Re: Possible transaction issue with LMDB

[Howard Chu <hyc@symas.com>]

William Brown wrote:
> Hi there,
>
> 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
> Commit X
> Open Read TXN B
> Open Write TXN Y, and change values of the DB
> Commit Y
> 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,

> B
> 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
be reused.

> This situation is more likely to arise on large batch writes, but could
> 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
> unpredictable results.
>
> 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/