Adding upstream version 1.22.1.upstream/1.22.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

7 files changed, 2781 insertions, 0 deletions

diff --git a/src/context/afterfunc_test.go b/src/context/afterfunc_test.go
new file mode 100644
index 0000000..7b75295
--- /dev/null
+++ b/src/context/afterfunc_test.go
@@ -0,0 +1,143 @@
+// Copyright 2023 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_test
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// afterFuncContext is a context that's not one of the types
+// defined in context.go, that supports registering AfterFuncs.
+type afterFuncContext struct {
+	mu         sync.Mutex
+	afterFuncs map[*byte]func()
+	done       chan struct{}
+	err        error
+}
+
+func newAfterFuncContext() context.Context {
+	return &afterFuncContext{}
+}
+
+func (c *afterFuncContext) Deadline() (time.Time, bool) {
+	return time.Time{}, false
+}
+
+func (c *afterFuncContext) Done() <-chan struct{} {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.done == nil {
+		c.done = make(chan struct{})
+	}
+	return c.done
+}
+
+func (c *afterFuncContext) Err() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.err
+}
+
+func (c *afterFuncContext) Value(key any) any {
+	return nil
+}
+
+func (c *afterFuncContext) AfterFunc(f func()) func() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	k := new(byte)
+	if c.afterFuncs == nil {
+		c.afterFuncs = make(map[*byte]func())
+	}
+	c.afterFuncs[k] = f
+	return func() bool {
+		c.mu.Lock()
+		defer c.mu.Unlock()
+		_, ok := c.afterFuncs[k]
+		delete(c.afterFuncs, k)
+		return ok
+	}
+}
+
+func (c *afterFuncContext) cancel(err error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err != nil {
+		return
+	}
+	c.err = err
+	for _, f := range c.afterFuncs {
+		go f()
+	}
+	c.afterFuncs = nil
+}
+
+func TestCustomContextAfterFuncCancel(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	ctx1, cancel := context.WithCancel(ctx0)
+	defer cancel()
+	ctx0.cancel(context.Canceled)
+	<-ctx1.Done()
+}
+
+func TestCustomContextAfterFuncTimeout(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	ctx1, cancel := context.WithTimeout(ctx0, veryLongDuration)
+	defer cancel()
+	ctx0.cancel(context.Canceled)
+	<-ctx1.Done()
+}
+
+func TestCustomContextAfterFuncAfterFunc(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	donec := make(chan struct{})
+	stop := context.AfterFunc(ctx0, func() {
+		close(donec)
+	})
+	defer stop()
+	ctx0.cancel(context.Canceled)
+	<-donec
+}
+
+func TestCustomContextAfterFuncUnregisterCancel(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	_, cancel1 := context.WithCancel(ctx0)
+	_, cancel2 := context.WithCancel(ctx0)
+	if got, want := len(ctx0.afterFuncs), 2; got != want {
+		t.Errorf("after WithCancel(ctx0): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	cancel1()
+	cancel2()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after canceling WithCancel(ctx0): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}
+
+func TestCustomContextAfterFuncUnregisterTimeout(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	_, cancel := context.WithTimeout(ctx0, veryLongDuration)
+	if got, want := len(ctx0.afterFuncs), 1; got != want {
+		t.Errorf("after WithTimeout(ctx0, d): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	cancel()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after canceling WithTimeout(ctx0, d): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}
+
+func TestCustomContextAfterFuncUnregisterAfterFunc(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	stop := context.AfterFunc(ctx0, func() {})
+	if got, want := len(ctx0.afterFuncs), 1; got != want {
+		t.Errorf("after AfterFunc(ctx0, f): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	stop()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after stopping AfterFunc(ctx0, f): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}
diff --git a/src/context/benchmark_test.go b/src/context/benchmark_test.go
new file mode 100644
index 0000000..144f473
--- /dev/null
+++ b/src/context/benchmark_test.go
@@ -0,0 +1,190 @@
+// 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_test
+
+import (
+	. "context"
+	"fmt"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+func BenchmarkCommonParentCancel(b *testing.B) {
+	root := WithValue(Background(), "key", "value")
+	shared, sharedcancel := WithCancel(root)
+	defer sharedcancel()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		x := 0
+		for pb.Next() {
+			ctx, cancel := WithCancel(shared)
+			if ctx.Value("key").(string) != "value" {
+				b.Fatal("should not be reached")
+			}
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+			cancel()
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+		}
+	})
+}
+
+func BenchmarkWithTimeout(b *testing.B) {
+	for concurrency := 40; concurrency <= 4e5; concurrency *= 100 {
+		name := fmt.Sprintf("concurrency=%d", concurrency)
+		b.Run(name, func(b *testing.B) {
+			benchmarkWithTimeout(b, concurrency)
+		})
+	}
+}
+
+func benchmarkWithTimeout(b *testing.B, concurrentContexts int) {
+	gomaxprocs := runtime.GOMAXPROCS(0)
+	perPContexts := concurrentContexts / gomaxprocs
+	root := Background()
+
+	// Generate concurrent contexts.
+	var wg sync.WaitGroup
+	ccf := make([][]CancelFunc, gomaxprocs)
+	for i := range ccf {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			cf := make([]CancelFunc, perPContexts)
+			for j := range cf {
+				_, cf[j] = WithTimeout(root, time.Hour)
+			}
+			ccf[i] = cf
+		}(i)
+	}
+	wg.Wait()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		wcf := make([]CancelFunc, 10)
+		for pb.Next() {
+			for i := range wcf {
+				_, wcf[i] = WithTimeout(root, time.Hour)
+			}
+			for _, f := range wcf {
+				f()
+			}
+		}
+	})
+	b.StopTimer()
+
+	for _, cf := range ccf {
+		for _, f := range cf {
+			f()
+		}
+	}
+}
+
+func BenchmarkCancelTree(b *testing.B) {
+	depths := []int{1, 10, 100, 1000}
+	for _, d := range depths {
+		b.Run(fmt.Sprintf("depth=%d", d), func(b *testing.B) {
+			b.Run("Root=Background", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					buildContextTree(Background(), d)
+				}
+			})
+			b.Run("Root=OpenCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					buildContextTree(ctx, d)
+					cancel()
+				}
+			})
+			b.Run("Root=ClosedCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					cancel()
+					buildContextTree(ctx, d)
+				}
+			})
+		})
+	}
+}
+
+func buildContextTree(root Context, depth int) {
+	for d := 0; d < depth; d++ {
+		root, _ = WithCancel(root)
+	}
+}
+
+func BenchmarkCheckCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	b.Run("Err", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ctx.Err()
+		}
+	})
+	b.Run("Done", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}
+
+func BenchmarkContextCancelDone(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	defer cancel()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}
+
+func BenchmarkDeepValueNewGoRoutine(b *testing.B) {
+	for _, depth := range []int{10, 20, 30, 50, 100} {
+		ctx := Background()
+		for i := 0; i < depth; i++ {
+			ctx = WithValue(ctx, i, i)
+		}
+
+		b.Run(fmt.Sprintf("depth=%d", depth), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				var wg sync.WaitGroup
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					ctx.Value(-1)
+				}()
+				wg.Wait()
+			}
+		})
+	}
+}
+
+func BenchmarkDeepValueSameGoRoutine(b *testing.B) {
+	for _, depth := range []int{10, 20, 30, 50, 100} {
+		ctx := Background()
+		for i := 0; i < depth; i++ {
+			ctx = WithValue(ctx, i, i)
+		}
+
+		b.Run(fmt.Sprintf("depth=%d", depth), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				ctx.Value(-1)
+			}
+		})
+	}
+}
diff --git a/src/context/context.go b/src/context/context.go
new file mode 100644
index 0000000..80e1787
--- /dev/null
+++ b/src/context/context.go
@@ -0,0 +1,790 @@
+// 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 defines the Context type, which carries deadlines,
+// cancellation signals, and other request-scoped values across API boundaries
+// and between processes.
+//
+// Incoming requests to a server should create a [Context], and outgoing
+// calls to servers should accept a Context. The chain of function
+// calls between them must propagate the Context, optionally replacing
+// it with a derived Context created using [WithCancel], [WithDeadline],
+// [WithTimeout], or [WithValue]. When a Context is canceled, all
+// Contexts derived from it are also canceled.
+//
+// The [WithCancel], [WithDeadline], and [WithTimeout] functions take a
+// Context (the parent) and return a derived Context (the child) and a
+// [CancelFunc]. Calling the CancelFunc cancels the child and its
+// children, removes the parent's reference to the child, and stops
+// any associated timers. Failing to call the CancelFunc leaks the
+// child and its children until the parent is canceled or the timer
+// fires. The go vet tool checks that CancelFuncs are used on all
+// control-flow paths.
+//
+// The [WithCancelCause] function returns a [CancelCauseFunc], which
+// takes an error and records it as the cancellation cause. Calling
+// [Cause] on the canceled context or any of its children retrieves
+// the cause. If no cause is specified, Cause(ctx) returns the same
+// value as ctx.Err().
+//
+// Programs that use Contexts should follow these rules to keep interfaces
+// consistent across packages and enable static analysis tools to check context
+// propagation:
+//
+// Do not store Contexts inside a struct type; instead, pass a Context
+// explicitly to each function that needs it. The Context should be the first
+// parameter, typically named ctx:
+//
+//	func DoSomething(ctx context.Context, arg Arg) error {
+//		// ... use ctx ...
+//	}
+//
+// Do not pass a nil [Context], even if a function permits it. Pass [context.TODO]
+// if you are unsure about which Context to use.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The same Context may be passed to functions running in different goroutines;
+// Contexts are safe for simultaneous use by multiple goroutines.
+//
+// See https://blog.golang.org/context for example code for a server that uses
+// Contexts.
+package context
+
+import (
+	"errors"
+	"internal/reflectlite"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// A Context carries a deadline, a cancellation signal, and other values across
+// API boundaries.
+//
+// Context's methods may be called by multiple goroutines simultaneously.
+type Context interface {
+	// Deadline returns the time when work done on behalf of this context
+	// should be canceled. Deadline returns ok==false when no deadline is
+	// set. Successive calls to Deadline return the same results.
+	Deadline() (deadline time.Time, ok bool)
+
+	// Done returns a channel that's closed when work done on behalf of this
+	// context should be canceled. Done may return nil if this context can
+	// never be canceled. Successive calls to Done return the same value.
+	// The close of the Done channel may happen asynchronously,
+	// after the cancel function returns.
+	//
+	// WithCancel arranges for Done to be closed when cancel is called;
+	// WithDeadline arranges for Done to be closed when the deadline
+	// expires; WithTimeout arranges for Done to be closed when the timeout
+	// elapses.
+	//
+	// Done is provided for use in select statements:
+	//
+	//  // Stream generates values with DoSomething and sends them to out
+	//  // until DoSomething returns an error or ctx.Done is closed.
+	//  func Stream(ctx context.Context, out chan<- Value) error {
+	//  	for {
+	//  		v, err := DoSomething(ctx)
+	//  		if err != nil {
+	//  			return err
+	//  		}
+	//  		select {
+	//  		case <-ctx.Done():
+	//  			return ctx.Err()
+	//  		case out <- v:
+	//  		}
+	//  	}
+	//  }
+	//
+	// See https://blog.golang.org/pipelines for more examples of how to use
+	// a Done channel for cancellation.
+	Done() <-chan struct{}
+
+	// If Done is not yet closed, Err returns nil.
+	// If Done is closed, Err returns a non-nil error explaining why:
+	// Canceled if the context was canceled
+	// or DeadlineExceeded if the context's deadline passed.
+	// After Err returns a non-nil error, successive calls to Err return the same error.
+	Err() error
+
+	// Value returns the value associated with this context for key, or nil
+	// if no value is associated with key. Successive calls to Value with
+	// the same key returns the same result.
+	//
+	// Use context values only for request-scoped data that transits
+	// processes and API boundaries, not for passing optional parameters to
+	// functions.
+	//
+	// A key identifies a specific value in a Context. Functions that wish
+	// to store values in Context typically allocate a key in a global
+	// variable then use that key as the argument to context.WithValue and
+	// Context.Value. A key can be any type that supports equality;
+	// packages should define keys as an unexported type to avoid
+	// collisions.
+	//
+	// Packages that define a Context key should provide type-safe accessors
+	// for the values stored using that key:
+	//
+	// 	// Package user defines a User type that's stored in Contexts.
+	// 	package user
+	//
+	// 	import "context"
+	//
+	// 	// User is the type of value stored in the Contexts.
+	// 	type User struct {...}
+	//
+	// 	// key is an unexported type for keys defined in this package.
+	// 	// This prevents collisions with keys defined in other packages.
+	// 	type key int
+	//
+	// 	// userKey is the key for user.User values in Contexts. It is
+	// 	// unexported; clients use user.NewContext and user.FromContext
+	// 	// instead of using this key directly.
+	// 	var userKey key
+	//
+	// 	// NewContext returns a new Context that carries value u.
+	// 	func NewContext(ctx context.Context, u *User) context.Context {
+	// 		return context.WithValue(ctx, userKey, u)
+	// 	}
+	//
+	// 	// FromContext returns the User value stored in ctx, if any.
+	// 	func FromContext(ctx context.Context) (*User, bool) {
+	// 		u, ok := ctx.Value(userKey).(*User)
+	// 		return u, ok
+	// 	}
+	Value(key any) any
+}
+
+// Canceled is the error returned by [Context.Err] when the context is canceled.
+var Canceled = errors.New("context canceled")
+
+// DeadlineExceeded is the error returned by [Context.Err] when the context's
+// deadline passes.
+var DeadlineExceeded error = deadlineExceededError{}
+
+type deadlineExceededError struct{}
+
+func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool   { return true }
+func (deadlineExceededError) Temporary() bool { return true }
+
+// An emptyCtx is never canceled, has no values, and has no deadline.
+// It is the common base of backgroundCtx and todoCtx.
+type emptyCtx struct{}
+
+func (emptyCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (emptyCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (emptyCtx) Err() error {
+	return nil
+}
+
+func (emptyCtx) Value(key any) any {
+	return nil
+}
+
+type backgroundCtx struct{ emptyCtx }
+
+func (backgroundCtx) String() string {
+	return "context.Background"
+}
+
+type todoCtx struct{ emptyCtx }
+
+func (todoCtx) String() string {
+	return "context.TODO"
+}
+
+// Background returns a non-nil, empty [Context]. It is never canceled, has no
+// values, and has no deadline. It is typically used by the main function,
+// initialization, and tests, and as the top-level Context for incoming
+// requests.
+func Background() Context {
+	return backgroundCtx{}
+}
+
+// TODO returns a non-nil, empty [Context]. Code should use context.TODO when
+// it's unclear which Context to use or it is not yet available (because the
+// surrounding function has not yet been extended to accept a Context
+// parameter).
+func TODO() Context {
+	return todoCtx{}
+}
+
+// A CancelFunc tells an operation to abandon its work.
+// A CancelFunc does not wait for the work to stop.
+// A CancelFunc may be called by multiple goroutines simultaneously.
+// After the first call, subsequent calls to a CancelFunc do nothing.
+type CancelFunc func()
+
+// WithCancel returns a copy of parent with a new Done channel. The returned
+// context's Done channel is closed when the returned cancel function is called
+// or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+	c := withCancel(parent)
+	return c, func() { c.cancel(true, Canceled, nil) }
+}
+
+// A CancelCauseFunc behaves like a [CancelFunc] but additionally sets the cancellation cause.
+// This cause can be retrieved by calling [Cause] on the canceled Context or on
+// any of its derived Contexts.
+//
+// If the context has already been canceled, CancelCauseFunc does not set the cause.
+// For example, if childContext is derived from parentContext:
+//   - if parentContext is canceled with cause1 before childContext is canceled with cause2,
+//     then Cause(parentContext) == Cause(childContext) == cause1
+//   - if childContext is canceled with cause2 before parentContext is canceled with cause1,
+//     then Cause(parentContext) == cause1 and Cause(childContext) == cause2
+type CancelCauseFunc func(cause error)
+
+// WithCancelCause behaves like [WithCancel] but returns a [CancelCauseFunc] instead of a [CancelFunc].
+// Calling cancel with a non-nil error (the "cause") records that error in ctx;
+// it can then be retrieved using Cause(ctx).
+// Calling cancel with nil sets the cause to Canceled.
+//
+// Example use:
+//
+//	ctx, cancel := context.WithCancelCause(parent)
+//	cancel(myError)
+//	ctx.Err() // returns context.Canceled
+//	context.Cause(ctx) // returns myError
+func WithCancelCause(parent Context) (ctx Context, cancel CancelCauseFunc) {
+	c := withCancel(parent)
+	return c, func(cause error) { c.cancel(true, Canceled, cause) }
+}
+
+func withCancel(parent Context) *cancelCtx {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	c := &cancelCtx{}
+	c.propagateCancel(parent, c)
+	return c
+}
+
+// Cause returns a non-nil error explaining why c was canceled.
+// The first cancellation of c or one of its parents sets the cause.
+// If that cancellation happened via a call to CancelCauseFunc(err),
+// then [Cause] returns err.
+// Otherwise Cause(c) returns the same value as c.Err().
+// Cause returns nil if c has not been canceled yet.
+func Cause(c Context) error {
+	if cc, ok := c.Value(&cancelCtxKey).(*cancelCtx); ok {
+		cc.mu.Lock()
+		defer cc.mu.Unlock()
+		return cc.cause
+	}
+	// There is no cancelCtxKey value, so we know that c is
+	// not a descendant of some Context created by WithCancelCause.
+	// Therefore, there is no specific cause to return.
+	// If this is not one of the standard Context types,
+	// it might still have an error even though it won't have a cause.
+	return c.Err()
+}
+
+// AfterFunc arranges to call f in its own goroutine after ctx is done
+// (cancelled or timed out).
+// If ctx is already done, AfterFunc calls f immediately in its own goroutine.
+//
+// Multiple calls to AfterFunc on a context operate independently;
+// one does not replace another.
+//
+// Calling the returned stop function stops the association of ctx with f.
+// It returns true if the call stopped f from being run.
+// If stop returns false,
+// either the context is done and f has been started in its own goroutine;
+// or f was already stopped.
+// The stop function does not wait for f to complete before returning.
+// If the caller needs to know whether f is completed,
+// it must coordinate with f explicitly.
+//
+// If ctx has a "AfterFunc(func()) func() bool" method,
+// AfterFunc will use it to schedule the call.
+func AfterFunc(ctx Context, f func()) (stop func() bool) {
+	a := &afterFuncCtx{
+		f: f,
+	}
+	a.cancelCtx.propagateCancel(ctx, a)
+	return func() bool {
+		stopped := false
+		a.once.Do(func() {
+			stopped = true
+		})
+		if stopped {
+			a.cancel(true, Canceled, nil)
+		}
+		return stopped
+	}
+}
+
+type afterFuncer interface {
+	AfterFunc(func()) func() bool
+}
+
+type afterFuncCtx struct {
+	cancelCtx
+	once sync.Once // either starts running f or stops f from running
+	f    func()
+}
+
+func (a *afterFuncCtx) cancel(removeFromParent bool, err, cause error) {
+	a.cancelCtx.cancel(false, err, cause)
+	if removeFromParent {
+		removeChild(a.Context, a)
+	}
+	a.once.Do(func() {
+		go a.f()
+	})
+}
+
+// A stopCtx is used as the parent context of a cancelCtx when
+// an AfterFunc has been registered with the parent.
+// It holds the stop function used to unregister the AfterFunc.
+type stopCtx struct {
+	Context
+	stop func() bool
+}
+
+// goroutines counts the number of goroutines ever created; for testing.
+var goroutines atomic.Int32
+
+// &cancelCtxKey is the key that a cancelCtx returns itself for.
+var cancelCtxKey int
+
+// parentCancelCtx returns the underlying *cancelCtx for parent.
+// It does this by looking up parent.Value(&cancelCtxKey) to find
+// the innermost enclosing *cancelCtx and then checking whether
+// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
+// has been wrapped in a custom implementation providing a
+// different done channel, in which case we should not bypass it.)
+func parentCancelCtx(parent Context) (*cancelCtx, bool) {
+	done := parent.Done()
+	if done == closedchan || done == nil {
+		return nil, false
+	}
+	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
+	if !ok {
+		return nil, false
+	}
+	pdone, _ := p.done.Load().(chan struct{})
+	if pdone != done {
+		return nil, false
+	}
+	return p, true
+}
+
+// removeChild removes a context from its parent.
+func removeChild(parent Context, child canceler) {
+	if s, ok := parent.(stopCtx); ok {
+		s.stop()
+		return
+	}
+	p, ok := parentCancelCtx(parent)
+	if !ok {
+		return
+	}
+	p.mu.Lock()
+	if p.children != nil {
+		delete(p.children, child)
+	}
+	p.mu.Unlock()
+}
+
+// A canceler is a context type that can be canceled directly. The
+// implementations are *cancelCtx and *timerCtx.
+type canceler interface {
+	cancel(removeFromParent bool, err, cause error)
+	Done() <-chan struct{}
+}
+
+// closedchan is a reusable closed channel.
+var closedchan = make(chan struct{})
+
+func init() {
+	close(closedchan)
+}
+
+// A cancelCtx can be canceled. When canceled, it also cancels any children
+// that implement canceler.
+type cancelCtx struct {
+	Context
+
+	mu       sync.Mutex            // protects following fields
+	done     atomic.Value          // of chan struct{}, created lazily, closed by first cancel call
+	children map[canceler]struct{} // set to nil by the first cancel call
+	err      error                 // set to non-nil by the first cancel call
+	cause    error                 // set to non-nil by the first cancel call
+}
+
+func (c *cancelCtx) Value(key any) any {
+	if key == &cancelCtxKey {
+		return c
+	}
+	return value(c.Context, key)
+}
+
+func (c *cancelCtx) Done() <-chan struct{} {
+	d := c.done.Load()
+	if d != nil {
+		return d.(chan struct{})
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	d = c.done.Load()
+	if d == nil {
+		d = make(chan struct{})
+		c.done.Store(d)
+	}
+	return d.(chan struct{})
+}
+
+func (c *cancelCtx) Err() error {
+	c.mu.Lock()
+	err := c.err
+	c.mu.Unlock()
+	return err
+}
+
+// propagateCancel arranges for child to be canceled when parent is.
+// It sets the parent context of cancelCtx.
+func (c *cancelCtx) propagateCancel(parent Context, child canceler) {
+	c.Context = parent
+
+	done := parent.Done()
+	if done == nil {
+		return // parent is never canceled
+	}
+
+	select {
+	case <-done:
+		// parent is already canceled
+		child.cancel(false, parent.Err(), Cause(parent))
+		return
+	default:
+	}
+
+	if p, ok := parentCancelCtx(parent); ok {
+		// parent is a *cancelCtx, or derives from one.
+		p.mu.Lock()
+		if p.err != nil {
+			// parent has already been canceled
+			child.cancel(false, p.err, p.cause)
+		} else {
+			if p.children == nil {
+				p.children = make(map[canceler]struct{})
+			}
+			p.children[child] = struct{}{}
+		}
+		p.mu.Unlock()
+		return
+	}
+
+	if a, ok := parent.(afterFuncer); ok {
+		// parent implements an AfterFunc method.
+		c.mu.Lock()
+		stop := a.AfterFunc(func() {
+			child.cancel(false, parent.Err(), Cause(parent))
+		})
+		c.Context = stopCtx{
+			Context: parent,
+			stop:    stop,
+		}
+		c.mu.Unlock()
+		return
+	}
+
+	goroutines.Add(1)
+	go func() {
+		select {
+		case <-parent.Done():
+			child.cancel(false, parent.Err(), Cause(parent))
+		case <-child.Done():
+		}
+	}()
+}
+
+type stringer interface {
+	String() string
+}
+
+func contextName(c Context) string {
+	if s, ok := c.(stringer); ok {
+		return s.String()
+	}
+	return reflectlite.TypeOf(c).String()
+}
+
+func (c *cancelCtx) String() string {
+	return contextName(c.Context) + ".WithCancel"
+}
+
+// cancel closes c.done, cancels each of c's children, and, if
+// removeFromParent is true, removes c from its parent's children.
+// cancel sets c.cause to cause if this is the first time c is canceled.
+func (c *cancelCtx) cancel(removeFromParent bool, err, cause error) {
+	if err == nil {
+		panic("context: internal error: missing cancel error")
+	}
+	if cause == nil {
+		cause = err
+	}
+	c.mu.Lock()
+	if c.err != nil {
+		c.mu.Unlock()
+		return // already canceled
+	}
+	c.err = err
+	c.cause = cause
+	d, _ := c.done.Load().(chan struct{})
+	if d == nil {
+		c.done.Store(closedchan)
+	} else {
+		close(d)
+	}
+	for child := range c.children {
+		// NOTE: acquiring the child's lock while holding parent's lock.
+		child.cancel(false, err, cause)
+	}
+	c.children = nil
+	c.mu.Unlock()
+
+	if removeFromParent {
+		removeChild(c.Context, c)
+	}
+}
+
+// WithoutCancel returns a copy of parent that is not canceled when parent is canceled.
+// The returned context returns no Deadline or Err, and its Done channel is nil.
+// Calling [Cause] on the returned context returns nil.
+func WithoutCancel(parent Context) Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	return withoutCancelCtx{parent}
+}
+
+type withoutCancelCtx struct {
+	c Context
+}
+
+func (withoutCancelCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (withoutCancelCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (withoutCancelCtx) Err() error {
+	return nil
+}
+
+func (c withoutCancelCtx) Value(key any) any {
+	return value(c, key)
+}
+
+func (c withoutCancelCtx) String() string {
+	return contextName(c.c) + ".WithoutCancel"
+}
+
+// WithDeadline returns a copy of the parent context with the deadline adjusted
+// to be no later than d. If the parent's deadline is already earlier than d,
+// WithDeadline(parent, d) is semantically equivalent to parent. The returned
+// [Context.Done] channel is closed when the deadline expires, when the returned
+// cancel function is called, or when the parent context's Done channel is
+// closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this [Context] complete.
+func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, d, nil)
+}
+
+// WithDeadlineCause behaves like [WithDeadline] but also sets the cause of the
+// returned Context when the deadline is exceeded. The returned [CancelFunc] does
+// not set the cause.
+func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, CancelFunc) {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
+		// The current deadline is already sooner than the new one.
+		return WithCancel(parent)
+	}
+	c := &timerCtx{
+		deadline: d,
+	}
+	c.cancelCtx.propagateCancel(parent, c)
+	dur := time.Until(d)
+	if dur <= 0 {
+		c.cancel(true, DeadlineExceeded, cause) // deadline has already passed
+		return c, func() { c.cancel(false, Canceled, nil) }
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err == nil {
+		c.timer = time.AfterFunc(dur, func() {
+			c.cancel(true, DeadlineExceeded, cause)
+		})
+	}
+	return c, func() { c.cancel(true, Canceled, nil) }
+}
+
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
+// implement Done and Err. It implements cancel by stopping its timer then
+// delegating to cancelCtx.cancel.
+type timerCtx struct {
+	cancelCtx
+	timer *time.Timer // Under cancelCtx.mu.
+
+	deadline time.Time
+}
+
+func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
+	return c.deadline, true
+}
+
+func (c *timerCtx) String() string {
+	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
+		c.deadline.String() + " [" +
+		time.Until(c.deadline).String() + "])"
+}
+
+func (c *timerCtx) cancel(removeFromParent bool, err, cause error) {
+	c.cancelCtx.cancel(false, err, cause)
+	if removeFromParent {
+		// Remove this timerCtx from its parent cancelCtx's children.
+		removeChild(c.cancelCtx.Context, c)
+	}
+	c.mu.Lock()
+	if c.timer != nil {
+		c.timer.Stop()
+		c.timer = nil
+	}
+	c.mu.Unlock()
+}
+
+// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this [Context] complete:
+//
+//	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
+//		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
+//		defer cancel()  // releases resources if slowOperation completes before timeout elapses
+//		return slowOperation(ctx)
+//	}
+func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
+	return WithDeadline(parent, time.Now().Add(timeout))
+}
+
+// WithTimeoutCause behaves like [WithTimeout] but also sets the cause of the
+// returned Context when the timeout expires. The returned [CancelFunc] does
+// not set the cause.
+func WithTimeoutCause(parent Context, timeout time.Duration, cause error) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, time.Now().Add(timeout), cause)
+}
+
+// WithValue returns a copy of parent in which the value associated with key is
+// val.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The provided key must be comparable and should not be of type
+// string or any other built-in type to avoid collisions between
+// packages using context. Users of WithValue should define their own
+// types for keys. To avoid allocating when assigning to an
+// interface{}, context keys often have concrete type
+// struct{}. Alternatively, exported context key variables' static
+// type should be a pointer or interface.
+func WithValue(parent Context, key, val any) Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	if key == nil {
+		panic("nil key")
+	}
+	if !reflectlite.TypeOf(key).Comparable() {
+		panic("key is not comparable")
+	}
+	return &valueCtx{parent, key, val}
+}
+
+// A valueCtx carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+type valueCtx struct {
+	Context
+	key, val any
+}
+
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v any) string {
+	switch s := v.(type) {
+	case stringer:
+		return s.String()
+	case string:
+		return s
+	}
+	return "<not Stringer>"
+}
+
+func (c *valueCtx) String() string {
+	return contextName(c.Context) + ".WithValue(type " +
+		reflectlite.TypeOf(c.key).String() +
+		", val " + stringify(c.val) + ")"
+}
+
+func (c *valueCtx) Value(key any) any {
+	if c.key == key {
+		return c.val
+	}
+	return value(c.Context, key)
+}
+
+func value(c Context, key any) any {
+	for {
+		switch ctx := c.(type) {
+		case *valueCtx:
+			if key == ctx.key {
+				return ctx.val
+			}
+			c = ctx.Context
+		case *cancelCtx:
+			if key == &cancelCtxKey {
+				return c
+			}
+			c = ctx.Context
+		case withoutCancelCtx:
+			if key == &cancelCtxKey {
+				// This implements Cause(ctx) == nil
+				// when ctx is created using WithoutCancel.
+				return nil
+			}
+			c = ctx.c
+		case *timerCtx:
+			if key == &cancelCtxKey {
+				return &ctx.cancelCtx
+			}
+			c = ctx.Context
+		case backgroundCtx, todoCtx:
+			return nil
+		default:
+			return c.Value(key)
+		}
+	}
+}
diff --git a/src/context/context_test.go b/src/context/context_test.go
new file mode 100644
index 0000000..57066c9
--- /dev/null
+++ b/src/context/context_test.go
@@ -0,0 +1,297 @@
+// 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
+
+// Tests in package context cannot depend directly on package testing due to an import cycle.
+// If your test does requires access to unexported members of the context package,
+// add your test below as `func XTestFoo(t testingT)` and add a `TestFoo` to x_test.go
+// that calls it. Otherwise, write a regular test in a test.go file in package context_test.
+
+import (
+	"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
+}
+
+const veryLongDuration = 1000 * time.Hour // an arbitrary upper bound on the test's running time
+
+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 -> afterChild
+	parent, cancel := WithCancel(Background())
+	cancelChild, stop := WithCancel(parent)
+	defer stop()
+	valueChild := WithValue(parent, "key", "value")
+	timerChild, stop := WithTimeout(valueChild, veryLongDuration)
+	defer stop()
+	afterStop := AfterFunc(parent, func() {})
+	defer afterStop()
+
+	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 three cancelable children.
+	pc := parent.(*cancelCtx)
+	cc := cancelChild.(*cancelCtx)
+	tc := timerChild.(*timerCtx)
+	pc.mu.Lock()
+	var ac *afterFuncCtx
+	for c := range pc.children {
+		if a, ok := c.(*afterFuncCtx); ok {
+			ac = a
+			break
+		}
+	}
+	if len(pc.children) != 3 || !contains(pc.children, cc) || !contains(pc.children, tc) || ac == nil {
+		t.Errorf("bad linkage: pc.children = %v, want %v, %v, and an afterFunc",
+			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)
+	}
+	if p, ok := parentCancelCtx(ac.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(afterChild.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 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)
+
+	ctx, _ = WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	stop := AfterFunc(ctx, func() {})
+	checkChildren("with AfterFunc child ", ctx, 1)
+	stop()
+	checkChildren("after stopping AfterFunc child ", ctx, 0)
+}
+
+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()
+}
diff --git a/src/context/example_test.go b/src/context/example_test.go
new file mode 100644
index 0000000..03333b5
--- /dev/null
+++ b/src/context/example_test.go
@@ -0,0 +1,263 @@
+// Copyright 2016 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_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sync"
+	"time"
+)
+
+var neverReady = make(chan struct{}) // never closed
+
+// This example demonstrates the use of a cancelable context to prevent a
+// goroutine leak. By the end of the example function, the goroutine started
+// by gen will return without leaking.
+func ExampleWithCancel() {
+	// gen generates integers in a separate goroutine and
+	// sends them to the returned channel.
+	// The callers of gen need to cancel the context once
+	// they are done consuming generated integers not to leak
+	// the internal goroutine started by gen.
+	gen := func(ctx context.Context) <-chan int {
+		dst := make(chan int)
+		n := 1
+		go func() {
+			for {
+				select {
+				case <-ctx.Done():
+					return // returning not to leak the goroutine
+				case dst <- n:
+					n++
+				}
+			}
+		}()
+		return dst
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel() // cancel when we are finished consuming integers
+
+	for n := range gen(ctx) {
+		fmt.Println(n)
+		if n == 5 {
+			break
+		}
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+}
+
+// This example passes a context with an arbitrary deadline to tell a blocking
+// function that it should abandon its work as soon as it gets to it.
+func ExampleWithDeadline() {
+	d := time.Now().Add(shortDuration)
+	ctx, cancel := context.WithDeadline(context.Background(), d)
+
+	// Even though ctx will be expired, it is good practice to call its
+	// cancellation function in any case. Failure to do so may keep the
+	// context and its parent alive longer than necessary.
+	defer cancel()
+
+	select {
+	case <-neverReady:
+		fmt.Println("ready")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err())
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example passes a context with a timeout to tell a blocking function that
+// it should abandon its work after the timeout elapses.
+func ExampleWithTimeout() {
+	// Pass a context with a timeout to tell a blocking function that it
+	// should abandon its work after the timeout elapses.
+	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
+	defer cancel()
+
+	select {
+	case <-neverReady:
+		fmt.Println("ready")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example demonstrates how a value can be passed to the context
+// and also how to retrieve it if it exists.
+func ExampleWithValue() {
+	type favContextKey string
+
+	f := func(ctx context.Context, k favContextKey) {
+		if v := ctx.Value(k); v != nil {
+			fmt.Println("found value:", v)
+			return
+		}
+		fmt.Println("key not found:", k)
+	}
+
+	k := favContextKey("language")
+	ctx := context.WithValue(context.Background(), k, "Go")
+
+	f(ctx, k)
+	f(ctx, favContextKey("color"))
+
+	// Output:
+	// found value: Go
+	// key not found: color
+}
+
+// This example uses AfterFunc to define a function which waits on a sync.Cond,
+// stopping the wait when a context is canceled.
+func ExampleAfterFunc_cond() {
+	waitOnCond := func(ctx context.Context, cond *sync.Cond, conditionMet func() bool) error {
+		stopf := context.AfterFunc(ctx, func() {
+			// We need to acquire cond.L here to be sure that the Broadcast
+			// below won't occur before the call to Wait, which would result
+			// in a missed signal (and deadlock).
+			cond.L.Lock()
+			defer cond.L.Unlock()
+
+			// If multiple goroutines are waiting on cond simultaneously,
+			// we need to make sure we wake up exactly this one.
+			// That means that we need to Broadcast to all of the goroutines,
+			// which will wake them all up.
+			//
+			// If there are N concurrent calls to waitOnCond, each of the goroutines
+			// will spuriously wake up O(N) other goroutines that aren't ready yet,
+			// so this will cause the overall CPU cost to be O(N²).
+			cond.Broadcast()
+		})
+		defer stopf()
+
+		// Since the wakeups are using Broadcast instead of Signal, this call to
+		// Wait may unblock due to some other goroutine's context becoming done,
+		// so to be sure that ctx is actually done we need to check it in a loop.
+		for !conditionMet() {
+			cond.Wait()
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
+		}
+
+		return nil
+	}
+
+	cond := sync.NewCond(new(sync.Mutex))
+
+	var wg sync.WaitGroup
+	for i := 0; i < 4; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+
+			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
+			defer cancel()
+
+			cond.L.Lock()
+			defer cond.L.Unlock()
+
+			err := waitOnCond(ctx, cond, func() bool { return false })
+			fmt.Println(err)
+		}()
+	}
+	wg.Wait()
+
+	// Output:
+	// context deadline exceeded
+	// context deadline exceeded
+	// context deadline exceeded
+	// context deadline exceeded
+}
+
+// This example uses AfterFunc to define a function which reads from a net.Conn,
+// stopping the read when a context is canceled.
+func ExampleAfterFunc_connection() {
+	readFromConn := func(ctx context.Context, conn net.Conn, b []byte) (n int, err error) {
+		stopc := make(chan struct{})
+		stop := context.AfterFunc(ctx, func() {
+			conn.SetReadDeadline(time.Now())
+			close(stopc)
+		})
+		n, err = conn.Read(b)
+		if !stop() {
+			// The AfterFunc was started.
+			// Wait for it to complete, and reset the Conn's deadline.
+			<-stopc
+			conn.SetReadDeadline(time.Time{})
+			return n, ctx.Err()
+		}
+		return n, err
+	}
+
+	listener, err := net.Listen("tcp", ":0")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer listener.Close()
+
+	conn, err := net.Dial(listener.Addr().Network(), listener.Addr().String())
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer conn.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
+	defer cancel()
+
+	b := make([]byte, 1024)
+	_, err = readFromConn(ctx, conn, b)
+	fmt.Println(err)
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example uses AfterFunc to define a function which combines
+// the cancellation signals of two Contexts.
+func ExampleAfterFunc_merge() {
+	// mergeCancel returns a context that contains the values of ctx,
+	// and which is canceled when either ctx or cancelCtx is canceled.
+	mergeCancel := func(ctx, cancelCtx context.Context) (context.Context, context.CancelFunc) {
+		ctx, cancel := context.WithCancelCause(ctx)
+		stop := context.AfterFunc(cancelCtx, func() {
+			cancel(context.Cause(cancelCtx))
+		})
+		return ctx, func() {
+			stop()
+			cancel(context.Canceled)
+		}
+	}
+
+	ctx1, cancel1 := context.WithCancelCause(context.Background())
+	defer cancel1(errors.New("ctx1 canceled"))
+
+	ctx2, cancel2 := context.WithCancelCause(context.Background())
+
+	mergedCtx, mergedCancel := mergeCancel(ctx1, ctx2)
+	defer mergedCancel()
+
+	cancel2(errors.New("ctx2 canceled"))
+	<-mergedCtx.Done()
+	fmt.Println(context.Cause(mergedCtx))
+
+	// Output:
+	// ctx2 canceled
+}
diff --git a/src/context/net_test.go b/src/context/net_test.go
new file mode 100644
index 0000000..a007689
--- /dev/null
+++ b/src/context/net_test.go
@@ -0,0 +1,21 @@
+// Copyright 2016 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_test
+
+import (
+	"context"
+	"net"
+	"testing"
+)
+
+func TestDeadlineExceededIsNetError(t *testing.T) {
+	err, ok := context.DeadlineExceeded.(net.Error)
+	if !ok {
+		t.Fatal("DeadlineExceeded does not implement net.Error")
+	}
+	if !err.Timeout() || !err.Temporary() {
+		t.Fatalf("Timeout() = %v, Temporary() = %v, want true, true", err.Timeout(), err.Temporary())
+	}
+}
diff --git a/src/context/x_test.go b/src/context/x_test.go
new file mode 100644
index 0000000..b1012fa
--- /dev/null
+++ b/src/context/x_test.go
@@ -0,0 +1,1077 @@
+// Copyright 2016 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_test
+
+import (
+	. "context"
+	"errors"
+	"fmt"
+	"math/rand"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// Each XTestFoo in context_test.go must be called from a TestFoo here to run.
+func TestParentFinishesChild(t *testing.T) {
+	XTestParentFinishesChild(t) // uses unexported context types
+}
+func TestChildFinishesFirst(t *testing.T) {
+	XTestChildFinishesFirst(t) // uses unexported context types
+}
+func TestCancelRemoves(t *testing.T) {
+	XTestCancelRemoves(t) // uses unexported context types
+}
+func TestCustomContextGoroutines(t *testing.T) {
+	XTestCustomContextGoroutines(t) // reads the context.goroutines counter
+}
+
+// The following are regular tests in package context_test.
+
+// 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 *testing.T) time.Duration {
+	deadline, ok := t.Deadline()
+	if !ok {
+		return 5 * time.Second
+	}
+
+	const arbitraryCleanupMargin = 1 * time.Second
+	return time.Until(deadline) - arbitraryCleanupMargin
+}
+func TestBackground(t *testing.T) {
+	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 TestTODO(t *testing.T) {
+	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 TestWithCancel(t *testing.T) {
+	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 testDeadline(c Context, name string, t *testing.T) {
+	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 TestDeadline(t *testing.T) {
+	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 TestTimeout(t *testing.T) {
+	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 TestCanceledTimeout(t *testing.T) {
+	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 TestValues(t *testing.T) {
+	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_test.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 TestAllocs(t *testing.T) {
+	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 testing.Short() {
+			numRuns = 10
+		}
+		if n := testing.AllocsPerRun(numRuns, test.f); n > limit {
+			t.Errorf("%s allocs = %f want %d", test.desc, n, int(limit))
+		}
+	}
+}
+
+func TestSimultaneousCancels(t *testing.T) {
+	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 TestInterlockedCancels(t *testing.T) {
+	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 TestLayersCancel(t *testing.T) {
+	testLayers(t, time.Now().UnixNano(), false)
+}
+
+func TestLayersTimeout(t *testing.T) {
+	testLayers(t, time.Now().UnixNano(), true)
+}
+
+func testLayers(t *testing.T, seed int64, testTimeout bool) {
+	t.Parallel()
+
+	r := rand.New(rand.NewSource(seed))
+	prefix := fmt.Sprintf("seed=%d", seed)
+	errorf := func(format string, a ...any) {
+		t.Errorf(prefix+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 TestWithCancelCanceledParent(t *testing.T) {
+	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 TestWithCancelSimultaneouslyCanceledParent(t *testing.T) {
+	// Cancel the parent goroutine concurrently with creating a child.
+	for i := 0; i < 100; i++ {
+		parent, pcancel := WithCancelCause(Background())
+		cause := fmt.Errorf("Because!")
+		go pcancel(cause)
+
+		c, _ := WithCancel(parent)
+		<-c.Done()
+		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 TestWithValueChecksKey(t *testing.T) {
+	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 TestInvalidDerivedFail(t *testing.T) {
+	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 TestDeadlineExceededSupportsTimeout(t *testing.T) {
+	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")
+	}
+}
+func TestCause(t *testing.T) {
+	var (
+		forever       = 1e6 * time.Second
+		parentCause   = fmt.Errorf("parentCause")
+		childCause    = fmt.Errorf("childCause")
+		tooSlow       = fmt.Errorf("tooSlow")
+		finishedEarly = fmt.Errorf("finishedEarly")
+	)
+	for _, test := range []struct {
+		name  string
+		ctx   func() 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,
+		},
+		{
+			name: "WithTimeout canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeout(Background(), forever)
+				cancel()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), 0, tooSlow)
+				cancel()
+				return ctx
+			},
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), forever, tooSlow)
+				cancel()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause stacked",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, 0, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause stacked canceled",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, forever, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   Canceled,
+			cause: finishedEarly,
+		},
+		{
+			name: "WithoutCancel",
+			ctx: func() Context {
+				return WithoutCancel(Background())
+			},
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "WithoutCancel canceled",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx = WithoutCancel(ctx)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "WithoutCancel timeout",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), 0, tooSlow)
+				ctx = WithoutCancel(ctx)
+				cancel()
+				return ctx
+			},
+			err:   nil,
+			cause: nil,
+		},
+	} {
+		test := test
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			ctx := test.ctx()
+			if got, want := ctx.Err(), test.err; want != got {
+				t.Errorf("ctx.Err() = %v want %v", got, want)
+			}
+			if got, want := Cause(ctx), test.cause; want != got {
+				t.Errorf("Cause(ctx) = %v want %v", got, want)
+			}
+		})
+	}
+}
+
+func TestCauseRace(t *testing.T) {
+	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()
+	}
+}
+
+func TestWithoutCancel(t *testing.T) {
+	key, value := "key", "value"
+	ctx := WithValue(Background(), key, value)
+	ctx = WithoutCancel(ctx)
+	if d, ok := ctx.Deadline(); !d.IsZero() || ok != false {
+		t.Errorf("ctx.Deadline() = %v, %v want zero, false", d, ok)
+	}
+	if done := ctx.Done(); done != nil {
+		t.Errorf("ctx.Deadline() = %v want nil", done)
+	}
+	if err := ctx.Err(); err != nil {
+		t.Errorf("ctx.Err() = %v want nil", err)
+	}
+	if v := ctx.Value(key); v != value {
+		t.Errorf("ctx.Value(%q) = %q want %q", key, v, value)
+	}
+}
+
+type customDoneContext struct {
+	Context
+	donec chan struct{}
+}
+
+func (c *customDoneContext) Done() <-chan struct{} {
+	return c.donec
+}
+
+func TestCustomContextPropagation(t *testing.T) {
+	cause := errors.New("TestCustomContextPropagation")
+	donec := make(chan struct{})
+	ctx1, cancel1 := WithCancelCause(Background())
+	ctx2 := &customDoneContext{
+		Context: ctx1,
+		donec:   donec,
+	}
+	ctx3, cancel3 := WithCancel(ctx2)
+	defer cancel3()
+
+	cancel1(cause)
+	close(donec)
+
+	<-ctx3.Done()
+	if got, want := ctx3.Err(), Canceled; got != want {
+		t.Errorf("child not canceled; got = %v, want = %v", got, want)
+	}
+	if got, want := Cause(ctx3), cause; got != want {
+		t.Errorf("child has wrong cause; got = %v, want = %v", got, want)
+	}
+}
+
+// customCauseContext is a custom Context used to test context.Cause.
+type customCauseContext struct {
+	mu   sync.Mutex
+	done chan struct{}
+	err  error
+
+	cancelChild CancelFunc
+}
+
+func (ccc *customCauseContext) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (ccc *customCauseContext) Done() <-chan struct{} {
+	ccc.mu.Lock()
+	defer ccc.mu.Unlock()
+	return ccc.done
+}
+
+func (ccc *customCauseContext) Err() error {
+	ccc.mu.Lock()
+	defer ccc.mu.Unlock()
+	return ccc.err
+}
+
+func (ccc *customCauseContext) Value(key any) any {
+	return nil
+}
+
+func (ccc *customCauseContext) cancel() {
+	ccc.mu.Lock()
+	ccc.err = Canceled
+	close(ccc.done)
+	cancelChild := ccc.cancelChild
+	ccc.mu.Unlock()
+
+	if cancelChild != nil {
+		cancelChild()
+	}
+}
+
+func (ccc *customCauseContext) setCancelChild(cancelChild CancelFunc) {
+	ccc.cancelChild = cancelChild
+}
+
+func TestCustomContextCause(t *testing.T) {
+	// Test if we cancel a custom context, Err and Cause return Canceled.
+	ccc := &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ccc.cancel()
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("Cause(ccc) = %v, want %v", got, Canceled)
+	}
+
+	// Test that if we pass a custom context to WithCancelCause,
+	// and then cancel that child context with a cause,
+	// that the cause of the child canceled context is correct
+	// but that the parent custom context is not canceled.
+	ccc = &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ctx, causeFunc := WithCancelCause(ccc)
+	cause := errors.New("TestCustomContextCause")
+	causeFunc(cause)
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != cause {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, cause)
+	}
+	if got := ccc.Err(); got != nil {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, nil)
+	}
+	if got := Cause(ccc); got != nil {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, nil)
+	}
+
+	// Test that if we now cancel the parent custom context,
+	// the cause of the child canceled context is still correct,
+	// and the parent custom context is canceled without a cause.
+	ccc.cancel()
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != cause {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, cause)
+	}
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, Canceled)
+	}
+
+	// Test that if we associate a custom context with a child,
+	// then canceling the custom context cancels the child.
+	ccc = &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ctx, cancelFunc := WithCancel(ccc)
+	ccc.setCancelChild(cancelFunc)
+	ccc.cancel()
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, Canceled)
+	}
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, Canceled)
+	}
+}
+
+func TestAfterFuncCalledAfterCancel(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+		t.Fatalf("AfterFunc called before context is done")
+	case <-time.After(shortDuration):
+	}
+	cancel()
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+	if stop() {
+		t.Fatalf("stop() = true, want false")
+	}
+}
+
+func TestAfterFuncCalledAfterTimeout(t *testing.T) {
+	ctx, cancel := WithTimeout(Background(), shortDuration)
+	defer cancel()
+	donec := make(chan struct{})
+	AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+}
+
+func TestAfterFuncCalledImmediately(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	donec := make(chan struct{})
+	AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called for already-canceled context")
+	}
+}
+
+func TestAfterFuncNotCalledAfterStop(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		close(donec)
+	})
+	if !stop() {
+		t.Fatalf("stop() = false, want true")
+	}
+	cancel()
+	select {
+	case <-donec:
+		t.Fatalf("AfterFunc called for already-canceled context")
+	case <-time.After(shortDuration):
+	}
+	if stop() {
+		t.Fatalf("stop() = true, want false")
+	}
+}
+
+// This test verifies that cancelling a context does not block waiting for AfterFuncs to finish.
+func TestAfterFuncCalledAsynchronously(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		// The channel send blocks until donec is read from.
+		donec <- struct{}{}
+	})
+	defer stop()
+	cancel()
+	// After cancel returns, read from donec and unblock the AfterFunc.
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+}