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https://issues.apache.org/jira/browse/CASSANDRA-17401?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17496911#comment-17496911
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Ivan Senic commented on CASSANDRA-17401:
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[~ifesdjeen] I have to admin that internals of Cassandra are not 100% familiar
to me. However, wouldn't there be a _PreparedQueryNotFoundException_ thrown in
the _ExecuteMessage_ or the _BatchMessage_ if the _QueryHandler.getPrepared_
returns null? I don't know how this is exception handled or how is the
re-preparing actually working.
Maybe I was not clear in my explanation above, but bottom line is that if the
race condition is uncovering the issue described, the
_QueryHandler.getPrepared_ will return null for the MD5 that was just returned
by the same class prepare method call. If you think this is acceptable, then
you can close the issue.
> 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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