Introduce Deferred Parent Cache Expiry (#569)

* Update platforms following rebase

* Introduce `.with_deferred_parent_expiration`

* Add mechanism to prevent nested blocks

This could have caused the cache to have been overwritten

* Ensure the thread variables are reset when exiting the block

* Add changelog entry

* Namespace `Thread.current` keys

* Add rdoc documentation

* Add version number

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+## 1.6.0
+
+- Introduce `.with_deferred_parent_expiration`, which takes a block and avoids duplicate parent cache expiry. (#569)
+
 ## 1.5.6
 
 - Minor performance improvements on association read

Gemfile.lock

@@ -86,6 +86,7 @@ GEM
 
 PLATFORMS
   arm64-darwin-22
+  arm64-darwin-23
   x86_64-linux
 
 DEPENDENCIES

lib/identity_cache.rb

@@ -60,6 +60,8 @@ class AssociationError < StandardError; end
 
   class InverseAssociationError < StandardError; end
 
+  class NestedDeferredParentBlockError < StandardError; end
+
   class UnsupportedScopeError < StandardError; end
 
   class UnsupportedAssociationError < StandardError; end
@@ -191,6 +193,44 @@ def fetch_multi(*keys)
       result
     end
 
+    # Executes a block with deferred parent expiration, ensuring that the parent
+    # records' cache expiration is deferred until the block completes. When the block
+    # completes, it triggers expiration of the primary index for the parent records.
+    # Raises a NestedDeferredParentBlockError if a deferred parent expiration block
+    # is already active on the current thread.
+    #
+    # == Parameters:
+    # No parameters.
+    #
+    # == Raises:
+    # NestedDeferredParentBlockError if a deferred parent expiration block is already active.
+    #
+    # == Yield:
+    # Runs the provided block with deferred parent expiration.
+    #
+    # == Returns:
+    # The result of executing the provided block.
+    #
+    # == Ensures:
+    # Cleans up thread-local variables related to deferred parent expiration regardless
+    # of whether the block raises an exception.
+    def with_deferred_parent_expiration
+      raise NestedDeferredParentBlockError if Thread.current[:idc_deferred_parent_expiration]
+
+      Thread.current[:idc_deferred_parent_expiration] = true
+      Thread.current[:idc_parent_records_for_cache_expiry] = Set.new
+
+      result = yield
+
+      Thread.current[:idc_deferred_parent_expiration] = nil
+      Thread.current[:idc_parent_records_for_cache_expiry].each(&:expire_primary_index)
+
+      result
+    ensure
+      Thread.current[:idc_deferred_parent_expiration] = nil
+      Thread.current[:idc_parent_records_for_cache_expiry].clear
+    end
+
     def with_fetch_read_only_records(value = true)
       old_value = Thread.current[:identity_cache_fetch_read_only_records]
       Thread.current[:identity_cache_fetch_read_only_records] = value

lib/identity_cache/parent_model_expiration.rb

@@ -47,6 +47,10 @@ def expire_parent_caches
       add_parents_to_cache_expiry_set(parents_to_expire)
       parents_to_expire.select! { |parent| parent.class.primary_cache_index_enabled }
       parents_to_expire.reduce(true) do |all_expired, parent|
+        if Thread.current[:idc_deferred_parent_expiration]
+          Thread.current[:idc_parent_records_for_cache_expiry] << parent
+          next parent
+        end
         parent.expire_primary_index && all_expired
       end
     end

test/index_cache_test.rb

@@ -166,6 +166,95 @@ def test_unique_cache_index_with_non_id_primary_key
     assert_equal(123, KeyedRecord.fetch_by_value("a").id)
   end
 
+  def test_with_deferred_parent_expiration_expires_parent_index_once
+    Item.send(:cache_has_many, :associated_records, embed: true)
+
+    @parent = Item.create!(title: "bob")
+    @records = @parent.associated_records.create!([{ name: "foo" }, { name: "bar" }, { name: "baz" }])
+
+    @memcached_spy = Spy.on(backend, :write).and_call_through
+
+    expected_item_expiration_count = Array(@parent).count
+    expected_associated_record_expiration_count = @records.count
+
+    expected_return_value = "Some text that we expect to see returned from the block"
+
+    result = IdentityCache.with_deferred_parent_expiration do
+      @parent.transaction do
+        @parent.associated_records.destroy_all
+      end
+      assert_equal(expected_associated_record_expiration_count, @memcached_spy.calls.count)
+      expected_return_value
+    end
+
+    expired_cache_keys = @memcached_spy.calls.map(&:args).map(&:first)
+    item_expiration_count = expired_cache_keys.count { _1.include?("Item") }
+    associated_record_expiration_count = expired_cache_keys.count { _1.include?("AssociatedRecord") }
+
+    assert_operator(@memcached_spy.calls.count, :>, 0)
+    assert_equal(expected_item_expiration_count, item_expiration_count)
+    assert_equal(expected_associated_record_expiration_count, associated_record_expiration_count)
+    assert_equal(expected_return_value, result)
+  end
+
+  def test_double_nested_deferred_parent_expiration_will_raise_error
+    Item.send(:cache_has_many, :associated_records, embed: true)
+
+    @parent = Item.create!(title: "bob")
+    @records = @parent.associated_records.create!([{ name: "foo" }, { name: "bar" }, { name: "baz" }])
+
+    @memcached_spy = Spy.on(backend, :write).and_call_through
+
+    assert_raises(IdentityCache::NestedDeferredParentBlockError) do
+      IdentityCache.with_deferred_parent_expiration do
+        IdentityCache.with_deferred_parent_expiration do
+          @parent.transaction do
+            @parent.associated_records.destroy_all
+          end
+        end
+      end
+    end
+
+    assert_equal(0, @memcached_spy.calls.count)
+  end
+
+  def test_deep_association_with_deferred_parent_expiration_expires_parent_once
+    AssociatedRecord.send(:has_many, :deeply_associated_records, dependent: :destroy)
+    Item.send(:cache_has_many, :associated_records, embed: true)
+
+    @parent = Item.create!(title: "bob")
+    @records = @parent.associated_records.create!([{ name: "foo" }, { name: "bar" }, { name: "baz" }])
+    @records.each do
+      _1.deeply_associated_records.create!([
+        { name: "a", item: @parent },
+        { name: "b", item: @parent },
+        { name: "c", item: @parent },
+      ])
+    end
+
+    @memcached_spy = Spy.on(backend, :write).and_call_through
+
+    expected_item_expiration_count = Array(@parent).count
+    expected_associated_record_expiration_count = @records.count
+    expected_deeply_associated_record_expiration_count = @records.flat_map(&:deeply_associated_records).count
+
+    IdentityCache.with_deferred_parent_expiration do
+      @parent.transaction do
+        @parent.associated_records.destroy_all
+      end
+    end
+
+    expired_cache_keys = @memcached_spy.calls.map(&:args).map(&:first)
+    item_expiration_count = expired_cache_keys.count { _1.include?("Item") }
+    associated_record_expiration_count = expired_cache_keys.count { _1.include?(":AssociatedRecord:") }
+    deeply_associated_record_expiration_count = expired_cache_keys.count { _1.include?("DeeplyAssociatedRecord") }
+
+    assert_operator(@memcached_spy.calls.count, :>, 0)
+    assert_equal(expected_item_expiration_count, item_expiration_count)
+    assert_equal(expected_associated_record_expiration_count, associated_record_expiration_count)
+    assert_equal(expected_deeply_associated_record_expiration_count, deeply_associated_record_expiration_count)
+  end
+
   private
 
   def cache_key(unique: false)