1 files changed, 103 insertions, 0 deletions
diff --git a/app/services/merge_requests/mergeability/logger.rb b/app/services/merge_requests/mergeability/logger.rb
new file mode 100644
index 00000000000..8b45d231e03
--- /dev/null
+++ b/app/services/merge_requests/mergeability/logger.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  module Mergeability
+    class Logger
+      include Gitlab::Utils::StrongMemoize
+
+      def initialize(merge_request:, destination: Gitlab::AppJsonLogger)
+        @destination = destination
+        @merge_request = merge_request
+      end
+
+      def commit
+        return unless enabled?
+
+        commit_logs
+      end
+
+      def instrument(mergeability_name:)
+        raise ArgumentError, 'block not given' unless block_given?
+
+        return yield unless enabled?
+
+        op_start_db_counters = current_db_counter_payload
+        op_started_at = current_monotonic_time
+
+        result = yield
+
+        observe("mergeability.#{mergeability_name}.duration_s", current_monotonic_time - op_started_at)
+
+        observe_sql_counters(mergeability_name, op_start_db_counters, current_db_counter_payload)
+
+        result
+      end
+
+      private
+
+      attr_reader :destination, :merge_request
+
+      def observe(name, value)
+        return unless enabled?
+
+        observations[name.to_s].push(value)
+      end
+
+      def commit_logs
+        attributes = Gitlab::ApplicationContext.current.merge({
+                                                                mergeability_project_id: merge_request.project.id
+                                                              })
+
+        attributes[:mergeability_merge_request_id] = merge_request.id
+        attributes.merge!(observations_hash)
+        attributes.compact!
+        attributes.stringify_keys!
+
+        destination.info(attributes)
+      end
+
+      def observations_hash
+        transformed = observations.transform_values do |values|
+          next if values.empty?
+
+          {
+            'values' => values
+          }
+        end.compact
+
+        transformed.each_with_object({}) do |key, hash|
+          key[1].each { |k, v| hash["#{key[0]}.#{k}"] = v }
+        end
+      end
+
+      def observations
+        strong_memoize(:observations) do
+          Hash.new { |hash, key| hash[key] = [] }
+        end
+      end
+
+      def observe_sql_counters(name, start_db_counters, end_db_counters)
+        end_db_counters.each do |key, value|
+          result = value - start_db_counters.fetch(key, 0)
+          next if result == 0
+
+          observe("mergeability.#{name}.#{key}", result)
+        end
+      end
+
+      def current_db_counter_payload
+        ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload
+      end
+
+      def enabled?
+        strong_memoize(:enabled) do
+          ::Feature.enabled?(:mergeability_checks_logger, merge_request.project)
+        end
+      end
+
+      def current_monotonic_time
+        ::Gitlab::Metrics::System.monotonic_time
+      end
+    end
+  end
+end