use #described_class, as suggested by RuboCop

spec/lib/port_pool_spec.rb

@@ -3,27 +3,27 @@ require 'rails_helper'
 describe PortPool do
   describe '.available_port' do
     it 'is synchronized' do
-      expect(PortPool.instance_variable_get(:@mutex)).to receive(:synchronize)
-      PortPool.available_port
+      expect(described_class.instance_variable_get(:@mutex)).to receive(:synchronize)
+      described_class.available_port
     end
 
     context 'when a port is available' do
       it 'returns the port' do
-        expect(PortPool.available_port).to be_a(Numeric)
+        expect(described_class.available_port).to be_a(Numeric)
       end
 
       it 'removes the port from the list of available ports' do
-        port = PortPool.available_port
-        expect(PortPool.instance_variable_get(:@available_ports)).not_to include(port)
+        port = described_class.available_port
+        expect(described_class.instance_variable_get(:@available_ports)).not_to include(port)
       end
     end
 
     context 'when no port is available' do
       it 'returns the port' do
-        available_ports = PortPool.instance_variable_get(:@available_ports)
-        PortPool.instance_variable_set(:@available_ports, [])
-        expect(PortPool.available_port).to be_nil
-        PortPool.instance_variable_set(:@available_ports, available_ports)
+        available_ports = described_class.instance_variable_get(:@available_ports)
+        described_class.instance_variable_set(:@available_ports, [])
+        expect(described_class.available_port).to be_nil
+        described_class.instance_variable_set(:@available_ports, available_ports)
       end
     end
   end
@@ -31,24 +31,24 @@ describe PortPool do
   describe '.release' do
     context 'when the port has been obtained earlier' do
       it 'adds the port to the list of available ports' do
-        port = PortPool.available_port
-        expect(PortPool.instance_variable_get(:@available_ports)).not_to include(port)
-        PortPool.release(port)
-        expect(PortPool.instance_variable_get(:@available_ports)).to include(port)
+        port = described_class.available_port
+        expect(described_class.instance_variable_get(:@available_ports)).not_to include(port)
+        described_class.release(port)
+        expect(described_class.instance_variable_get(:@available_ports)).to include(port)
       end
     end
 
     context 'when the port has not been obtained earlier' do
       it 'does not add the port to the list of available ports' do
-        port = PortPool.instance_variable_get(:@available_ports).sample
-        expect { PortPool.release(port) }.not_to change { PortPool.instance_variable_get(:@available_ports).length }
+        port = described_class.instance_variable_get(:@available_ports).sample
+        expect { described_class.release(port) }.not_to change { described_class.instance_variable_get(:@available_ports).length }
       end
     end
 
     context 'when the port is not included in the port range' do
       it 'does not add the port to the list of available ports' do
         port = nil
-        expect { PortPool.release(port) }.not_to change { PortPool.instance_variable_get(:@available_ports).length }
+        expect { described_class.release(port) }.not_to change { described_class.instance_variable_get(:@available_ports).length }
       end
     end
   end