Documentation
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Overview ¶
Package context defines the Context type, which carries deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes. As of Go 1.7 this package is available in the standard library under the name context, and migrating to it can be done automatically with go fix.
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.
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. This is discussed further in https://go.dev/blog/context-and-structs. 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://go.dev/blog/context for example code for a server that uses Contexts.
Index ¶
Constants ¶
This section is empty.
Variables ¶
var Canceled = context.Canceled
Canceled is the error returned by [Context.Err] when the context is canceled for some reason other than its deadline passing.
var DeadlineExceeded = context.DeadlineExceeded
DeadlineExceeded is the error returned by [Context.Err] when the context is canceled due to its deadline passing.
Functions ¶
func WithCancel ¶
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)
WithCancel returns a derived context that points to the parent context but has 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 WithDeadline ¶
func WithDeadline(parent Context, d time.Time) (Context, CancelFunc)
WithDeadline returns a derived context that points to the parent context but has 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 WithTimeout ¶
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)
}
Types ¶
type CancelFunc ¶
type CancelFunc = context.CancelFunc
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 Context ¶
A Context carries a deadline, a cancellation signal, and other values across API boundaries.
Context's methods may be called by multiple goroutines simultaneously.
func Background ¶
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 ¶
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 ¶
WithValue returns a derived context that points to the parent Context. In the derived context, 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.
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