Merge branch '32562-dynamic-db-pool-size-in-puma' into 'master'

Dynamically set db pool size in Puma

Closes #32562

See merge request gitlab-org/gitlab!19286

changelogs/unreleased/32562-dynamic-db-pool-size-in-puma.yml

+---
+title: 'Puma only: database connection pool now always >= number of worker threads'
+merge_request: 19286
+author:
+type: performance

config/initializers/database_config.rb

+# frozen_string_literal: true
+
+# when running on puma, scale connection pool size with the number
+# of threads per worker process
+if defined?(::Puma)
+  db_config = Gitlab::Database.config ||
+      Rails.application.config.database_configuration[Rails.env]
+  puma_options = Puma.cli_config.options
+
+  # We use either the maximum number of threads per worker process, or
+  # the user specified value, whichever is larger.
+  desired_pool_size = [db_config['pool'].to_i, puma_options[:max_threads]].max
+
+  db_config['pool'] = desired_pool_size
+
+  # recreate the connection pool from the new config
+  ActiveRecord::Base.establish_connection(db_config)
+end

spec/initializers/database_config_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Database config initializer' do
+  subject do
+    load Rails.root.join('config/initializers/database_config.rb')
+  end
+
+  before do
+    allow(ActiveRecord::Base).to receive(:establish_connection)
+  end
+
+  context "when using Puma" do
+    let(:puma) { double('puma') }
+    let(:puma_options) { { max_threads: 8 } }
+
+    before do
+      stub_const("Puma", puma)
+      allow(puma).to receive_message_chain(:cli_config, :options).and_return(puma_options)
+    end
+
+    context "and no existing pool size is set" do
+      before do
+        stub_database_config(pool_size: nil)
+      end
+
+      it "sets it to the max number of worker threads" do
+        expect { subject }.to change { Gitlab::Database.config['pool'] }.from(nil).to(8)
+      end
+    end
+
+    context "and the existing pool size is smaller than the max number of worker threads" do
+      before do
+        stub_database_config(pool_size: 7)
+      end
+
+      it "sets it to the max number of worker threads" do
+        expect { subject }.to change { Gitlab::Database.config['pool'] }.from(7).to(8)
+      end
+    end
+
+    context "and the existing pool size is larger than the max number of worker threads" do
+      before do
+        stub_database_config(pool_size: 9)
+      end
+
+      it "keeps the configured pool size" do
+        expect { subject }.not_to change { Gitlab::Database.config['pool'] }
+      end
+    end
+  end
+
+  context "when not using Puma" do
+    before do
+      stub_database_config(pool_size: 7)
+    end
+
+    it "does nothing" do
+      expect { subject }.not_to change { Gitlab::Database.config['pool'] }
+    end
+  end
+
+  def stub_database_config(pool_size:)
+    config = {
+      'adapter' => 'postgresql',
+      'host' => 'db.host.com',
+      'pool' => pool_size
+    }.compact
+
+    allow(Gitlab::Database).to receive(:config).and_return(config)
+  end
+end