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Posted to commits@cassandra.apache.org by "Ekaterina Dimitrova (Jira)" <ji...@apache.org> on 2022/02/23 14:37:00 UTC
[jira] [Commented] (CASSANDRA-17401) Race condition in QueryProcessor causes just prepared statement not to be in the prepared statements cache
[ https://issues.apache.org/jira/browse/CASSANDRA-17401?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17496820#comment-17496820 ]
Ekaterina Dimitrova commented on CASSANDRA-17401:
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[~ifesdjeen] , [~marcuse] , do you mind to take a look, please? Looking at the links provided, seems related to the issues you were working on?
> Race condition in QueryProcessor causes just prepared statement not to be in the prepared statements cache
> ----------------------------------------------------------------------------------------------------------
>
> Key: CASSANDRA-17401
> URL: https://issues.apache.org/jira/browse/CASSANDRA-17401
> Project: Cassandra
> Issue Type: Bug
> Reporter: Ivan Senic
> Priority: Normal
>
> The changes in the [QueryProcessor#prepare|https://github.com/apache/cassandra/blame/cassandra-4.0.2/src/java/org/apache/cassandra/cql3/QueryProcessor.java#L575-L638] method that were introduced in versions *4.0.2* and *3.11.12* can cause a race condition between two threads trying to concurrently prepare the same statement. This race condition can cause removing of a prepared statement from the cache, after one of the threads has received the result of the prepare and eventually uses MD5Digest to call [QueryProcessor#getPrepared|https://github.com/apache/cassandra/blame/cassandra-4.0.2/src/java/org/apache/cassandra/cql3/QueryProcessor.java#L212-L215].
> The race condition looks like this:
> * Thread1 enters _prepare_ method and resolves _safeToReturnCached_ as false
> * Thread1 executes eviction of hashes
> * Thread2 enters _prepare_ method and resolves _safeToReturnCached_ as false
> * Thread1 prepares the statement and caches it
> * Thread1 returns the result of the prepare
> * Thread2 executes eviction of hashes
> * Thread1 tries to execute the prepared statement with the received MD5Digest, but statement is not in the cache as it was evicted by Thread2
> I tried to reproduce this by using a Java driver, but hitting this case from a client side is highly unlikely and I can not simulate the needed race condition. However, we can easily reproduce this in Stargate (details [here|https://github.com/stargate/stargate/pull/1647]), as it's closer to QueryProcessor.
>
> Reproducing this in a unit test is fairly easy. I am happy to showcase this if needed.
>
> Note that the issue can occur only when safeToReturnCached is resolved as false.
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