Remove GITLAB_LB_CONFIGURE_CONNECTION feature flag

The connection pool size reconfigure was moved
to load balancer setup. This is safe and we can
remove the legacy code.

config/initializers/database_config.rb

@@ -10,10 +10,6 @@ Gitlab.ee do
   end
 end
 
-unless Gitlab::Utils.to_boolean(ENV["GITLAB_LB_CONFIGURE_CONNECTION"], default: true)
-  ActiveRecord::Base.establish_connection(Gitlab::Database.main.db_config_with_default_pool_size)
-end
-
 Gitlab.ee do
   if Gitlab::Runtime.sidekiq? && Gitlab::Geo.geo_database_configured?
     Rails.configuration.geo_database['pool'] = Gitlab::Database.default_pool_size

lib/gitlab/database/connection.rb

@@ -59,20 +59,6 @@ module Gitlab
         adapter_name.casecmp('postgresql') == 0
       end
 
-      # TODO: To be removed with GITLAB_LB_CONFIGURE_CONNECTION
-      def db_config_with_default_pool_size
-        db_config_object = scope.connection_db_config
-        config = db_config_object
-          .configuration_hash
-          .merge(pool: Database.default_pool_size)
-
-        ActiveRecord::DatabaseConfigurations::HashConfig.new(
-          db_config_object.env_name,
-          db_config_object.name,
-          config
-        )
-      end
-
       # Check whether the underlying database is in read-only mode
       def db_read_only?
         pg_is_in_recovery =

lib/gitlab/database/load_balancing/setup.rb

@@ -22,17 +22,10 @@ module Gitlab
         def configure_connection
           db_config_object = @model.connection_db_config
 
-          hash =
-            if Gitlab::Utils.to_boolean(ENV["GITLAB_LB_CONFIGURE_CONNECTION"], default: true)
-              db_config_object.configuration_hash.merge(
+          hash = db_config_object.configuration_hash.merge(
             prepared_statements: false,
             pool: Gitlab::Database.default_pool_size
           )
-            else
-              db_config_object.configuration_hash.merge(
-                prepared_statements: false
-              )
-            end
 
           hash_config = ActiveRecord::DatabaseConfigurations::HashConfig.new(
             db_config_object.env_name,

spec/initializers/database_config_spec.rb

@@ -7,13 +7,6 @@ RSpec.describe 'Database config initializer', :reestablished_active_record_base
     load Rails.root.join('config/initializers/database_config.rb')
   end
 
-  before do
-    allow(Gitlab::Runtime).to receive(:max_threads).and_return(max_threads)
-    stub_env('GITLAB_LB_CONFIGURE_CONNECTION', 'false')
-  end
-
-  let(:max_threads) { 8 }
-
   it 'retains the correct database name for the connection' do
     previous_db_name = Gitlab::Database.main.scope.connection.pool.db_config.name
 
@@ -22,52 +15,7 @@ RSpec.describe 'Database config initializer', :reestablished_active_record_base
     expect(Gitlab::Database.main.scope.connection.pool.db_config.name).to eq(previous_db_name)
   end
 
-  context 'when no custom headroom is specified' do
-    it 'sets the pool size based on the number of worker threads' do
-      old = ActiveRecord::Base.connection_db_config.pool
-
-      expect(old).not_to eq(18)
-
-      expect { subject }
-        .to change { ActiveRecord::Base.connection_db_config.pool }
-        .from(old)
-        .to(18)
-    end
-
-    it 'overwrites custom pool settings' do
-      config = Gitlab::Database.main.config.merge(pool: 42)
-
-      allow(Gitlab::Database.main).to receive(:config).and_return(config)
-      subject
-
-      expect(ActiveRecord::Base.connection_db_config.pool).to eq(18)
-    end
-
-    context 'when GITLAB_LB_CONFIGURE_CONNECTION=true' do
-      before do
-        stub_env('GITLAB_LB_CONFIGURE_CONNECTION', 'true')
-      end
-
   it 'does not overwrite custom pool settings' do
     expect { subject }.not_to change { ActiveRecord::Base.connection_db_config.pool }
   end
-    end
-  end
-
-  context "when specifying headroom through an ENV variable" do
-    let(:headroom) { 15 }
-
-    before do
-      stub_env("DB_POOL_HEADROOM", headroom)
-    end
-
-    it "adds headroom on top of the calculated size" do
-      old = ActiveRecord::Base.connection_db_config.pool
-
-      expect { subject }
-        .to change { ActiveRecord::Base.connection_db_config.pool }
-        .from(old)
-        .to(23)
-    end
-  end
 end

spec/lib/gitlab/database/connection_spec.rb

@@ -112,20 +112,6 @@ RSpec.describe Gitlab::Database::Connection do
     end
   end
 
-  describe '#db_config_with_default_pool_size' do
-    it 'returns db_config with our default pool size' do
-      allow(Gitlab::Database).to receive(:default_pool_size).and_return(9)
-
-      expect(connection.db_config_with_default_pool_size.pool).to eq(9)
-    end
-
-    it 'returns db_config with the correct database name' do
-      db_name = connection.scope.connection.pool.db_config.name
-
-      expect(connection.db_config_with_default_pool_size.name).to eq(db_name)
-    end
-  end
-
   describe '#db_read_only?' do
     it 'detects a read-only database' do
       allow(connection.scope.connection)