testy: add a virtual clock helper for tests.

Signed-off-by: David Anderson <dave@natulte.net>

testy/clock.go

@@ -0,0 +1,44 @@
+// 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 testy
+
+import "time"
+
+// Clock is a testing clock that advances every time its Now method is
+// called, beginning at Start.
+//
+// The zero value starts virtual time at an arbitrary value recorded
+// in Start on the first call to Now, and increments by one second
+// between calls to Now.
+type Clock struct {
+	// Start is the first value returned by Now.
+	Start time.Time
+	// Step is how much to advance with each Now call.
+	// Zero means 1 second.
+	Step time.Duration
+	// Present is the time that the next Now call will receive.
+	Present time.Time
+}
+
+// Now returns the virtual clock's current time, and avances it
+// according to its step configuration.
+func (c *Clock) Now() time.Time {
+	if c.Start.IsZero() {
+		c.Start = time.Now()
+		c.Present = c.Start
+	}
+	step := c.Step
+	if step == 0 {
+		step = time.Second
+	}
+	ret := c.Present
+	c.Present = c.Present.Add(step)
+	return ret
+}
+
+// Reset rewinds the virtual clock to its start time.
+func (c *Clock) Reset() {
+	c.Present = c.Start
+}

testy/doc.go

@@ -0,0 +1,6 @@
+// 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 testy provides utilities for use in unit tests.
+package testy