From 08782b92f787d3e7cd8ee60f2fff462a7872e60b Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Thu, 29 Apr 2021 14:43:02 -0700
Subject: [PATCH] tstest: add WaitFor helper

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 tstest/doc.go    |  6 ------
 tstest/tstest.go | 31 +++++++++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 6 deletions(-)
 delete mode 100644 tstest/doc.go
 create mode 100644 tstest/tstest.go

diff --git a/tstest/doc.go b/tstest/doc.go
deleted file mode 100644
index 72e63b7db..000000000
--- a/tstest/doc.go
+++ /dev/null
@@ -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
diff --git a/tstest/tstest.go b/tstest/tstest.go
new file mode 100644
index 000000000..0bda2d6cb
--- /dev/null
+++ b/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
+}