Make recursive association hydration thread-safe (#568) #598
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This PR addresses a rare thread safety issue in IdentityCache's recursive association hydration, where concurrent access to the same record's dehydrated instance variables could cause a NameError when trying to remove an already-deleted ivar.
Problem
As described in issue #568, multiple threads calling fetch_relation (or similar cached accessors) on the same record can trigger a race condition:
Race condition scenario:
This error manifests as:
NameError: instance variable @dehydrated_relation not definedSolution
Introduce a mutex to synchronize hydration, ensuring only one thread can hydrate a dehydrated association at a time. This prevents the race without over-serializing unrelated operations.