Quartz

A Go time testing library for writing deterministic unit tests

Our high level goal is to write unit tests that

1. execute quickly
2. don't flake
3. are straightforward to write and understand

For tests to execute quickly without flakes, we want to focus on determinism: the test should run the same each time, and it should be easy to force the system into a known state (no races) before executing test assertions. time.Sleep, runtime.Gosched(), and polling/Eventually are all symptoms of an inability to do this easily.

Usage

Clock interface

In your application code, maintain a reference to a quartz.Clock instance to start timers and tickers, instead of the bare time standard library.

import "github.com/coder/quartz"

type Component struct {
	...

	// for testing
	clock quartz.Clock
}

Whenever you would call into time to start a timer or ticker, call Component's clock instead.

In production, set this clock to quartz.NewReal() to create a clock that just transparently passes through to the standard time library.

Mocking

In your tests, you can use a *Mock to control the tickers and timers your code under test gets.

import (
	"testing"
	"github.com/coder/quartz"
)

func TestComponent(t *testing.T) {
	mClock := quartz.NewMock(t)
	comp := &Component{
		...
		clock: mClock,
	}
}

The *Mock clock starts at Jan 1, 2024, 00:00 UTC by default, but you can set any start time you'd like prior to your test.

mClock := quartz.NewMock(t)
mClock.Set(time.Date(2021, 6, 18, 12, 0, 0, 0, time.UTC)) // June 18, 2021 @ 12pm UTC

Advancing the clock

Once you begin setting timers or tickers, you cannot change the time backward, only advance it forward. You may continue to use Set(), but it is often easier and clearer to use Advance().

For example, with a timer:

fired := false

tmr := mClock.AfterFunc(time.Second, func() {
  fired = true
})
mClock.Advance(time.Second)

When you call Advance() it immediately moves the clock forward the given amount, and triggers any tickers or timers that are scheduled to happen at that time. Any triggered events happen on separate goroutines, so do not immediately assert the results:

fired := false

tmr := mClock.AfterFunc(time.Second, func() {
  fired = true
})
mClock.Advance(time.Second)

// RACE CONDITION, DO NOT DO THIS!
if !fired {
  t.Fatal("didn't fire")
}

Advance() (and Set() for that matter) return an AdvanceWaiter object you can use to wait for all triggered events to complete.

fired := false
// set a test timeout so we don't wait the default `go test` timeout for a failure
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)

tmr := mClock.AfterFunc(time.Second, func() {
  fired = true
})

w := mClock.Advance(time.Second)
err := w.Wait(ctx)
if err != nil {
  t.Fatal("AfterFunc f never completed")
}
if !fired {
  t.Fatal("didn't fire")
}

The construction of waiting for the triggered events and failing the test if they don't complete is very common, so there is a shorthand:

w := mClock.Advance(time.Second)
err := w.Wait(ctx)
if err != nil {
  t.Fatal("AfterFunc f never completed")
}

is equivalent to:

w := mClock.Advance(time.Second)
w.MustWait(ctx)

or even more briefly:

mClock.Advance(time.Second).MustWait(ctx)

Advance only to the next event

One important restriction on advancing the clock is that you may only advance forward to the next timer or ticker event and no further. The following will result in a test failure:

func TestAdvanceTooFar(t *testing.T) {
	ctx, cancel := context.WithTimeout(10*time.Second)
	defer cancel()
	mClock := quartz.NewMock(t)
	var firedAt time.Time
	mClock.AfterFunc(time.Second, func() {
		firedAt := mClock.Now()
	})
	mClock.Advance(2*time.Second).MustWait(ctx)
}

This is a deliberate design decision to allow Advance() to immediately and synchronously move the clock forward (even without calling Wait() on returned waiter). This helps meet Quartz's design goals of writing deterministic and easy to understand unit tests. It also allows the clock to be advanced, deterministically during the execution of a tick or timer function, as explained in the next sections on Traps.

Advancing multiple events can be accomplished via looping. E.g. if you have a 1-second ticker

for i := 0; i < 10; i++ {
	mClock.Advance(time.Second).MustWait(ctx)
}

will advance 10 ticks.

If you don't know or don't want to compute the time to the next event, you can use AdvanceNext().

d, w := mClock.AdvanceNext()
w.MustWait(ctx)
// d contains the duration we advanced

d, ok := Peek() returns the duration until the next event, if any (ok is true). You can use this to advance a specific time, regardless of the tickers and timer events:

desired := time.Minute // time to advance
for desired > 0 {
	p, ok := mClock.Peek()
	if !ok || p > desired {
		mClock.Advance(desired).MustWait(ctx)
		break
	}
	mClock.Advance(p).MustWait(ctx)
	desired -= p
}

Traps

A trap allows you to match specific calls into the library while mocking, block their return, inspect their arguments, then release them to allow them to return. They help you write deterministic unit tests even when the code under test executes asynchronously from the test.

You set your traps prior to executing code under test, and then wait for them to be triggered.

func TestTrap(t *testing.T) {
	ctx, cancel := context.WithTimeout(10*time.Second)
	defer cancel()
	mClock := quartz.NewMock(t)
	trap := mClock.Trap().AfterFunc()
	defer trap.Close() // stop trapping AfterFunc calls

	count := 0
	go mClock.AfterFunc(time.Hour, func(){
		count++
	})
	call := trap.MustWait(ctx)
	call.Release()
	if call.Duration != time.Hour {
		t.Fatal("wrong duration")
	}

	// Now that the async call to AfterFunc has occurred, we can advance the clock to trigger it
	mClock.Advance(call.Duration).MustWait(ctx)
	if count != 1 {
		t.Fatal("wrong count")
	}
}

In this test, the trap serves 2 purposes. Firstly, it allows us to capture and assert the duration passed to the AfterFunc call. Secondly, it prevents a race between setting the timer and advancing it. Since these things happen on different goroutines, if Advance() completes before AfterFunc() is called, then the timer never pops in this test.

Any untrapped calls immediately complete using the current time, and calling Close() on a trap causes the mock clock to stop trapping those calls.

You may also Advance() the clock between trapping a call and releasing it. The call uses the current (mocked) time at the moment it is released.

func TestTrap2(t *testing.T) {
	ctx, cancel := context.WithTimeout(10*time.Second)
	defer cancel()
	mClock := quartz.NewMock(t)
	trap := mClock.Trap().Now()
	defer trap.Close() // stop trapping AfterFunc calls

	var logs []string
	done := make(chan struct{})
	go func(clk quartz.Clock){
		defer close(done)
		start := clk.Now()
		phase1()
		p1end := clk.Now()
		logs = append(fmt.Sprintf("Phase 1 took %s", p1end.Sub(start).String()))
		phase2()
		p2end := clk.Now()
		logs = append(fmt.Sprintf("Phase 2 took %s", p2end.Sub(p1end).String()))
	}(mClock)

	// start
	trap.MustWait(ctx).Release()
	// phase 1
	call := trap.MustWait(ctx)
	mClock.Advance(3*time.Second).MustWait(ctx)
	call.Release()
	// phase 2
	call = trap.MustWait(ctx)
	mClock.Advance(5*time.Second).MustWait(ctx)
	call.Release()

	<-done
	// Now logs contains []string{"Phase 1 took 3s", "Phase 2 took 5s"}
}

Tags

When multiple goroutines in the code under test call into the Clock, you can use tags to distinguish them in your traps.

trap := mClock.Trap.Now("foo") // traps any calls that contain "foo"
defer trap.Close()

foo := make(chan time.Time)
go func(){
	foo <- mClock.Now("foo", "bar")
}()
baz := make(chan time.Time)
go func(){
	baz <- mClock.Now("baz")
}()
call := trap.MustWait(ctx)
mClock.Advance(time.Second).MustWait(ctx)
call.Release()
// call.Tags contains []string{"foo", "bar"}

gotFoo := <-foo // 1s after start
gotBaz := <-baz // ?? never trapped, so races with Advance()

Tags appear as an optional suffix on all Clock methods (type ...string) and are ignored entirely by the real clock. They also appear on all methods on returned timers and tickers.

Recommended Patterns

Options

We use the Option pattern to inject the mock clock for testing, keeping the call signature in production clean. The option pattern is compatible with other optional fields as well.

type Option func(*Thing)

// WithTestClock is used in tests to inject a mock Clock
func WithTestClock(clk quartz.Clock) Option {
	return func(t *Thing) {
		t.clock = clk
	}
}

func NewThing(<required args>, opts ...Option) *Thing {
	t := &Thing{
		...
		clock: quartz.NewReal()
	}
	for _, o := range opts {
	  o(t)
	}
	return t
}

In tests, this becomes

func TestThing(t *testing.T) {
	mClock := quartz.NewMock(t)
	thing := NewThing(<required args>, WithTestClock(mClock))
	...
}

Tagging convention

Tag your Clock method calls as:

func (c *Component) Method() {
	now := c.clock.Now("Component", "Method")
}

or

func (c *Component) Method() {
	start := c.clock.Now("Component", "Method", "start")
	...
	end := c.clock.Now("Component", "Method", "end")
}

This makes it much less likely that code changes that introduce new components or methods will spoil existing unit tests.

Why another time testing library?

Writing good unit tests for components and functions that use the time package is difficult, even though several open source libraries exist. In building Quartz, we took some inspiration from

• github.com/benbjohnson/clock
• Tailscale's tstest.Clock
• github.com/aspenmesh/tock

Quartz shares the high level design of a Clock interface that closely resembles the functions in the time standard library, and a "real" clock passes thru to the standard library in production, while a mock clock gives precise control in testing.

As mentioned in our introduction, our high level goal is to write unit tests that

1. execute quickly
2. don't flake
3. are straightforward to write and understand

For several reasons, this is a tall order when it comes to code that depends on time, and we found the existing libraries insufficient for our goals.

Preventing test flakes

The following example comes from the README from benbjohnson/clock:

mock := clock.NewMock()
count := 0

// Kick off a timer to increment every 1 mock second.
go func() {
	ticker := mock.Ticker(1 * time.Second)
	for {
		<-ticker.C
		count++
	}
}()
runtime.Gosched()

// Move the clock forward 10 seconds.
mock.Add(10 * time.Second)

// This prints 10.
fmt.Println(count)

The first race condition is fairly obvious: moving the clock forward 10 seconds may generate 10 ticks on the ticker.C channel, but there is no guarantee that count++ executes before fmt.Println(count).

The second race condition is more subtle, but runtime.Gosched() is the tell. Since the ticker is started on a separate goroutine, there is no guarantee that mock.Ticker() executes before mock.Add(). runtime.Gosched() is an attempt to get this to happen, but it makes no hard promises. On a busy system, especially when running tests in parallel, this can flake, advance the time 10 seconds first, then start the ticker and never generate a tick.

Let's talk about how Quartz tackles these problems.

In our experience, an extremely common use case is creating a ticker then doing a 2-arm select with ticks in one and context expiring in another, i.e.

t := time.NewTicker(duration)
for {
	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-t.C:
		err := do()
		if err != nil {
			return err
		}
	}
}

In Quartz, we refactor this to be more compact and testing friendly:

t := clock.TickerFunc(ctx, duration, do)
return t.Wait()

This affords the mock Clock the ability to explicitly know when processing of a tick is finished because it's wrapped in the function passed to TickerFunc (do() in this example).

In Quartz, when you advance the clock, you are returned an object you can Wait() on to ensure all ticks and timers triggered are finished. This solves the first race condition in the example.

(As an aside, we still support a traditional standard library-style Ticker. You may find it useful if you want to keep your code as close as possible to the standard library, or if you need to use the channel in a larger select block. In that case, you'll have to find some other mechanism to sync tick processing to your test code.)

To prevent race conditions related to the starting of the ticker, Quartz allows you to set "traps" for calls that access the clock.

func TestTicker(t *testing.T) {
	mClock := quartz.NewMock(t)
	trap := mClock.Trap().TickerFunc()
	defer trap.Close() // stop trapping at end
	go runMyTicker(mClock) // async calls TickerFunc()
	call := trap.Wait(context.Background()) // waits for a call and blocks its return
	call.Release() // allow the TickerFunc() call to return
	// optionally check the duration using call.Duration
	// Move the clock forward 1 tick
	mClock.Advance(time.Second).MustWait(context.Background())
	// assert results of the tick
}

Trapping and then releasing the call to TickerFunc() ensures the ticker is started at a deterministic time, so our calls to Advance() will have a predictable effect.

Take a look at TestExampleTickerFunc in example_test.go for a complete worked example.

Complex time dependence

Another difficult issue to handle when unit testing is when some code under test makes multiple calls that depend on the time, and you want to simulate some time passing between them.

A very basic example is measuring how long something took:

var measurement time.Duration
go func(clock quartz.Clock) {
	start := clock.Now()
	doSomething()
	measurement = clock.Since(start)
}(mClock)

// how to get measurement to be, say, 5 seconds?

The two calls into the clock happen asynchronously, so we need to be able to advance the clock after the first call to Now() but before the call to Since(). Doing this with the libraries we mentioned above means that you have to be able to mock out or otherwise block the completion of doSomething().

But, with the trap functionality we mentioned in the previous section, you can deterministically control the time each call sees.

trap := mClock.Trap().Since()
var measurement time.Duration
go func(clock quartz.Clock) {
	start := clock.Now()
	doSomething()
	measurement = clock.Since(start)
}(mClock)

c := trap.Wait(ctx)
mClock.Advance(5*time.Second)
c.Release()

We wait until we trap the clock.Since() call, which implies that clock.Now() has completed, then advance the mock clock 5 seconds. Finally, we release the clock.Since() call. Any changes to the clock that happen before we release the call will be included in the time used for the clock.Since() call.

As a more involved example, consider an inactivity timeout: we want something to happen if there is no activity recorded for some period, say 10 minutes in the following example:

type InactivityTimer struct {
	mu sync.Mutex
	activity time.Time
	clock quartz.Clock
}

func (i *InactivityTimer) Start() {
	i.mu.Lock()
	defer i.mu.Unlock()
	next := i.clock.Until(i.activity.Add(10*time.Minute))
	t := i.clock.AfterFunc(next, func() {
		i.mu.Lock()
		defer i.mu.Unlock()
		next := i.clock.Until(i.activity.Add(10*time.Minute))
		if next == 0 {
			i.timeoutLocked()
			return
		}
		t.Reset(next)
	})
}

The actual contents of timeoutLocked() doesn't matter for this example, and assume there are other functions that record the latest activity.

We found that some time testing libraries hold a lock on the mock clock while calling the function passed to AfterFunc, resulting in a deadlock if you made clock calls from within.

Others allow this sort of thing, but don't have the flexibility to test edge cases. There is a subtle bug in our Start() function. The timer may pop a little late, and/or some measurable real time may elapse before Until() gets called inside the AfterFunc. If there hasn't been activity, next might be negative.

To test this in Quartz, we'll use a trap. We only want to trap the inner Until() call, not the initial one, so to make testing easier we can "tag" the call we want. Like this:

func (i *InactivityTimer) Start() {
	i.mu.Lock()
	defer i.mu.Unlock()
	next := i.clock.Until(i.activity.Add(10*time.Minute))
	t := i.clock.AfterFunc(next, func() {
		i.mu.Lock()
		defer i.mu.Unlock()
		next := i.clock.Until(i.activity.Add(10*time.Minute), "inner")
		if next == 0 {
			i.timeoutLocked()
			return
		}
		t.Reset(next)
	})
}

All Quartz Clock functions, and functions on returned timers and tickers support zero or more string tags that allow traps to match on them.

func TestInactivityTimer_Late(t *testing.T) {
	// set a timeout on the test itself, so that if Wait functions get blocked, we don't have to
	// wait for the default test timeout of 10 minutes.
	ctx, cancel := context.WithTimeout(10*time.Second)
	defer cancel()
	mClock := quartz.NewMock(t)
	trap := mClock.Trap.Until("inner")
	defer trap.Close()

	it := &InactivityTimer{
		activity: mClock.Now(),
		clock: mClock,
	}
	it.Start()

	// Trigger the AfterFunc
	w := mClock.Advance(10*time.Minute)
	c := trap.Wait(ctx)
	// Advance the clock a few ms to simulate a busy system
	mClock.Advance(3*time.Millisecond)
	c.Release() // Until() returns
	w.MustWait(ctx) // Wait for the AfterFunc to wrap up

	// Assert that the timeoutLocked() function was called
}

This test case will fail with our bugged implementation, since the triggered AfterFunc won't call timeoutLocked() and instead will reset the timer with a negative number. The fix is easy, use next <= 0 as the comparison.