ptr at the time of the crash points to db->map_base which is the
> initial skiplist header => Kaboom.
> 
> When I apply the patch:
> 
> @@ -1292,6 +1292,11 @@
>      ptr = find_node(db, key, keylen, updateoffsets);
>      if (ptr == db->map_base ||
>         !db->compar(KEY(ptr), KEYLEN(ptr), key, keylen)) {
> +/* start glad */
> +if(ptr == db->map_base) {
> +       ptr = DUMMY_PTR(db);
> +}
> +/* end glad */
>         /* gotcha */
>         offset = ptr - db->map_base;
> 
> things doesn't SIGV and output from
> 
> 	/usr/lib/cyrus-imapd/cyr_dbtool /var/lib/imap/statuscache.db skiplist show
> 
> looks sensible -- it is partly garbled on my nonpatched RHEL4 
> x32 production system.


I have CC'd this to the cyrus development list, as there are probably people with
more skiplist and 64 bit programming experience who can tell you if this is the
right approach, and hopefully push the fix into the next Cyrus release!

Bron.
-- 
  Bron Gondwana
  brong at fastmail.fm