Start server with fixed port when --port argument is provided

It should immediately error when the port is occupied, instead of automatically finding the next port.

local/http/http.go

@@ -45,6 +45,7 @@ type ServerCallback func(w http.ResponseWriter, r *http.Request, env map[string]
 type Server struct {
 	DocumentRoot  string
 	Callback      ServerCallback
+	Port          int
 	PreferredPort int
 	PKCS12        string
 	AllowHTTP     bool
@@ -59,7 +60,7 @@ type Server struct {
 
 // Start starts the server
 func (s *Server) Start(errChan chan error) (int, error) {
-	ln, port, err := process.CreateListener(s.PreferredPort)
+	ln, port, err := process.CreateListener(s.Port, s.PreferredPort)
 	if err != nil {
 		return port, errors.WithStack(err)
 	}

local/process/listener.go

@@ -29,33 +29,40 @@ import (
 // CreateListener creates a listener on a port
 // Pass a preferred port (will increment by 1 if port is not available)
 // or pass 0 to auto-find any available port
-func CreateListener(preferredPort int) (net.Listener, int, error) {
+func CreateListener(port, preferredPort int) (net.Listener, int, error) {
 	var ln net.Listener
 	var err error
-	port := preferredPort
+	tryPort := preferredPort
 	max := 50
+	if port > 0 {
+		tryPort = port
+		max = 1
+	}
 	for {
 		// we really want to test availability on 127.0.0.1
-		ln, err = net.Listen("tcp", "127.0.0.1:"+strconv.Itoa(port))
+		ln, err = net.Listen("tcp", "127.0.0.1:"+strconv.Itoa(tryPort))
 		if err == nil {
 			ln.Close()
 			// but then, we want to listen to as many local IP's as possible
-			ln, err = net.Listen("tcp", ":"+strconv.Itoa(port))
+			ln, err = net.Listen("tcp", ":"+strconv.Itoa(tryPort))
 			if err == nil {
 				break
 			}
 		}
+		if port > 0 {
+			return nil, 0, errors.Wrapf(err, "unable to listen on port %d", port)
+		}
 		if preferredPort == 0 {
 			return nil, 0, errors.Wrap(err, "unable to find an available port")
 		}
 		max--
 		if max == 0 {
-			return nil, 0, errors.Wrapf(err, "unable to find an available port (from %d to %d)", preferredPort, port)
+			return nil, 0, errors.Wrapf(err, "unable to find an available port (from %d to %d)", preferredPort, tryPort)
 		}
-		port++
+		tryPort++
 	}
-	if preferredPort == 0 {
-		port = ln.Addr().(*net.TCPAddr).Port
+	if port == 0 && preferredPort == 0 {
+		tryPort = ln.Addr().(*net.TCPAddr).Port
 	}
-	return ln, port, nil
+	return ln, tryPort, nil
 }

local/project/config.go

@@ -36,6 +36,7 @@ type Config struct {
 	ProjectDir    string
 	DocumentRoot  string `yaml:"document_root"`
 	Passthru      string `yaml:"passthru"`
+	Port          int    `yaml:"port"`
 	PreferredPort int    `yaml:"preferred_port"`
 	PKCS12        string `yaml:"p12"`
 	Logger        zerolog.Logger
@@ -85,9 +86,9 @@ func NewConfigFromContext(c *console.Context, projectDir string) (*Config, *File
 		config.Passthru = c.String("passthru")
 	}
 	if c.IsSet("port") {
-		config.PreferredPort = c.Int("port")
+		config.Port = c.Int("port")
 	}
-	if config.PreferredPort == 0 {
+	if config.Port == 0 {
 		config.PreferredPort = 8000
 	}
 	if c.IsSet("allow-http") {

local/project/project.go

@@ -55,6 +55,7 @@ func New(c *Config) (*Project, error) {
 		projectDir: c.ProjectDir,
 		HTTP: &lhttp.Server{
 			DocumentRoot:  documentRoot,
+			Port:          c.Port,
 			PreferredPort: c.PreferredPort,
 			Logger:        c.Logger,
 			PKCS12:        c.PKCS12,