src/context/context_test.go

// Copyright 2014 The Go 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 context

import (
	"errors"
	"fmt"
	"math/rand"
	"runtime"
	"strings"
	"sync"
	"time"
)

type testingT interface {
	Deadline() (time.Time, bool)
	Error(args ...any)
	Errorf(format string, args ...any)
	Fail()
	FailNow()
	Failed() bool
	Fatal(args ...any)
	Fatalf(format string, args ...any)
	Helper()
	Log(args ...any)
	Logf(format string, args ...any)
	Name() string
	Parallel()
	Skip(args ...any)
	SkipNow()
	Skipf(format string, args ...any)
	Skipped() bool
}

// otherContext is a Context that's not one of the types defined in context.go.
// This lets us test code paths that differ based on the underlying type of the
// Context.
type otherContext struct {
	Context
}

const (
	shortDuration    = 1 * time.Millisecond // a reasonable duration to block in a test
	veryLongDuration = 1000 * time.Hour     // an arbitrary upper bound on the test's running time
)

// quiescent returns an arbitrary duration by which the program should have
// completed any remaining work and reached a steady (idle) state.
func quiescent(t testingT) time.Duration {
	deadline, ok := t.Deadline()
	if !ok {
		return 5 * time.Second
	}

	const arbitraryCleanupMargin = 1 * time.Second
	return time.Until(deadline) - arbitraryCleanupMargin
}

func XTestBackground(t testingT) {
	c := Background()
	if c == nil {
		t.Fatalf("Background returned nil")
	}
	select {
	case x := <-c.Done():
		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
	default:
	}
	if got, want := fmt.Sprint(c), "context.Background"; got != want {
		t.Errorf("Background().String() = %q want %q", got, want)
	}
}

func XTestTODO(t testingT) {
	c := TODO()
	if c == nil {
		t.Fatalf("TODO returned nil")
	}
	select {
	case x := <-c.Done():
		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
	default:
	}
	if got, want := fmt.Sprint(c), "context.TODO"; got != want {
		t.Errorf("TODO().String() = %q want %q", got, want)
	}
}

func XTestWithCancel(t testingT) {
	c1, cancel := WithCancel(Background())

	if got, want := fmt.Sprint(c1), "context.Background.WithCancel"; got != want {
		t.Errorf("c1.String() = %q want %q", got, want)
	}

	o := otherContext{c1}
	c2, _ := WithCancel(o)
	contexts := []Context{c1, o, c2}

	for i, c := range contexts {
		if d := c.Done(); d == nil {
			t.Errorf("c[%d].Done() == %v want non-nil", i, d)
		}
		if e := c.Err(); e != nil {
			t.Errorf("c[%d].Err() == %v want nil", i, e)
		}

		select {
		case x := <-c.Done():
			t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
		default:
		}
	}

	cancel() // Should propagate synchronously.
	for i, c := range contexts {
		select {
		case <-c.Done():
		default:
			t.Errorf("<-c[%d].Done() blocked, but shouldn't have", i)
		}
		if e := c.Err(); e != Canceled {
			t.Errorf("c[%d].Err() == %v want %v", i, e, Canceled)
		}
	}
}

func contains(m map[canceler]struct{}, key canceler) bool {
	_, ret := m[key]
	return ret
}

func XTestParentFinishesChild(t testingT) {
	// Context tree:
	// parent -> cancelChild
	// parent -> valueChild -> timerChild
	parent, cancel := WithCancel(Background())
	cancelChild, stop := WithCancel(parent)
	defer stop()
	valueChild := WithValue(parent, "key", "value")
	timerChild, stop := WithTimeout(valueChild, veryLongDuration)
	defer stop()

	select {
	case x := <-parent.Done():
		t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
	case x := <-cancelChild.Done():
		t.Errorf("<-cancelChild.Done() == %v want nothing (it should block)", x)
	case x := <-timerChild.Done():
		t.Errorf("<-timerChild.Done() == %v want nothing (it should block)", x)
	case x := <-valueChild.Done():
		t.Errorf("<-valueChild.Done() == %v want nothing (it should block)", x)
	default:
	}

	// The parent's children should contain the two cancelable children.
	pc := parent.(*cancelCtx)
	cc := cancelChild.(*cancelCtx)
	tc := timerChild.(*timerCtx)
	pc.mu.Lock()
	if len(pc.children) != 2 || !contains(pc.children, cc) || !contains(pc.children, tc) {
		t.Errorf("bad linkage: pc.children = %v, want %v and %v",
			pc.children, cc, tc)
	}
	pc.mu.Unlock()

	if p, ok := parentCancelCtx(cc.Context); !ok || p != pc {
		t.Errorf("bad linkage: parentCancelCtx(cancelChild.Context) = %v, %v want %v, true", p, ok, pc)
	}
	if p, ok := parentCancelCtx(tc.Context); !ok || p != pc {
		t.Errorf("bad linkage: parentCancelCtx(timerChild.Context) = %v, %v want %v, true", p, ok, pc)
	}

	cancel()

	pc.mu.Lock()
	if len(pc.children) != 0 {
		t.Errorf("pc.cancel didn't clear pc.children = %v", pc.children)
	}
	pc.mu.Unlock()

	// parent and children should all be finished.
	check := func(ctx Context, name string) {
		select {
		case <-ctx.Done():
		default:
			t.Errorf("<-%s.Done() blocked, but shouldn't have", name)
		}
		if e := ctx.Err(); e != Canceled {
			t.Errorf("%s.Err() == %v want %v", name, e, Canceled)
		}
	}
	check(parent, "parent")
	check(cancelChild, "cancelChild")
	check(valueChild, "valueChild")
	check(timerChild, "timerChild")

	// WithCancel should return a canceled context on a canceled parent.
	precanceledChild := WithValue(parent, "key", "value")
	select {
	case <-precanceledChild.Done():
	default:
		t.Errorf("<-precanceledChild.Done() blocked, but shouldn't have")
	}
	if e := precanceledChild.Err(); e != Canceled {
		t.Errorf("precanceledChild.Err() == %v want %v", e, Canceled)
	}
}

func XTestChildFinishesFirst(t testingT) {
	cancelable, stop := WithCancel(Background())
	defer stop()
	for _, parent := range []Context{Background(), cancelable} {
		child, cancel := WithCancel(parent)

		select {
		case x := <-parent.Done():
			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
		case x := <-child.Done():
			t.Errorf("<-child.Done() == %v want nothing (it should block)", x)
		default:
		}

		cc := child.(*cancelCtx)
		pc, pcok := parent.(*cancelCtx) // pcok == false when parent == Background()
		if p, ok := parentCancelCtx(cc.Context); ok != pcok || (ok && pc != p) {
			t.Errorf("bad linkage: parentCancelCtx(cc.Context) = %v, %v want %v, %v", p, ok, pc, pcok)
		}

		if pcok {
			pc.mu.Lock()
			if len(pc.children) != 1 || !contains(pc.children, cc) {
				t.Errorf("bad linkage: pc.children = %v, cc = %v", pc.children, cc)
			}
			pc.mu.Unlock()
		}

		cancel()

		if pcok {
			pc.mu.Lock()
			if len(pc.children) != 0 {
				t.Errorf("child's cancel didn't remove self from pc.children = %v", pc.children)
			}
			pc.mu.Unlock()
		}

		// child should be finished.
		select {
		case <-child.Done():
		default:
			t.Errorf("<-child.Done() blocked, but shouldn't have")
		}
		if e := child.Err(); e != Canceled {
			t.Errorf("child.Err() == %v want %v", e, Canceled)
		}

		// parent should not be finished.
		select {
		case x := <-parent.Done():
			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
		default:
		}
		if e := parent.Err(); e != nil {
			t.Errorf("parent.Err() == %v want nil", e)
		}
	}
}

func testDeadline(c Context, name string, t testingT) {
	t.Helper()
	d := quiescent(t)
	timer := time.NewTimer(d)
	defer timer.Stop()
	select {
	case <-timer.C:
		t.Fatalf("%s: context not timed out after %v", name, d)
	case <-c.Done():
	}
	if e := c.Err(); e != DeadlineExceeded {
		t.Errorf("%s: c.Err() == %v; want %v", name, e, DeadlineExceeded)
	}
}

func XTestDeadline(t testingT) {
	t.Parallel()

	c, _ := WithDeadline(Background(), time.Now().Add(shortDuration))
	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
		t.Errorf("c.String() = %q want prefix %q", got, prefix)
	}
	testDeadline(c, "WithDeadline", t)

	c, _ = WithDeadline(Background(), time.Now().Add(shortDuration))
	o := otherContext{c}
	testDeadline(o, "WithDeadline+otherContext", t)

	c, _ = WithDeadline(Background(), time.Now().Add(shortDuration))
	o = otherContext{c}
	c, _ = WithDeadline(o, time.Now().Add(veryLongDuration))
	testDeadline(c, "WithDeadline+otherContext+WithDeadline", t)

	c, _ = WithDeadline(Background(), time.Now().Add(-shortDuration))
	testDeadline(c, "WithDeadline+inthepast", t)

	c, _ = WithDeadline(Background(), time.Now())
	testDeadline(c, "WithDeadline+now", t)
}

func XTestTimeout(t testingT) {
	t.Parallel()

	c, _ := WithTimeout(Background(), shortDuration)
	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
		t.Errorf("c.String() = %q want prefix %q", got, prefix)
	}
	testDeadline(c, "WithTimeout", t)

	c, _ = WithTimeout(Background(), shortDuration)
	o := otherContext{c}
	testDeadline(o, "WithTimeout+otherContext", t)

	c, _ = WithTimeout(Background(), shortDuration)
	o = otherContext{c}
	c, _ = WithTimeout(o, veryLongDuration)
	testDeadline(c, "WithTimeout+otherContext+WithTimeout", t)
}

func XTestCanceledTimeout(t testingT) {
	c, _ := WithTimeout(Background(), time.Second)
	o := otherContext{c}
	c, cancel := WithTimeout(o, veryLongDuration)
	cancel() // Should propagate synchronously.
	select {
	case <-c.Done():
	default:
		t.Errorf("<-c.Done() blocked, but shouldn't have")
	}
	if e := c.Err(); e != Canceled {
		t.Errorf("c.Err() == %v want %v", e, Canceled)
	}
}

type key1 int
type key2 int

var k1 = key1(1)
var k2 = key2(1) // same int as k1, different type
var k3 = key2(3) // same type as k2, different int

func XTestValues(t testingT) {
	check := func(c Context, nm, v1, v2, v3 string) {
		if v, ok := c.Value(k1).(string); ok == (len(v1) == 0) || v != v1 {
			t.Errorf(`%s.Value(k1).(string) = %q, %t want %q, %t`, nm, v, ok, v1, len(v1) != 0)
		}
		if v, ok := c.Value(k2).(string); ok == (len(v2) == 0) || v != v2 {
			t.Errorf(`%s.Value(k2).(string) = %q, %t want %q, %t`, nm, v, ok, v2, len(v2) != 0)
		}
		if v, ok := c.Value(k3).(string); ok == (len(v3) == 0) || v != v3 {
			t.Errorf(`%s.Value(k3).(string) = %q, %t want %q, %t`, nm, v, ok, v3, len(v3) != 0)
		}
	}

	c0 := Background()
	check(c0, "c0", "", "", "")

	c1 := WithValue(Background(), k1, "c1k1")
	check(c1, "c1", "c1k1", "", "")

	if got, want := fmt.Sprint(c1), `context.Background.WithValue(type context.key1, val c1k1)`; got != want {
		t.Errorf("c.String() = %q want %q", got, want)
	}

	c2 := WithValue(c1, k2, "c2k2")
	check(c2, "c2", "c1k1", "c2k2", "")

	c3 := WithValue(c2, k3, "c3k3")
	check(c3, "c2", "c1k1", "c2k2", "c3k3")

	c4 := WithValue(c3, k1, nil)
	check(c4, "c4", "", "c2k2", "c3k3")

	o0 := otherContext{Background()}
	check(o0, "o0", "", "", "")

	o1 := otherContext{WithValue(Background(), k1, "c1k1")}
	check(o1, "o1", "c1k1", "", "")

	o2 := WithValue(o1, k2, "o2k2")
	check(o2, "o2", "c1k1", "o2k2", "")

	o3 := otherContext{c4}
	check(o3, "o3", "", "c2k2", "c3k3")

	o4 := WithValue(o3, k3, nil)
	check(o4, "o4", "", "c2k2", "")
}

func XTestAllocs(t testingT, testingShort func() bool, testingAllocsPerRun func(int, func()) float64) {
	bg := Background()
	for _, test := range []struct {
		desc       string
		f          func()
		limit      float64
		gccgoLimit float64
	}{
		{
			desc:       "Background()",
			f:          func() { Background() },
			limit:      0,
			gccgoLimit: 0,
		},
		{
			desc: fmt.Sprintf("WithValue(bg, %v, nil)", k1),
			f: func() {
				c := WithValue(bg, k1, nil)
				c.Value(k1)
			},
			limit:      3,
			gccgoLimit: 3,
		},
		{
			desc: "WithTimeout(bg, 1*time.Nanosecond)",
			f: func() {
				c, _ := WithTimeout(bg, 1*time.Nanosecond)
				<-c.Done()
			},
			limit:      12,
			gccgoLimit: 15,
		},
		{
			desc: "WithCancel(bg)",
			f: func() {
				c, cancel := WithCancel(bg)
				cancel()
				<-c.Done()
			},
			limit:      5,
			gccgoLimit: 8,
		},
		{
			desc: "WithTimeout(bg, 5*time.Millisecond)",
			f: func() {
				c, cancel := WithTimeout(bg, 5*time.Millisecond)
				cancel()
				<-c.Done()
			},
			limit:      8,
			gccgoLimit: 25,
		},
	} {
		limit := test.limit
		if runtime.Compiler == "gccgo" {
			// gccgo does not yet do escape analysis.
			// TODO(iant): Remove this when gccgo does do escape analysis.
			limit = test.gccgoLimit
		}
		numRuns := 100
		if testingShort() {
			numRuns = 10
		}
		if n := testingAllocsPerRun(numRuns, test.f); n > limit {
			t.Errorf("%s allocs = %f want %d", test.desc, n, int(limit))
		}
	}
}

func XTestSimultaneousCancels(t testingT) {
	root, cancel := WithCancel(Background())
	m := map[Context]CancelFunc{root: cancel}
	q := []Context{root}
	// Create a tree of contexts.
	for len(q) != 0 && len(m) < 100 {
		parent := q[0]
		q = q[1:]
		for i := 0; i < 4; i++ {
			ctx, cancel := WithCancel(parent)
			m[ctx] = cancel
			q = append(q, ctx)
		}
	}
	// Start all the cancels in a random order.
	var wg sync.WaitGroup
	wg.Add(len(m))
	for _, cancel := range m {
		go func(cancel CancelFunc) {
			cancel()
			wg.Done()
		}(cancel)
	}

	d := quiescent(t)
	stuck := make(chan struct{})
	timer := time.AfterFunc(d, func() { close(stuck) })
	defer timer.Stop()

	// Wait on all the contexts in a random order.
	for ctx := range m {
		select {
		case <-ctx.Done():
		case <-stuck:
			buf := make([]byte, 10<<10)
			n := runtime.Stack(buf, true)
			t.Fatalf("timed out after %v waiting for <-ctx.Done(); stacks:\n%s", d, buf[:n])
		}
	}
	// Wait for all the cancel functions to return.
	done := make(chan struct{})
	go func() {
		wg.Wait()
		close(done)
	}()
	select {
	case <-done:
	case <-stuck:
		buf := make([]byte, 10<<10)
		n := runtime.Stack(buf, true)
		t.Fatalf("timed out after %v waiting for cancel functions; stacks:\n%s", d, buf[:n])
	}
}

func XTestInterlockedCancels(t testingT) {
	parent, cancelParent := WithCancel(Background())
	child, cancelChild := WithCancel(parent)
	go func() {
		<-parent.Done()
		cancelChild()
	}()
	cancelParent()
	d := quiescent(t)
	timer := time.NewTimer(d)
	defer timer.Stop()
	select {
	case <-child.Done():
	case <-timer.C:
		buf := make([]byte, 10<<10)
		n := runtime.Stack(buf, true)
		t.Fatalf("timed out after %v waiting for child.Done(); stacks:\n%s", d, buf[:n])
	}
}

func XTestLayersCancel(t testingT) {
	testLayers(t, time.Now().UnixNano(), false)
}

func XTestLayersTimeout(t testingT) {
	testLayers(t, time.Now().UnixNano(), true)
}

func testLayers(t testingT, seed int64, testTimeout bool) {
	t.Parallel()

	r := rand.New(rand.NewSource(seed))
	errorf := func(format string, a ...any) {
		t.Errorf(fmt.Sprintf("seed=%d: %s", seed, format), a...)
	}
	const (
		minLayers = 30
	)
	type value int
	var (
		vals      []*value
		cancels   []CancelFunc
		numTimers int
		ctx       = Background()
	)
	for i := 0; i < minLayers || numTimers == 0 || len(cancels) == 0 || len(vals) == 0; i++ {
		switch r.Intn(3) {
		case 0:
			v := new(value)
			ctx = WithValue(ctx, v, v)
			vals = append(vals, v)
		case 1:
			var cancel CancelFunc
			ctx, cancel = WithCancel(ctx)
			cancels = append(cancels, cancel)
		case 2:
			var cancel CancelFunc
			d := veryLongDuration
			if testTimeout {
				d = shortDuration
			}
			ctx, cancel = WithTimeout(ctx, d)
			cancels = append(cancels, cancel)
			numTimers++
		}
	}
	checkValues := func(when string) {
		for _, key := range vals {
			if val := ctx.Value(key).(*value); key != val {
				errorf("%s: ctx.Value(%p) = %p want %p", when, key, val, key)
			}
		}
	}
	if !testTimeout {
		select {
		case <-ctx.Done():
			errorf("ctx should not be canceled yet")
		default:
		}
	}
	if s, prefix := fmt.Sprint(ctx), "context.Background."; !strings.HasPrefix(s, prefix) {
		t.Errorf("ctx.String() = %q want prefix %q", s, prefix)
	}
	t.Log(ctx)
	checkValues("before cancel")
	if testTimeout {
		d := quiescent(t)
		timer := time.NewTimer(d)
		defer timer.Stop()
		select {
		case <-ctx.Done():
		case <-timer.C:
			errorf("ctx should have timed out after %v", d)
		}
		checkValues("after timeout")
	} else {
		cancel := cancels[r.Intn(len(cancels))]
		cancel()
		select {
		case <-ctx.Done():
		default:
			errorf("ctx should be canceled")
		}
		checkValues("after cancel")
	}
}

func XTestCancelRemoves(t testingT) {
	checkChildren := func(when string, ctx Context, want int) {
		if got := len(ctx.(*cancelCtx).children); got != want {
			t.Errorf("%s: context has %d children, want %d", when, got, want)
		}
	}

	ctx, _ := WithCancel(Background())
	checkChildren("after creation", ctx, 0)
	_, cancel := WithCancel(ctx)
	checkChildren("with WithCancel child ", ctx, 1)
	cancel()
	checkChildren("after canceling WithCancel child", ctx, 0)

	ctx, _ = WithCancel(Background())
	checkChildren("after creation", ctx, 0)
	_, cancel = WithTimeout(ctx, 60*time.Minute)
	checkChildren("with WithTimeout child ", ctx, 1)
	cancel()
	checkChildren("after canceling WithTimeout child", ctx, 0)
}

func XTestWithCancelCanceledParent(t testingT) {
	parent, pcancel := WithCancelCause(Background())
	cause := fmt.Errorf("Because!")
	pcancel(cause)

	c, _ := WithCancel(parent)
	select {
	case <-c.Done():
	default:
		t.Errorf("child not done immediately upon construction")
	}
	if got, want := c.Err(), Canceled; got != want {
		t.Errorf("child not canceled; got = %v, want = %v", got, want)
	}
	if got, want := Cause(c), cause; got != want {
		t.Errorf("child has wrong cause; got = %v, want = %v", got, want)
	}
}

func XTestWithValueChecksKey(t testingT) {
	panicVal := recoveredValue(func() { WithValue(Background(), []byte("foo"), "bar") })
	if panicVal == nil {
		t.Error("expected panic")
	}
	panicVal = recoveredValue(func() { WithValue(Background(), nil, "bar") })
	if got, want := fmt.Sprint(panicVal), "nil key"; got != want {
		t.Errorf("panic = %q; want %q", got, want)
	}
}

func XTestInvalidDerivedFail(t testingT) {
	panicVal := recoveredValue(func() { WithCancel(nil) })
	if panicVal == nil {
		t.Error("expected panic")
	}
	panicVal = recoveredValue(func() { WithDeadline(nil, time.Now().Add(shortDuration)) })
	if panicVal == nil {
		t.Error("expected panic")
	}
	panicVal = recoveredValue(func() { WithValue(nil, "foo", "bar") })
	if panicVal == nil {
		t.Error("expected panic")
	}
}

func recoveredValue(fn func()) (v any) {
	defer func() { v = recover() }()
	fn()
	return
}

func XTestDeadlineExceededSupportsTimeout(t testingT) {
	i, ok := DeadlineExceeded.(interface {
		Timeout() bool
	})
	if !ok {
		t.Fatal("DeadlineExceeded does not support Timeout interface")
	}
	if !i.Timeout() {
		t.Fatal("wrong value for timeout")
	}
}

type myCtx struct {
	Context
}

type myDoneCtx struct {
	Context
}

func (d *myDoneCtx) Done() <-chan struct{} {
	c := make(chan struct{})
	return c
}

func XTestCustomContextGoroutines(t testingT) {
	g := goroutines.Load()
	checkNoGoroutine := func() {
		t.Helper()
		now := goroutines.Load()
		if now != g {
			t.Fatalf("%d goroutines created", now-g)
		}
	}
	checkCreatedGoroutine := func() {
		t.Helper()
		now := goroutines.Load()
		if now != g+1 {
			t.Fatalf("%d goroutines created, want 1", now-g)
		}
		g = now
	}

	_, cancel0 := WithCancel(&myDoneCtx{Background()})
	cancel0()
	checkCreatedGoroutine()

	_, cancel0 = WithTimeout(&myDoneCtx{Background()}, veryLongDuration)
	cancel0()
	checkCreatedGoroutine()

	checkNoGoroutine()
	defer checkNoGoroutine()

	ctx1, cancel1 := WithCancel(Background())
	defer cancel1()
	checkNoGoroutine()

	ctx2 := &myCtx{ctx1}
	ctx3, cancel3 := WithCancel(ctx2)
	defer cancel3()
	checkNoGoroutine()

	_, cancel3b := WithCancel(&myDoneCtx{ctx2})
	defer cancel3b()
	checkCreatedGoroutine() // ctx1 is not providing Done, must not be used

	ctx4, cancel4 := WithTimeout(ctx3, veryLongDuration)
	defer cancel4()
	checkNoGoroutine()

	ctx5, cancel5 := WithCancel(ctx4)
	defer cancel5()
	checkNoGoroutine()

	cancel5()
	checkNoGoroutine()

	_, cancel6 := WithTimeout(ctx5, veryLongDuration)
	defer cancel6()
	checkNoGoroutine()

	// Check applied to canceled context.
	cancel6()
	cancel1()
	_, cancel7 := WithCancel(ctx5)
	defer cancel7()
	checkNoGoroutine()
}

func XTestCause(t testingT) {
	var (
		parentCause = fmt.Errorf("parentCause")
		childCause  = fmt.Errorf("childCause")
	)
	for _, test := range []struct {
		name  string
		ctx   Context
		err   error
		cause error
	}{
		{
			name:  "Background",
			ctx:   Background(),
			err:   nil,
			cause: nil,
		},
		{
			name:  "TODO",
			ctx:   TODO(),
			err:   nil,
			cause: nil,
		},
		{
			name: "WithCancel",
			ctx: func() Context {
				ctx, cancel := WithCancel(Background())
				cancel()
				return ctx
			}(),
			err:   Canceled,
			cause: Canceled,
		},
		{
			name: "WithCancelCause",
			ctx: func() Context {
				ctx, cancel := WithCancelCause(Background())
				cancel(parentCause)
				return ctx
			}(),
			err:   Canceled,
			cause: parentCause,
		},
		{
			name: "WithCancelCause nil",
			ctx: func() Context {
				ctx, cancel := WithCancelCause(Background())
				cancel(nil)
				return ctx
			}(),
			err:   Canceled,
			cause: Canceled,
		},
		{
			name: "WithCancelCause: parent cause before child",
			ctx: func() Context {
				ctx, cancelParent := WithCancelCause(Background())
				ctx, cancelChild := WithCancelCause(ctx)
				cancelParent(parentCause)
				cancelChild(childCause)
				return ctx
			}(),
			err:   Canceled,
			cause: parentCause,
		},
		{
			name: "WithCancelCause: parent cause after child",
			ctx: func() Context {
				ctx, cancelParent := WithCancelCause(Background())
				ctx, cancelChild := WithCancelCause(ctx)
				cancelChild(childCause)
				cancelParent(parentCause)
				return ctx
			}(),
			err:   Canceled,
			cause: childCause,
		},
		{
			name: "WithCancelCause: parent cause before nil",
			ctx: func() Context {
				ctx, cancelParent := WithCancelCause(Background())
				ctx, cancelChild := WithCancel(ctx)
				cancelParent(parentCause)
				cancelChild()
				return ctx
			}(),
			err:   Canceled,
			cause: parentCause,
		},
		{
			name: "WithCancelCause: parent cause after nil",
			ctx: func() Context {
				ctx, cancelParent := WithCancelCause(Background())
				ctx, cancelChild := WithCancel(ctx)
				cancelChild()
				cancelParent(parentCause)
				return ctx
			}(),
			err:   Canceled,
			cause: Canceled,
		},
		{
			name: "WithCancelCause: child cause after nil",
			ctx: func() Context {
				ctx, cancelParent := WithCancel(Background())
				ctx, cancelChild := WithCancelCause(ctx)
				cancelParent()
				cancelChild(childCause)
				return ctx
			}(),
			err:   Canceled,
			cause: Canceled,
		},
		{
			name: "WithCancelCause: child cause before nil",
			ctx: func() Context {
				ctx, cancelParent := WithCancel(Background())
				ctx, cancelChild := WithCancelCause(ctx)
				cancelChild(childCause)
				cancelParent()
				return ctx
			}(),
			err:   Canceled,
			cause: childCause,
		},
		{
			name: "WithTimeout",
			ctx: func() Context {
				ctx, cancel := WithTimeout(Background(), 0)
				cancel()
				return ctx
			}(),
			err:   DeadlineExceeded,
			cause: DeadlineExceeded,
		},
	} {
		if got, want := test.ctx.Err(), test.err; want != got {
			t.Errorf("%s: ctx.Err() = %v want %v", test.name, got, want)
		}
		if got, want := Cause(test.ctx), test.cause; want != got {
			t.Errorf("%s: Cause(ctx) = %v want %v", test.name, got, want)
		}
	}
}

func XTestCauseRace(t testingT) {
	cause := errors.New("TestCauseRace")
	ctx, cancel := WithCancelCause(Background())
	go func() {
		cancel(cause)
	}()
	for {
		// Poll Cause, rather than waiting for Done, to test that
		// access to the underlying cause is synchronized properly.
		if err := Cause(ctx); err != nil {
			if err != cause {
				t.Errorf("Cause returned %v, want %v", err, cause)
			}
			break
		}
		runtime.Gosched()
	}
}