William Brown wrote:
While doing some integration testing of LMDB I noticed that there may
be an issue with out of order transaction handling.
May be? Do you have sample code that demonstrates the existence of the issue?
The scenario is:
Open Read TXN A
Open Write TXN X, and change values of the DB
Open Read TXN B
Open Write TXN Y, and change values of the DB
Open Read TXN C
Abort/Close TXN B.
At this point, because of the page touch between A -> B and B -> C,
What page touch? Read txns don't do any such thing,
now believes that the pages of A are the "last time" they are available
as they were all subsequently copied for TXN C. The pages of A are then
added to the freelists when B closes.
That's not how it works at all. The freelist is only modified when write txns
commit. Notthing changes when a read txn ends, either by commit or abort.
When TXN A is read from the data
may have been altered due to future writes as LMDB attempts to use
previously allocated pages first.
That's not how it works. As long as read txn A is open none of its pages will
This situation is more likely to arise on large batch writes, but
manifest with smaller series of writes. This would be a silent issue,
as the over-written pages may be valid, and could cause data to
"silently vanish" inside of the read transaction A leading to
I hope that this report helps you to diagnose and resolve the issue.
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/