Fallback when the port isn't actually available

This code will test the randomly assigned port to make sure it can be
used before returning it. It finds another port if there is a problem.

lib/capybara/server.rb

@@ -105,10 +105,23 @@ module Capybara
     end
 
     def find_available_port(host)
-      server = TCPServer.new(host, 0)
-      server.addr[1]
-    ensure
-      server&.close
+      port = 0
+      while port.zero?
+        begin
+          server = TCPServer.new(host, port)
+          port = server.addr[1]
+        ensure
+          server&.close
+        end
+        begin
+          server = TCPServer.new(host, port)
+        rescue Errno::EADDRINUSE
+          port = 0
+        ensure
+          server&.close
+        end
+      end
+      port
     end
   end
 end

spec/server_spec.rb

@@ -81,12 +81,12 @@ RSpec.describe Capybara::Server do
   end
 
   it 'should handle that getting available ports fails randomly' do
-    expect {
-      100000.times do |count|
-        port = Capybara::Server.new(Object.new).send(:find_available_port, "0.0.0.0")
-        use_port("0.0.0.0", port)
+    expect do
+      100000.times do
+        port = described_class.new(Object.new).send(:find_available_port, '0.0.0.0')
+        use_port('0.0.0.0', port)
       end
-    }.not_to raise_error
+    end.not_to raise_error
   end
 
   it 'should return its #base_url' do