Default pool size in Gitlab::Database.config

Fix max_threads for Puma with no cli_config

config/initializers/database_config.rb

@@ -20,25 +20,15 @@ Gitlab.ee do
   end
 end
 
-# We configure the database connection pool size automatically based on the
-# configured concurrency. We also add some headroom, to make sure we don't run
-# out of connections when more threads besides the 'user-facing' ones are
-# running.
-#
-# Read more about this in doc/development/database/client_side_connection_pool.md
-
-headroom = (ENV["DB_POOL_HEADROOM"].presence || 10).to_i
-calculated_pool_size = Gitlab::Runtime.max_threads + headroom
-
 db_config = Gitlab::Database.config ||
             Rails.application.config.database_configuration[Rails.env]
 
-db_config['pool'] = calculated_pool_size
+db_config['pool'] = Gitlab::Database.default_pool_size
 ActiveRecord::Base.establish_connection(db_config)
 
 Gitlab.ee do
   if Gitlab::Runtime.sidekiq? && Gitlab::Geo.geo_database_configured?
-    Rails.configuration.geo_database['pool'] = calculated_pool_size
+    Rails.configuration.geo_database['pool'] = Gitlab::Database.default_pool_size
     Geo::TrackingBase.establish_connection(Rails.configuration.geo_database)
   end
 end

lib/gitlab/database.rb

@@ -43,10 +43,27 @@ module Gitlab
     # It does not include the default public schema
     EXTRA_SCHEMAS = [DYNAMIC_PARTITIONS_SCHEMA, STATIC_PARTITIONS_SCHEMA].freeze
 
+    DEFAULT_POOL_HEADROOM = 10
+
+    # We configure the database connection pool size automatically based on the
+    # configured concurrency. We also add some headroom, to make sure we don't run
+    # out of connections when more threads besides the 'user-facing' ones are
+    # running.
+    #
+    # Read more about this in doc/development/database/client_side_connection_pool.md
+    def self.default_pool_size
+      headroom = (ENV["DB_POOL_HEADROOM"].presence || DEFAULT_POOL_HEADROOM).to_i
+
+      Gitlab::Runtime.max_threads + headroom
+    end
+
     def self.config
       default_config_hash = ActiveRecord::Base.configurations.find_db_config(Rails.env)&.config || {}
 
-      default_config_hash.with_indifferent_access
+      default_config_hash.with_indifferent_access.tap do |hash|
+        # Match config/initializers/database_config.rb
+        hash[:pool] ||= default_pool_size
+      end
     end
 
     def self.username

lib/gitlab/runtime.rb

@@ -87,7 +87,7 @@ module Gitlab
       def max_threads
         threads = 1 # main thread
 
-        if puma?
+        if puma? && Puma.respond_to?(:cli_config)
           threads += Puma.cli_config.options[:max_threads]
         elsif sidekiq?
           # An extra thread for the poller in Sidekiq Cron:

spec/lib/gitlab/database_spec.rb

@@ -15,10 +15,30 @@ RSpec.describe Gitlab::Database do
     end
   end
 
+  describe '.default_pool_size' do
+    before do
+      allow(Gitlab::Runtime).to receive(:max_threads).and_return(7)
+    end
+
+    it 'returns the max thread size plus a fixed headroom of 10' do
+      expect(described_class.default_pool_size).to eq(17)
+    end
+
+    it 'returns the max thread size plus a DB_POOL_HEADROOM if this env var is present' do
+      stub_env('DB_POOL_HEADROOM', '7')
+
+      expect(described_class.default_pool_size).to eq(14)
+    end
+  end
+
   describe '.config' do
     it 'returns a HashWithIndifferentAccess' do
       expect(described_class.config).to be_an_instance_of(HashWithIndifferentAccess)
     end
+
+    it 'returns a default pool size' do
+      expect(described_class.config).to include(pool: described_class.default_pool_size)
+    end
   end
 
   describe '.adapter_name' do

spec/lib/gitlab/runtime_spec.rb

@@ -42,7 +42,19 @@ RSpec.describe Gitlab::Runtime do
     end
   end
 
-  context "puma" do
+  # Puma has no cli_config method unless `puma/cli` is required
+  context "puma without cli_config" do
+    let(:puma_type) { double('::Puma') }
+
+    before do
+      stub_const('::Puma', puma_type)
+      stub_env('ACTION_CABLE_IN_APP', 'false')
+    end
+
+    it_behaves_like "valid runtime", :puma, 1
+  end
+
+  context "puma with cli_config" do
     let(:puma_type) { double('::Puma') }
     let(:max_workers) { 2 }