test/integration: Make localPort reusable as FindFreeLocalPort

test/integration/framework/master_utils.go

@@ -22,6 +22,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	goruntime "runtime"
+	"strconv"
 	"sync"
 	"testing"
 	"time"
@@ -509,3 +510,25 @@ func RunParallel(task Task, numTasks, numWorkers int) {
 	wg.Wait()
 	close(semCh)
 }
+
+// FindFreeLocalPort returns the number of an available port number on
+// the loopback interface.  Useful for determining the port to launch
+// a server on.  Error handling required - there is a non-zero chance
+// that the returned port number will be bound by another process
+// after this function returns.
+func FindFreeLocalPort() (int, error) {
+	l, err := net.Listen("tcp", ":0")
+	if err != nil {
+		return 0, err
+	}
+	defer l.Close()
+	_, portStr, err := net.SplitHostPort(l.Addr().String())
+	if err != nil {
+		return 0, err
+	}
+	port, err := strconv.Atoi(portStr)
+	if err != nil {
+		return 0, err
+	}
+	return port, nil
+}