Refactor `Kemal.run` to allow custom port binding (#463)

spec/run_spec.cr

@@ -1,20 +1,48 @@
 require "./spec_helper"
 
+private def run(code)
+  code = <<-CR
+    require "./src/kemal"
+    #{code}
+    CR
+  String.build do |stdout|
+    stderr = String.build do |stderr|
+      Process.new("crystal", ["eval"], input: IO::Memory.new(code), output: stdout, error: stderr).wait
+    end
+    unless stderr.empty?
+      fail(stderr)
+    end
+  end
+end
+
 describe "Run" do
   it "runs a code block after starting" do
-    Kemal.config.env = "test"
-    make_me_true = false
-    Kemal.run do
-      make_me_true = true
-      Kemal.stop
-    end
-    make_me_true.should eq true
+    run(<<-CR).should eq "started\nstopped\n"
+      Kemal.config.env = "test"
+      Kemal.run do
+        puts "started"
+        Kemal.stop
+        puts "stopped"
+      end
+      CR
   end
 
   it "runs without a block being specified" do
-    Kemal.config.env = "test"
-    Kemal.run
-    Kemal.config.running.should eq true
-    Kemal.stop
+    run(<<-CR).should eq "[test] Kemal is ready to lead at http://0.0.0.0:3000\ntrue\n"
+      Kemal.config.env = "test"
+      Kemal.run
+      puts Kemal.config.running
+      CR
+  end
+
+  it "allows custom HTTP::Server bind" do
+    run(<<-CR).should eq "[test] Kemal is ready to lead at http://127.0.0.1:3000, http://0.0.0.0:3001\n"
+      Kemal.config.env = "test"
+      Kemal.run do |config|
+        server = config.server.not_nil!
+        server.bind_tcp "127.0.0.1", 3000, reuse_port: true
+        server.bind_tcp "0.0.0.0", 3001, reuse_port: true
+      end
+      CR
   end
 end

src/kemal.cr

@@ -9,9 +9,7 @@ require "./kemal/helpers/*"
 module Kemal
   # Overload of `self.run` with the default startup logging.
   def self.run(port : Int32?)
-    self.run port do
-      log "[#{config.env}] Kemal is ready to lead at #{config.scheme}://#{config.host_binding}:#{config.port}"
-    end
+    self.run(port) { }
   end
 
   # Overload of `self.run` without port.
@@ -68,7 +66,22 @@ module Kemal
     config.running = true
 
     yield config
-    server.listen(config.host_binding, config.port) if config.env != "test"
+
+    # Abort if block called `Kemal.stop`
+    return unless config.running
+
+    unless server.each_address { |_| break true }
+      server.bind_tcp(config.host_binding, config.port)
+    end
+
+    display_startup_message(config, server)
+
+    server.listen unless config.env == "test"
+  end
+
+  def self.display_startup_message(config, server)
+    addresses = server.addresses.map { |address| "#{config.scheme}://#{address}" }.join ", "
+    log "[#{config.env}] Kemal is ready to lead at #{addresses}"
   end
 
   def self.stop