tstest: add WaitFor helper

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/doc.go

@@ -1,6 +0,0 @@
-// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Package tstest provides utilities for use in unit tests.
-package tstest

tstest/tstest.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package tstest provides utilities for use in unit tests.
+package tstest
+
+import (
+	"context"
+	"time"
+
+	"tailscale.com/logtail/backoff"
+	"tailscale.com/types/logger"
+)
+
+// WaitFor retries try for up to maxWait.
+// It returns nil once try returns nil the first time.
+// If maxWait passes without success, it returns try's last error.
+func WaitFor(maxWait time.Duration, try func() error) error {
+	bo := backoff.NewBackoff("wait-for", logger.Discard, maxWait/4)
+	deadline := time.Now().Add(maxWait)
+	var err error
+	for time.Now().Before(deadline) {
+		err = try()
+		if err == nil {
+			break
+		}
+		bo.BackOff(context.Background(), err)
+	}
+	return err
+}