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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{}
func AfterFunc ¶ 1.21
func AfterFunc(ctx Context, f func()) (stop func() bool)
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.
▸ Example (Cond)
▸ Example (Connection)
▸ Example (Merge)
func Cause ¶ 1.20
func Cause(c Context) error
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 WithCancel ¶ 1.7
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)
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.
▸ Example
func WithCancelCause ¶ 1.20
func WithCancelCause(parent Context) (ctx Context, cancel CancelCauseFunc)
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 WithDeadline ¶ 1.7
func WithDeadline(parent Context, d time.Time) (Context, CancelFunc)
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.
▸ Example
func WithDeadlineCause ¶ 1.21
func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, CancelFunc)
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 WithTimeout ¶ 1.7
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)
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) }
▸ Example
func WithTimeoutCause ¶ 1.21
func WithTimeoutCause(parent Context, timeout time.Duration, cause error) (Context, CancelFunc)
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.
type CancelCauseFunc ¶ 1.20
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)
type CancelFunc ¶ 1.7
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()
type Context ¶ 1.7
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 }
func Background ¶ 1.7
func Background() Context
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 TODO ¶ 1.7
func TODO() Context
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 WithValue ¶ 1.7
func WithValue(parent Context, key, val any) Context
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.
▸ Example
func WithoutCancel ¶ 1.21
func WithoutCancel(parent Context) Context
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.