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// 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
// (canceled 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 reflectlite.TypeOf(v).String()
}
func (c *valueCtx) String() string {
return contextName(c.Context) + ".WithValue(" +
stringify(c.key) + ", " +
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)
}
}
}