Overview

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:

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 files

Variables

  1. var Canceled = errors.("context canceled")

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

  1. var DeadlineExceeded error = deadlineExceededError{}

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

  1. type CancelFunc func()

type 

  1. type Context interface {
  2.     // Deadline returns the time when work done on behalf of this context
  3.     // should be canceled. Deadline returns ok==false when no deadline is
  4.     // set. Successive calls to Deadline return the same results.
  5.     Deadline() (deadline .Time, ok )
  6.  
  7.     // Done returns a channel that's closed when work done on behalf of this
  8.     // context should be canceled. Done may return nil if this context can
  9.     // never be canceled. Successive calls to Done return the same value.
  10.     //
  11.     // WithCancel arranges for Done to be closed when cancel is called;
  12.     // WithDeadline arranges for Done to be closed when the deadline
  13.     // expires; WithTimeout arranges for Done to be closed when the timeout
  14.     // elapses.
  15.     //
  16.     // Done is provided for use in select statements:
  17.     //
  18.     //  // Stream generates values with DoSomething and sends them to out
  19.     //  // until DoSomething returns an error or ctx.Done is closed.
  20.     //  func Stream(ctx context.Context, out chan<- Value) error {
  21.     //      for {
  22.     //          v, err := DoSomething(ctx)
  23.     //          if err != nil {
  24.     //              return err
  25.     //          }
  26.     //          select {
  27.     //          case <-ctx.Done():
  28.     //              return ctx.Err()
  29.     //          case out <- v:
  30.     //          }
  31.     //      }
  32.     //  }
  33.     //
  34.     // See https://blog.golang.org/pipelines for more examples of how to use
  35.     // a Done channel for cancelation.
  36.     Done() <-chan struct{}
  37.     // If Done is not yet closed, Err returns nil.
  38.     // If Done is closed, Err returns a non-nil error explaining why:
  39.     // Canceled if the context was canceled
  40.     // or DeadlineExceeded if the context's deadline passed.
  41.     // After Err returns a non-nil error, successive calls to Err return the same error.
  42.     Err() error
  43.  
  44.     // Value returns the value associated with this context for key, or nil
  45.     // if no value is associated with key. Successive calls to Value with
  46.     // the same key returns the same result.
  47.     //
  48.     // Use context values only for request-scoped data that transits
  49.     // processes and API boundaries, not for passing optional parameters to
  50.     // functions.
  51.     //
  52.     // A key identifies a specific value in a Context. Functions that wish
  53.     // to store values in Context typically allocate a key in a global
  54.     // variable then use that key as the argument to context.WithValue and
  55.     // Context.Value. A key can be any type that supports equality;
  56.     // packages should define keys as an unexported type to avoid
  57.     // collisions.
  58.     //
  59.     // Packages that define a Context key should provide type-safe accessors
  60.     // for the values stored using that key:
  61.     //
  62.     //     // Package user defines a User type that's stored in Contexts.
  63.     //     package user
  64.     //
  65.     //     import "context"
  66.     //
  67.     //     // User is the type of value stored in the Contexts.
  68.     //     type User struct {...}
  69.     //
  70.     //     // key is an unexported type for keys defined in this package.
  71.     //     // This prevents collisions with keys defined in other packages.
  72.     //     type key int
  73.     //
  74.     //     // userKey is the key for user.User values in Contexts. It is
  75.     //     // unexported; clients use user.NewContext and user.FromContext
  76.     //     // instead of using this key directly.
  77.     //     var userKey key
  78.     //
  79.     //     // NewContext returns a new Context that carries value u.
  80.     //     func NewContext(ctx context.Context, u *User) context.Context {
  81.     //         return context.WithValue(ctx, userKey, u)
  82.     //     }
  83.     //
  84.     //     // FromContext returns the User value stored in ctx, if any.
  85.     //     func FromContext(ctx context.Context) (*User, bool) {
  86.     //         u, ok := ctx.Value(userKey).(*User)
  87.     //         return u, ok
  88.     //     }
  89.     Value(key interface{}) interface{}
  90. }

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

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

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 

  1. 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.

  1. func WithCancel(parent Context) (ctx , 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:

  1. // sends them to the returned channel.
  2. // The callers of gen need to cancel the context once
  3. // they are done consuming generated integers not to leak
  4. // the internal goroutine started by gen.
  5. gen := func(ctx context.Context) <-chan int {
  6.     dst := make(chan int)
  7.     n := 1
  8.     go func() {
  9.         for {
  10.             select {
  11.             case <-ctx.Done():
  12.                 return // returning not to leak the goroutine
  13.             case dst <- n:
  14.                 n++
  15.             }
  16.         }
  18.     return dst
  19. }
  21. ctx, cancel := context.WithCancel(context.Background())
  22. defer cancel() // cancel when we are finished consuming integers
  24. for n := range gen(ctx) {
  25.     fmt.Println(n)
  26.     if n == 5 {
  27.         break
  28.     }
  29. }
  30. // Output:
  31. // 1
  32. // 2
  33. // 3
  34. // 4

func WithDeadline

  1. func WithDeadline(parent Context, d .Time) (, 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:

  1. func WithTimeout(parent Context, timeout .Duration) (, 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:

  1. func slowOperationWithTimeout(ctx context.Context) (Result, error) {
  2.     ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
  3.     defer cancel()  // releases resources if slowOperation completes before timeout elapses
  4.     return slowOperation(ctx)
  5. }


Example:

  1. // Pass a context with a timeout to tell a blocking function that it
  2. // should abandon its work after the timeout elapses.
  3. ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
  4. defer cancel()
  6. select {
  7. case <-time.After(1 * time.Second):
  8.     fmt.Println("overslept")
  9. case <-ctx.Done():
  10.     fmt.Println(ctx.Err()) // prints "context deadline exceeded"
  11. }
  14. // 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: