Make recursive association hydration thread-safe (#568)

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- Fix rare race condition causing `NameError` (missing `@dehydrated_*` ivar) when hydrating recursively
+  embedded associations by synchronizing hydration to be thread-safe.
+
 ## 1.6.3
 
 - Split the `with_deferred_parent_expiration` and `with_deferred_parent_expiration`. (#578)

lib/identity_cache/cached/recursive/association.rb

@@ -7,6 +7,7 @@ class Association < Cached::Association # :nodoc:
         def initialize(name, reflection:)
           super
           @dehydrated_variable_name = :"@dehydrated_#{name}"
+          @hydration_mutex = Mutex.new
         end
 
         attr_reader :dehydrated_variable_name
@@ -29,10 +30,7 @@ def read(record)
             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)
+              hydrate_safely(record, assoc)
             else
               assoc.load_target
             end
@@ -76,6 +74,21 @@ def embedded_recursively?
 
         private
 
+        def hydrate_safely(record, assoc)
+          @hydration_mutex.synchronize do
+            if record.instance_variable_defined?(records_variable_name)
+              return record.instance_variable_get(records_variable_name)
+            end
+
+            dehydrated_target = record.instance_variable_get(dehydrated_variable_name)
+            association_target = hydrate_association_target(assoc.klass, dehydrated_target)
+            if record.instance_variable_defined?(dehydrated_variable_name)
+              record.remove_instance_variable(dehydrated_variable_name)
+            end
+            set_with_inverse(record, association_target)
+          end
+        end
+
         def set_inverse(record, association_target)
           return if association_target.nil?
 

test/cached/recursive/has_many_test.rb

@@ -45,6 +45,33 @@ def test_embedded_recursively
         def test_embedded_by_reference
           refute_predicate(has_many, :embedded_by_reference?)
         end
+
+        def test_thread_safety_hydration
+          # Simulate race condition by setting up a dehydrated record
+          record = AssociatedRecord.new
+          dehydrated_data = [{ "id" => 1, "name" => "Test" }] # Mock dehydrated data
+          record.instance_variable_set(has_many.dehydrated_variable_name, dehydrated_data)
+
+          # Simulate multiple threads trying to hydrate concurrently
+          threads = []
+          errors = []
+
+          10.times do
+            threads << Thread.new do
+              begin
+                # Call the cached accessor which triggers hydration
+                record.fetch_deeply_associated_records
+              rescue => e
+                errors << e
+              end
+            end
+          end
+
+          threads.each(&:join)
+
+          # Assert no NameError occurred (which would indicate the race condition)
+          assert_empty(errors, "Race condition detected: #{errors.map(&:message).join(', ')}")
+        end
       end
     end
   end