Documentation
¶
Overview ¶
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.
Index ¶
- Variables
- func Cause(c Context) error
- func WithCancel(parent Context) (ctx Context, cancel CancelFunc)
- func WithCancelCause(parent Context) (ctx Context, cancel CancelCauseFunc)
- func WithDeadline(parent Context, d time.Time) (Context, CancelFunc)
- func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)
- type CancelCauseFunc
- type CancelFunc
- type Context
Examples ¶
Constants ¶
This section is empty.
Variables ¶
var Canceled = errors.New("context canceled")
Canceled is the error returned by Context.Err when the context is canceled.
var DeadlineExceeded error = deadlineExceededError{}
DeadlineExceeded is the error returned by Context.Err when the context's deadline passes.
Functions ¶
func Cause ¶ added in go1.20
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 ¶
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 ¶
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.
package main
import (
"context"
"fmt"
)
func main() {
// 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
func WithCancelCause ¶ added in go1.20
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 ¶
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's 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 ¶
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.
package main
import (
"context"
"fmt"
"time"
)
const shortDuration = 1 * time.Millisecond
func main() {
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 <-time.After(1 * time.Second):
fmt.Println("overslept")
case <-ctx.Done():
fmt.Println(ctx.Err())
}
}
Output: context deadline exceeded