Integration tests: scan for open port

Previously our integration tests bound to port 53, and, if that failed,
fell back to binding to port 3553.

This commit introduces code to scan for an open port and uses that,
which lays the foundation for potentially parallelizing the integration
tests.

src/sslip.io-dns-server/integration_test.go

@@ -1,6 +1,7 @@
 package main_test
 
 import (
+	"log"
 	"net"
 	"os/exec"
 	"strconv"
@@ -17,17 +18,10 @@ import (
 var err error
 var serverCmd *exec.Cmd
 var serverSession *Session
-var port = 53
+var port = getFreePort()
 
 var _ = BeforeSuite(func() {
 	// Try to bind to the privileged first (for macOS), the fall back to unprivileged
-	conn, err := net.ListenUDP("udp", &net.UDPAddr{Port: port})
-	if err != nil {
-		port = 3553 // unprivileged, a.k.a. "Red Box Recorder ADP" from /etc/services
-	} else {
-		err = conn.Close()
-		Expect(err).ToNot(HaveOccurred())
-	}
 	serverPath, err := Build("main.go")
 	Expect(err).ToNot(HaveOccurred())
 	serverCmd = exec.Command(serverPath, "-port", strconv.Itoa(port))
@@ -444,3 +438,31 @@ var _ = Describe("sslip.io-dns-server", func() {
 		)
 	})
 })
+
+var listenPort = 1023 // lowest unprivileged port - 1 (immediately incremented)
+
+// getFreePort should always succeed unless something awful has happened, e.g. port exhaustion
+func getFreePort() int {
+	for {
+		listenPort += 1
+		switch {
+		case listenPort > 65535:
+			listenPort = 1023 // we've reached the highest port, start over
+		case isPortFree(listenPort):
+			return listenPort
+		}
+	}
+}
+
+func isPortFree(port int) bool {
+	conn, err := net.ListenUDP("udp", &net.UDPAddr{Port: port})
+	if err != nil {
+		return false
+	}
+	err = conn.Close()
+	if err != nil {
+		log.Printf("I couldn't close port %d", port)
+		return false
+	}
+	return true
+}