(ITS#7635) lmdb race condition in read vs write txns

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (84.127.144.82)
Submitted by: hyc


It's possible for a write txn to update a meta page while a reader is creating
its snapshot of the meta page. In most cases this situation is harmless, but if
the update raises the mm_last_pg field it may later cause an assert if the
reader still has the old value of mm_last_pg, but accesses a DB whose pages are
beyond the old value.

A fix has been tested and will be committed shortly.