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

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.