Instrument sub-transactions created using ApplicationRecord

app/models/application_record.rb

@@ -86,4 +86,17 @@ class ApplicationRecord < ActiveRecord::Base
     values = enum_mod.definition.transform_values { |v| v[:value] }
     enum(enum_mod.key => values)
   end
+
+  def self.transaction(**options, &block)
+    if options[:requires_new] && track_subtransactions?
+      ::Gitlab::Database::Metrics.subtransactions_increment(self.name)
+    end
+
+    super(**options, &block)
+  end
+
+  def self.track_subtransactions?
+    ::Feature.enabled?(:active_record_subtransactions_counter, type: :ops, default_enabled: :yaml) &&
+      connection.transaction_open?
+  end
 end

config/feature_flags/ops/active_record_subtransactions_counter.yml

+---
+name: active_record_subtransactions_counter
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66477
+rollout_issue_url:
+milestone: '14.1'
+type: ops
+group: group::pipeline execution
+default_enabled: false

lib/gitlab/database/load_balancing/host.rb

@@ -42,9 +42,9 @@ module Gitlab
         # timeout - The time after which the pool should be forcefully
         #           disconnected.
         def disconnect!(timeout = 120)
-          start_time = Metrics::System.monotonic_time
+          start_time = ::Gitlab::Metrics::System.monotonic_time
 
-          while (Metrics::System.monotonic_time - start_time) <= timeout
+          while (::Gitlab::Metrics::System.monotonic_time - start_time) <= timeout
             break if pool.connections.none?(&:in_use?)
 
             sleep(2)

lib/gitlab/database/metrics.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    class Metrics
+      extend ::Gitlab::Utils::StrongMemoize
+
+      class << self
+        def subtransactions_increment(model_name)
+          subtransactions_counter.increment(model: model_name)
+        end
+
+        private
+
+        def subtransactions_counter
+          strong_memoize(:subtransactions_counter) do
+            name = :gitlab_active_record_subtransactions_total
+            comment = 'Total amount of subtransactions created by ActiveRecord'
+
+            ::Gitlab::Metrics.counter(name, comment)
+          end
+        end
+      end
+    end
+  end
+end

spec/models/application_record_spec.rb

@@ -105,6 +105,50 @@ RSpec.describe ApplicationRecord do
     end
   end
 
+  describe '.transaction', :delete do
+    it 'opens a new transaction' do
+      expect(described_class.connection.transaction_open?).to be false
+
+      Project.transaction do
+        expect(Project.connection.transaction_open?).to be true
+
+        Project.transaction(requires_new: true) do
+          expect(Project.connection.transaction_open?).to be true
+        end
+      end
+    end
+
+    it 'does not increment a counter when a transaction is not nested' do
+      expect(described_class.connection.transaction_open?).to be false
+
+      expect(::Gitlab::Database::Metrics)
+        .not_to receive(:subtransactions_increment)
+
+      Project.transaction do
+        expect(Project.connection.transaction_open?).to be true
+      end
+
+      Project.transaction(requires_new: true) do
+        expect(Project.connection.transaction_open?).to be true
+      end
+    end
+
+    it 'increments a counter when a nested transaction is created' do
+      expect(described_class.connection.transaction_open?).to be false
+
+      expect(::Gitlab::Database::Metrics)
+        .to receive(:subtransactions_increment)
+        .with('Project')
+        .once
+
+      Project.transaction do
+        Project.transaction(requires_new: true) do
+          expect(Project.connection.transaction_open?).to be true
+        end
+      end
+    end
+  end
+
   describe '.with_fast_read_statement_timeout' do
     context 'when the query runs faster than configured timeout' do
       it 'executes the query without error' do