fix: do port availability check only on macOS. (#3654)

On macOS, if a process already listens on 127.0.0.1:PORT, net.Listen() falls back
to IPv6 address ie minio will start listening on IPv6 address whereas another
(non-)minio process is listening on IPv4 of given port.
To avoid this error sutiation we check for port availability only for macOS.

Note: checkPortAvailability() tries to listen on given port and closes it.
It is possible to have a disconnected client in this tiny window of time.

cmd/checkport.go

@@ -22,25 +22,13 @@ import (
 	"syscall"
 )
 
-// Make sure that none of the other processes are listening on the
-// specified port on any of the interfaces.
-//
-// On linux if a process is listening on 127.0.0.1:9000 then Listen()
-// on ":9000" fails with the error "port already in use".
-// However on Mac OSX Listen() on ":9000" falls back to the IPv6 address.
-// This causes confusion on Mac OSX that minio server is not reachable
-// on 127.0.0.1 even though minio server is running. So before we start
-// the minio server we make sure that the port is free on each tcp network.
-//
-// Port is string on purpose here.
-// https://github.com/golang/go/issues/16142#issuecomment-245912773
-//
-// "Keep in mind that ports in Go are strings: https://play.golang.org/p/zk2WEri_E9"
-//                 - @bradfitz
-func checkPortAvailability(portStr string) error {
+// checkPortAvailability - check if given port is already in use.
+// Note: The check method tries to listen on given port and closes it.
+// It is possible to have a disconnected client in this tiny window of time.
+func checkPortAvailability(port string) error {
 	network := [3]string{"tcp", "tcp4", "tcp6"}
 	for _, n := range network {
-		l, err := net.Listen(n, net.JoinHostPort("", portStr))
+		l, err := net.Listen(n, net.JoinHostPort("", port))
 		if err != nil {
 			if isAddrInUse(err) {
 				// Return error if another process is listening on the

cmd/server-main.go

@@ -352,10 +352,15 @@ func getHostPort(address string) (host, port string, err error) {
 		return "", "", err
 	}
 
-	// Check if port is available.
+	if runtime.GOOS == "darwin" {
+		// On macOS, if a process already listens on 127.0.0.1:PORT, net.Listen() falls back
+		// to IPv6 address ie minio will start listening on IPv6 address whereas another
+		// (non-)minio process is listening on IPv4 of given port.
+		// To avoid this error sutiation we check for port availability only for macOS.
 		if err = checkPortAvailability(port); err != nil {
 			return "", "", err
 		}
+	}
 
 	// Success.
 	return host, port, nil