Add test helpers to find free tcp ports and wait for ports to become available.

tools/testtool/freeport.go

@@ -0,0 +1,57 @@
+package testtool
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"time"
+)
+
+// GetFreePort returns free local tcp port.
+func GetFreePort() (string, error) {
+	addr, err := net.ResolveTCPAddr("tcp", "localhost:0")
+	if err != nil {
+		return "", err
+	}
+
+	l, err := net.ListenTCP("tcp", addr)
+	if err != nil {
+		return "", err
+	}
+	defer func() { _ = l.Close() }()
+
+	p := l.Addr().(*net.TCPAddr).Port
+
+	return strconv.Itoa(p), nil
+}
+
+// WaitForPort tries to connect to given local port with constant backoff.
+//
+// Can be canceled via ctx.
+func WaitForPort(ctx context.Context, port string) {
+	t := time.NewTicker(time.Millisecond * 100)
+	defer t.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			if err := portIsReady(port); err != nil {
+				_, _ = fmt.Fprintf(os.Stderr, "waiting for port: %s\n", err)
+				break
+			}
+			return
+		}
+	}
+}
+
+func portIsReady(port string) error {
+	conn, err := net.Dial("tcp", net.JoinHostPort("localhost", port))
+	if err != nil {
+		return err
+	}
+	return conn.Close()
+}

tools/testtool/freeport_test.go

@@ -0,0 +1,27 @@
+package testtool
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetFreePort(t *testing.T) {
+	p, err := GetFreePort()
+	require.NoError(t, err)
+	require.NotEmpty(t, p)
+}
+
+func TestWaitForPort(t *testing.T) {
+	p, err := GetFreePort()
+	require.NoError(t, err)
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+
+	WaitForPort(ctx, p)
+
+	require.Error(t, ctx.Err())
+}