Add context methods to Clock interface

This allows for using timeout/deadline functionality built in to
context.Context with a custom clock implementation.

clock.go

@@ -54,6 +54,22 @@ func (c defaultClock) AfterFunc(d time.Duration, f func()) StopTimer {
 	return time.AfterFunc(d, f)
 }
 
+func (c defaultClock) ContextWithDeadline(ctx context.Context, t time.Time) (context.Context, context.CancelFunc) {
+	return context.WithDeadline(ctx, t)
+}
+
+func (c defaultClock) ContextWithDeadlineCause(ctx context.Context, t time.Time, cause error) (context.Context, context.CancelFunc) {
+	return context.WithDeadlineCause(ctx, t, cause)
+}
+
+func (c defaultClock) ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc) {
+	return context.WithTimeout(ctx, d)
+}
+
+func (c defaultClock) ContextWithTimeoutCause(ctx context.Context, d time.Duration, cause error) (context.Context, context.CancelFunc) {
+	return context.WithTimeoutCause(ctx, d, cause)
+}
+
 // DefaultClock returns a clock that minimally wraps the `time` package
 func DefaultClock() Clock {
 	return defaultClock{}
@@ -80,4 +96,17 @@ type Clock interface {
 	// The callback function f will be executed after the interval d has
 	// elapsed, unless the returned timer's Stop() method is called first.
 	AfterFunc(d time.Duration, f func()) StopTimer
+
+	// ContextWithDeadline behaves like context.WithDeadline, but it uses the
+	// clock to determine the when the deadline has expired.
+	ContextWithDeadline(ctx context.Context, t time.Time) (context.Context, context.CancelFunc)
+	// ContextWithDeadlineCause behaves like context.WithDeadlineCause, but it
+	// uses the clock to determine the when the deadline has expired.
+	ContextWithDeadlineCause(ctx context.Context, t time.Time, cause error) (context.Context, context.CancelFunc)
+	// ContextWithTimeout behaves like context.WithTimeout, but it uses the
+	// clock to determine the when the timeout has elapsed.
+	ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc)
+	// ContextWithTimeoutCause behaves like context.WithTimeoutCause, but it
+	// uses the clock to determine the when the timeout has elapsed.
+	ContextWithTimeoutCause(ctx context.Context, d time.Duration, cause error) (context.Context, context.CancelFunc)
 }

fake/fake_clock.go

@@ -3,6 +3,7 @@ package fake
 import (
 	"context"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	clocks "github.com/vimeo/go-clocks"
@@ -410,3 +411,59 @@ func (f *Clock) AwaitTimerAborts(n int) {
 func (f *Clock) WaitAfterFuncs() {
 	f.cbsWG.Wait()
 }
+
+type deadlineContext struct {
+	context.Context
+	timedOut atomic.Bool
+	deadline time.Time
+}
+
+func (d *deadlineContext) Deadline() (time.Time, bool) {
+	return d.deadline, true
+}
+
+func (d *deadlineContext) Err() error {
+	if d.timedOut.Load() {
+		return context.DeadlineExceeded
+	}
+	return d.Context.Err()
+}
+
+func (f *Clock) ContextWithDeadlineCause(ctx context.Context, t time.Time, cause error) (context.Context, context.CancelFunc) {
+	cctx, cancelCause := context.WithCancelCause(ctx)
+	dctx := &deadlineContext{
+		Context:  cctx,
+		deadline: t,
+	}
+	dur := f.Until(t)
+	if dur <= 0 {
+		dctx.timedOut.CompareAndSwap(false, true)
+		cancelCause(cause)
+		return dctx, func() {
+			cancelCause(context.Canceled)
+		}
+	}
+	stop := f.AfterFunc(dur, func() {
+		if cctx.Err() == nil {
+			dctx.timedOut.CompareAndSwap(false, true)
+		}
+		cancelCause(cause)
+	})
+	cancel := func() {
+		cancelCause(context.Canceled)
+		stop.Stop()
+	}
+	return dctx, cancel
+}
+
+func (c *Clock) ContextWithDeadline(ctx context.Context, t time.Time) (context.Context, context.CancelFunc) {
+	return c.ContextWithDeadlineCause(ctx, t, nil)
+}
+
+func (c *Clock) ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc) {
+	return c.ContextWithDeadlineCause(ctx, c.Now().Add(d), nil)
+}
+
+func (c *Clock) ContextWithTimeoutCause(ctx context.Context, d time.Duration, cause error) (context.Context, context.CancelFunc) {
+	return c.ContextWithDeadlineCause(ctx, c.Now().Add(d), cause)
+}

offset/offset_clock.go

@@ -49,6 +49,22 @@ func (o *Clock) AfterFunc(d time.Duration, f func()) clocks.StopTimer {
 	return o.inner.AfterFunc(d, f)
 }
 
+func (o *Clock) ContextWithDeadline(ctx context.Context, t time.Time) (context.Context, context.CancelFunc) {
+	return o.inner.ContextWithDeadline(ctx, t.Add(o.offset))
+}
+
+func (o *Clock) ContextWithDeadlineCause(ctx context.Context, t time.Time, cause error) (context.Context, context.CancelFunc) {
+	return o.inner.ContextWithDeadlineCause(ctx, t.Add(o.offset), cause)
+}
+
+func (o *Clock) ContextWithTimeout(ctx context.Context, d time.Duration) (context.Context, context.CancelFunc) {
+	return o.inner.ContextWithTimeout(ctx, d+o.offset)
+}
+
+func (o *Clock) ContextWithTimeoutCause(ctx context.Context, d time.Duration, cause error) (context.Context, context.CancelFunc) {
+	return o.inner.ContextWithTimeoutCause(ctx, d+o.offset, cause)
+}
+
 // NewOffsetClock creates an OffsetClock. offset is added to all absolute times.
 func NewOffsetClock(inner clocks.Clock, offset time.Duration) *Clock {
 	return &Clock{