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Thread safety when calling fetch_relation #568

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@madejejej

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@madejejej

Hi!

We utilize a global, in-memory cache with TTL for one of our models per process. We also wanted to use identity_cache so that the whole object tree is fetched with one network call.

However, we receive rare errors when we call fetch_association with the following stack trace:

NameError instance variable @dehydrated_relation not defined

/usr/local/bundle/ruby/3.3.0/gems/identity_cache-1.5.6/lib/identity_cache/cached/recursive/association.rb:34
/usr/local/bundle/ruby/3.3.0/gems/identity_cache-1.5.6/lib/identity_cache/cached/recursive/association.rb:19

which points to this piece of code:

def read(record)
assoc = record.association(name)
if !assoc.loaded? && assoc.target.blank? && (record.send(:loaded_by_idc?) || assoc.klass.should_use_cache?)
if record.instance_variable_defined?(records_variable_name)
record.instance_variable_get(records_variable_name)
elsif record.instance_variable_defined?(dehydrated_variable_name)
dehydrated_target = record.instance_variable_get(dehydrated_variable_name)
association_target = hydrate_association_target(assoc.klass, dehydrated_target)
record.remove_instance_variable(dehydrated_variable_name)
set_with_inverse(record, association_target)
else
assoc.load_target
end
else
assoc.load_target
end
end

Since the code has a guard clause: record.instance_variable_defined?(dehydrated_variable_name), the only way this can happen is that another thread is concurrently executing the same code and has already removed this instance variable.

Are you open to contributions to fix concurrency issues? So far we've only seen this error popping up dozens of times on ~50M requests, however, there might be more that we haven't seen yet or are failing silently 👀

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