Library/Homebrew: add free_port test helper

Co-Authored-By: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/dev-cmd/test.rb

@@ -36,6 +36,7 @@ module Homebrew
     test_args.parse
 
     require "formula_assertions"
+    require "formula_free_port"
 
     args.resolved_formulae.each do |f|
       # Cannot test uninstalled formulae

Library/Homebrew/formula_free_port.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module Homebrew
+  module FreePort
+    require "socket"
+
+    def free_port
+      server = TCPServer.new 0
+      _, port, = server.addr
+      server.close
+
+      port
+    end
+  end
+end

Library/Homebrew/test.rb

@@ -7,6 +7,7 @@ require "extend/ENV"
 require "timeout"
 require "debrew"
 require "formula_assertions"
+require "formula_free_port"
 require "fcntl"
 require "socket"
 require "cli/parser"
@@ -26,6 +27,7 @@ begin
 
   formula = Homebrew.args.resolved_formulae.first
   formula.extend(Homebrew::Assertions)
+  formula.extend(Homebrew::FreePort)
   formula.extend(Debrew::Formula) if Homebrew.args.debug?
 
   # tests can also return false to indicate failure

Library/Homebrew/test/formula_free_port_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require "socket"
+require "formula_free_port"
+
+module Homebrew
+  describe FreePort do
+    include described_class
+
+    describe "#free_port" do
+      # IANA suggests user port from 1024 to 49151
+      # and dynamic port for 49152 to 65535
+      # http://www.iana.org/assignments/port-numbers
+      MIN_PORT = 1024
+      MAX_PORT = 65535
+
+      it "returns a free TCP/IP port" do
+        port = free_port
+
+        expect(port).to be_between(MIN_PORT, MAX_PORT)
+        expect { TCPServer.new(port).close }.not_to raise_error
+      end
+    end
+  end
+end