1 files changed, 84 insertions, 0 deletions
diff --git a/lib/gitlab/repository_cache_adapter.rb b/lib/gitlab/repository_cache_adapter.rb
new file mode 100644
index 00000000000..7f64a8c9e46
--- /dev/null
+++ b/lib/gitlab/repository_cache_adapter.rb
@@ -0,0 +1,84 @@
+module Gitlab
+  module RepositoryCacheAdapter
+    extend ActiveSupport::Concern
+
+    class_methods do
+      # Wraps around the given method and caches its output in Redis and an instance
+      # variable.
+      #
+      # This only works for methods that do not take any arguments.
+      def cache_method(name, fallback: nil, memoize_only: false)
+        original = :"_uncached_#{name}"
+
+        alias_method(original, name)
+
+        define_method(name) do
+          cache_method_output(name, fallback: fallback, memoize_only: memoize_only) do
+            __send__(original) # rubocop:disable GitlabSecurity/PublicSend
+          end
+        end
+      end
+    end
+
+    # RepositoryCache to be used. Should be overridden by the including class
+    def cache
+      raise NotImplementedError
+    end
+
+    # Caches the supplied block both in a cache and in an instance variable.
+    #
+    # The cache key and instance variable are named the same way as the value of
+    # the `key` argument.
+    #
+    # This method will return `nil` if the corresponding instance variable is also
+    # set to `nil`. This ensures we don't keep yielding the block when it returns
+    # `nil`.
+    #
+    # key - The name of the key to cache the data in.
+    # fallback - A value to fall back to in the event of a Git error.
+    def cache_method_output(key, fallback: nil, memoize_only: false, &block)
+      ivar = cache_instance_variable_name(key)
+
+      if instance_variable_defined?(ivar)
+        instance_variable_get(ivar)
+      else
+        # If the repository doesn't exist and a fallback was specified we return
+        # that value inmediately. This saves us Rugged/gRPC invocations.
+        return fallback unless fallback.nil? || cache.repository.exists?
+
+        begin
+          value =
+            if memoize_only
+              yield
+            else
+              cache.fetch(key, &block)
+            end
+
+          instance_variable_set(ivar, value)
+        rescue Gitlab::Git::Repository::NoRepository
+          # Even if the above `#exists?` check passes these errors might still
+          # occur (for example because of a non-existing HEAD). We want to
+          # gracefully handle this and not cache anything
+          fallback
+        end
+      end
+    end
+
+    # Expires the caches of a specific set of methods
+    def expire_method_caches(methods)
+      methods.each do |key|
+        cache.expire(key)
+
+        ivar = cache_instance_variable_name(key)
+
+        remove_instance_variable(ivar) if instance_variable_defined?(ivar)
+      end
+    end
+
+    private
+
+    def cache_instance_variable_name(key)
+      :"@#{key.to_s.tr('?!', '')}"
+    end
+  end
+end