1 files changed, 111 insertions, 0 deletions
diff --git a/lib/gitlab/cache/helpers.rb b/lib/gitlab/cache/helpers.rb
new file mode 100644
index 00000000000..7b11d6bc9ff
--- /dev/null
+++ b/lib/gitlab/cache/helpers.rb
@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Cache
+    module Helpers
+      # @return [ActiveSupport::Duration]
+      DEFAULT_EXPIRY = 1.day
+
+      # @return [ActiveSupport::Cache::Store]
+      def cache
+        Rails.cache
+      end
+
+      def render_cached(obj_or_collection, with:, cache_context: -> (_) { current_user&.cache_key }, expires_in: Gitlab::Cache::Helpers::DEFAULT_EXPIRY, **presenter_args)
+        json =
+          if obj_or_collection.is_a?(Enumerable)
+            cached_collection(
+              obj_or_collection,
+              presenter: with,
+              presenter_args: presenter_args,
+              context: cache_context,
+              expires_in: expires_in
+            )
+          else
+            cached_object(
+              obj_or_collection,
+              presenter: with,
+              presenter_args: presenter_args,
+              context: cache_context,
+              expires_in: expires_in
+            )
+          end
+
+        render Gitlab::Json::PrecompiledJson.new(json)
+      end
+
+      private
+
+      # Optionally uses a `Proc` to add context to a cache key
+      #
+      # @param object [Object] must respond to #cache_key
+      # @param context [Proc] a proc that will be called with the object as an argument, and which should return a
+      #                       string or array of strings to be combined into the cache key
+      # @return [String]
+      def contextual_cache_key(presenter, object, context)
+        return object.cache_key if context.nil?
+
+        [presenter.class.name, object.cache_key, context.call(object)].flatten.join(":")
+      end
+
+      # Used for fetching or rendering a single object
+      #
+      # @param object [Object] the object to render
+      # @param presenter [Grape::Entity]
+      # @param presenter_args [Hash] keyword arguments to be passed to the entity
+      # @param context [Proc]
+      # @param expires_in [ActiveSupport::Duration, Integer] an expiry time for the cache entry
+      # @return [String]
+      def cached_object(object, presenter:, presenter_args:, context:, expires_in:)
+        cache.fetch(contextual_cache_key(presenter, object, context), expires_in: expires_in) do
+          Gitlab::Json.dump(presenter.represent(object, **presenter_args).as_json)
+        end
+      end
+
+      # Used for fetching or rendering multiple objects
+      #
+      # @param objects [Enumerable<Object>] the objects to render
+      # @param presenter [Grape::Entity]
+      # @param presenter_args [Hash] keyword arguments to be passed to the entity
+      # @param context [Proc]
+      # @param expires_in [ActiveSupport::Duration, Integer] an expiry time for the cache entry
+      # @return [Array<String>]
+      def cached_collection(collection, presenter:, presenter_args:, context:, expires_in:)
+        json = fetch_multi(presenter, collection, context: context, expires_in: expires_in) do |obj|
+          Gitlab::Json.dump(presenter.represent(obj, **presenter_args).as_json)
+        end
+
+        json.values
+      end
+
+      # An adapted version of ActiveSupport::Cache::Store#fetch_multi.
+      #
+      # The original method only provides the missing key to the block,
+      # not the missing object, so we have to create a map of cache keys
+      # to the objects to allow us to pass the object to the missing value
+      # block.
+      #
+      # The result is that this is functionally identical to `#fetch`.
+      def fetch_multi(presenter, *objs, context:, **kwargs)
+        objs.flatten!
+        map = multi_key_map(presenter, objs, context: context)
+
+        # TODO: `contextual_cache_key` should be constructed based on the guideline https://docs.gitlab.com/ee/development/redis.html#multi-key-commands.
+        Gitlab::Instrumentation::RedisClusterValidator.allow_cross_slot_commands do
+          cache.fetch_multi(*map.keys, **kwargs) do |key|
+            yield map[key]
+          end
+        end
+      end
+
+      # @param objects [Enumerable<Object>] objects which _must_ respond to `#cache_key`
+      # @param context [Proc] a proc that can be called to help generate each cache key
+      # @return [Hash]
+      def multi_key_map(presenter, objects, context:)
+        objects.index_by do |object|
+          contextual_cache_key(presenter, object, context)
+        end
+      end
+    end
+  end
+end