Adding upstream version 1.22.1.upstream/1.22.1

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

1 files changed, 790 insertions, 0 deletions

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)
+		}
+	}
+}