Dynamically set db pool size in Puma

Before Puma, we had 1 rails process talk to the database
at any given time, which meant a connection pool size of 1
was sufficient to support that model. With Puma and its worker
thread pool, there can now be many threads simultaneously
trying to talk to the database. This change makes sure that
we allow the db pool size to scale along with the maximum
number of worker threads configured.

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