Re: (ITS#7774) LMDB assertion failure during Postfix cache cleanup

[hyc@symas.com]

Wietse Venema wrote:
> Hallvard Breien Furuseth:
>>> Sofar the abstraction layer already hides the LMDB-specific MAP_FULL
>>> and MAP_RESIZED error conditions. If this abstraction layer needs
>>> additional code in order to maintain MDB cursor sanity, then please
>>> educate me.
>>
>> ldmb.h says --
>>
>> mdb_env_set_mapsize():
>>      "It may be called at later times if no transactions are active in
>>      this process. Note that the library does not check for this condition,
>>      the caller must ensure it explicitly."
>> MDB_NOLOCK:
>>      "[caller] must ensure that no readers are using old transactions
>>      while a writer is active. The simplest approach is to use an
>>      exclusive lock so that no readers may be active at all when a writer
>>      begins."
>>
>> That's why I talked about saving the cursor position and restoring it -
>> cursors are per-transaction and you need a new transaction.
>
> I can remember the last key from mdb_cursor_get() and set the cursor
> to that key. There does not appear to be a cursor "save" operation
> in the API documentation http://symas.com/mdb/doc/group__mdb.html.

Hallvard is suggesting exactly that - remember the key. There is no cursor 
"save" operation, any saved state would be meaningless after one write 
transaction.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/