[jira] [Created] (CASSANDRA-17401) Race condition in QueryProcessor causes just prepared statement not to be in the prepared statements cache

["Ivan Senic (Jira)" <ji...@apache.org>]

Ivan Senic created CASSANDRA-17401:
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             Summary: 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


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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