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From Benoit Tellier <be...@minet.net>
Subject [PATCH] Fix CassandraModSeqProvider
Date Wed, 17 Dec 2014 18:05:20 GMT

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Hi,

I found in the current Cassandra implementation a possible data race
during modseq generation. It is located in james-mailbox project and in
cassandra subproject.

The previous implementation, relying on Cassandra counters, first
increments the modseq for the current mailbox in a first request to
Cassandra, and in a second time is reading the new modseq value, using a
second request.

If two mails for the same mailbox are received on different servers at
the same time, it is possible that both server proceed to the first
operation in the same time, setting ModSeq value from n to n+2. During
the second operation, they will return both the n+2 value, and both
mails will have the same modseq.



To solve the bug, I used Cassandra's lightweight tansactions :
 - I created a table dedicated to modseq storage ( I needed a separated
table as in unit tests modseq operations are used with non initialised
mailbox )
 - We ensure that an entry exist for this mailbox
 - We read and update the value ( in two operations ) until our
modification is applied ( using Cassandra lightweight transactions )
 - I added a max retry parameter so that we can't dead lock. Upon
failure ( too many tries ), we throw a mailbox exception.



You will find the corresponding patch as an attachment to this current
e-mail,

Sincerly yours,

Benoit Tellier
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