# package context

`import "context"`

Package context defines the Context type, which carries deadlines, cancelation 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.

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.

context.go

## Variables

Canceled is the error returned by Context.Err when the context is canceled.

DeadlineExceeded is the error returned by Context.Err when the context's deadline passes.

## type CancelFunc

A CancelFunc tells an operation to abandon its work. A CancelFunc does not wait for the work to stop. After the first call, subsequent calls to a CancelFunc do nothing.

## type Context

A Context carries a deadline, a cancelation signal, and other values across API boundaries.

Context's methods may be called by multiple goroutines simultaneously.

### func Background

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

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). TODO is recognized by static analysis tools that determine whether Contexts are propagated correctly in a program.

### func WithCancel

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.

Code:play

    // 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 WithDeadline

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 a arbitrary deadline to tell a blocking function that it should abandon its work as soon as it gets to it.

Code:play

    d := time.Now().Add(50 * time.Millisecond)
    ctx, cancel := context.WithDeadline(context.Background(), d)

    // Even though ctx will be expired, it is good practice to call its
    // cancelation 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

### func WithTimeout

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

This example passes a context with a timeout to tell a blocking function that it should abandon its work after the timeout elapses.

Code:play

    // Pass a context with a timeout to tell a blocking function that it
    // should abandon its work after the timeout elapses.
    ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
    defer cancel()

    select {
    case <-time.After(1 * time.Second):
        fmt.Println("overslept")
    case <-ctx.Done():
        fmt.Println(ctx.Err()) // prints "context deadline exceeded"
    }

Output:

    context deadline exceeded

### func WithValue

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

Code:

    type favContextKey string

    f := func(ctx context.Context, k favContextKey) {
        if v := ctx.Value(k); v != nil {
            fmt.Println("found value:", v)
            return
        }
        fmt.Println("key not found:", k)
    }

    k := favContextKey("language")
    ctx := context.WithValue(context.Background(), k, "Go")

    f(ctx, k)
    f(ctx, favContextKey("color"))

Output:

    found value: Go
    key not found: color