448 lines
14 KiB
Go
448 lines
14 KiB
Go
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// Copyright 2014 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package context defines the Context type, which carries deadlines,
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// cancelation signals, and other request-scoped values across API boundaries
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// and between processes.
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//
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// Incoming requests to a server should create a Context, and outgoing calls to
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// servers should accept a Context. The chain of function calls between must
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// propagate the Context, optionally replacing it with a modified copy created
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// using WithDeadline, WithTimeout, WithCancel, or WithValue.
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//
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// Programs that use Contexts should follow these rules to keep interfaces
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// consistent across packages and enable static analysis tools to check context
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// propagation:
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//
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// Do not store Contexts inside a struct type; instead, pass a Context
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// explicitly to each function that needs it. The Context should be the first
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// parameter, typically named ctx:
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//
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// func DoSomething(ctx context.Context, arg Arg) error {
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// // ... use ctx ...
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// }
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//
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// Do not pass a nil Context, even if a function permits it. Pass context.TODO
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// if you are unsure about which Context to use.
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//
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// Use context Values only for request-scoped data that transits processes and
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// APIs, not for passing optional parameters to functions.
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//
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// The same Context may be passed to functions running in different goroutines;
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// Contexts are safe for simultaneous use by multiple goroutines.
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//
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// See http://blog.golang.org/context for example code for a server that uses
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// Contexts.
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package context
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import (
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"errors"
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"fmt"
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"sync"
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"time"
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)
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// A Context carries a deadline, a cancelation signal, and other values across
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// API boundaries.
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//
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// Context's methods may be called by multiple goroutines simultaneously.
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type Context interface {
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// Deadline returns the time when work done on behalf of this context
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// should be canceled. Deadline returns ok==false when no deadline is
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// set. Successive calls to Deadline return the same results.
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Deadline() (deadline time.Time, ok bool)
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// Done returns a channel that's closed when work done on behalf of this
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// context should be canceled. Done may return nil if this context can
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// never be canceled. Successive calls to Done return the same value.
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//
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// WithCancel arranges for Done to be closed when cancel is called;
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// WithDeadline arranges for Done to be closed when the deadline
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// expires; WithTimeout arranges for Done to be closed when the timeout
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// elapses.
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//
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// Done is provided for use in select statements:
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//
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// // Stream generates values with DoSomething and sends them to out
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// // until DoSomething returns an error or ctx.Done is closed.
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// func Stream(ctx context.Context, out <-chan Value) error {
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// for {
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// v, err := DoSomething(ctx)
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// if err != nil {
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// return err
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// }
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// select {
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// case <-ctx.Done():
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// return ctx.Err()
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// case out <- v:
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// }
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// }
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// }
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//
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// See http://blog.golang.org/pipelines for more examples of how to use
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// a Done channel for cancelation.
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Done() <-chan struct{}
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// Err returns a non-nil error value after Done is closed. Err returns
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// Canceled if the context was canceled or DeadlineExceeded if the
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// context's deadline passed. No other values for Err are defined.
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// After Done is closed, successive calls to Err return the same value.
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Err() error
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// Value returns the value associated with this context for key, or nil
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// if no value is associated with key. Successive calls to Value with
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// the same key returns the same result.
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//
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// Use context values only for request-scoped data that transits
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// processes and API boundaries, not for passing optional parameters to
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// functions.
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//
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// A key identifies a specific value in a Context. Functions that wish
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// to store values in Context typically allocate a key in a global
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// variable then use that key as the argument to context.WithValue and
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// Context.Value. A key can be any type that supports equality;
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// packages should define keys as an unexported type to avoid
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// collisions.
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//
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// Packages that define a Context key should provide type-safe accessors
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// for the values stores using that key:
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//
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// // Package user defines a User type that's stored in Contexts.
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// package user
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//
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// import "golang.org/x/net/context"
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//
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// // User is the type of value stored in the Contexts.
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// type User struct {...}
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//
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// // key is an unexported type for keys defined in this package.
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// // This prevents collisions with keys defined in other packages.
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// type key int
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//
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// // userKey is the key for user.User values in Contexts. It is
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// // unexported; clients use user.NewContext and user.FromContext
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// // instead of using this key directly.
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// var userKey key = 0
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//
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// // NewContext returns a new Context that carries value u.
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// func NewContext(ctx context.Context, u *User) context.Context {
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// return context.WithValue(ctx, userKey, u)
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// }
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//
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// // FromContext returns the User value stored in ctx, if any.
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// func FromContext(ctx context.Context) (*User, bool) {
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// u, ok := ctx.Value(userKey).(*User)
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// return u, ok
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// }
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Value(key interface{}) interface{}
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}
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// Canceled is the error returned by Context.Err when the context is canceled.
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var Canceled = errors.New("context canceled")
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// DeadlineExceeded is the error returned by Context.Err when the context's
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// deadline passes.
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var DeadlineExceeded = errors.New("context deadline exceeded")
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// An emptyCtx is never canceled, has no values, and has no deadline. It is not
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// struct{}, since vars of this type must have distinct addresses.
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type emptyCtx int
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func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
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return
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}
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func (*emptyCtx) Done() <-chan struct{} {
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return nil
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}
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func (*emptyCtx) Err() error {
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return nil
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}
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func (*emptyCtx) Value(key interface{}) interface{} {
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return nil
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}
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func (e *emptyCtx) String() string {
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switch e {
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case background:
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return "context.Background"
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case todo:
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return "context.TODO"
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}
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return "unknown empty Context"
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}
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var (
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background = new(emptyCtx)
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todo = new(emptyCtx)
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)
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// Background returns a non-nil, empty Context. It is never canceled, has no
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// values, and has no deadline. It is typically used by the main function,
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// initialization, and tests, and as the top-level Context for incoming
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// requests.
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func Background() Context {
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return background
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}
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// TODO returns a non-nil, empty Context. Code should use context.TODO when
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// it's unclear which Context to use or it is not yet available (because the
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// surrounding function has not yet been extended to accept a Context
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// parameter). TODO is recognized by static analysis tools that determine
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// whether Contexts are propagated correctly in a program.
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func TODO() Context {
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return todo
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}
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// A CancelFunc tells an operation to abandon its work.
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// A CancelFunc does not wait for the work to stop.
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// After the first call, subsequent calls to a CancelFunc do nothing.
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type CancelFunc func()
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// WithCancel returns a copy of parent with a new Done channel. The returned
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// context's Done channel is closed when the returned cancel function is called
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// or when the parent context's Done channel is closed, whichever happens first.
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//
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// Canceling this context releases resources associated with it, so code should
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// call cancel as soon as the operations running in this Context complete.
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func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
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c := newCancelCtx(parent)
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propagateCancel(parent, &c)
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return &c, func() { c.cancel(true, Canceled) }
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}
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// newCancelCtx returns an initialized cancelCtx.
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func newCancelCtx(parent Context) cancelCtx {
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return cancelCtx{
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Context: parent,
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done: make(chan struct{}),
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}
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}
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// propagateCancel arranges for child to be canceled when parent is.
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func propagateCancel(parent Context, child canceler) {
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if parent.Done() == nil {
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return // parent is never canceled
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}
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if p, ok := parentCancelCtx(parent); ok {
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p.mu.Lock()
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if p.err != nil {
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// parent has already been canceled
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child.cancel(false, p.err)
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} else {
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if p.children == nil {
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p.children = make(map[canceler]bool)
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}
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p.children[child] = true
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}
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p.mu.Unlock()
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} else {
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go func() {
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select {
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case <-parent.Done():
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child.cancel(false, parent.Err())
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case <-child.Done():
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}
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}()
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}
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}
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// parentCancelCtx follows a chain of parent references until it finds a
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// *cancelCtx. This function understands how each of the concrete types in this
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// package represents its parent.
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func parentCancelCtx(parent Context) (*cancelCtx, bool) {
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for {
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switch c := parent.(type) {
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case *cancelCtx:
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return c, true
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case *timerCtx:
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return &c.cancelCtx, true
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case *valueCtx:
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parent = c.Context
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default:
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return nil, false
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}
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}
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}
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// removeChild removes a context from its parent.
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func removeChild(parent Context, child canceler) {
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p, ok := parentCancelCtx(parent)
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if !ok {
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return
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}
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p.mu.Lock()
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if p.children != nil {
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delete(p.children, child)
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}
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p.mu.Unlock()
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}
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// A canceler is a context type that can be canceled directly. The
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// implementations are *cancelCtx and *timerCtx.
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type canceler interface {
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cancel(removeFromParent bool, err error)
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Done() <-chan struct{}
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}
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// A cancelCtx can be canceled. When canceled, it also cancels any children
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// that implement canceler.
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type cancelCtx struct {
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Context
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done chan struct{} // closed by the first cancel call.
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mu sync.Mutex
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children map[canceler]bool // set to nil by the first cancel call
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err error // set to non-nil by the first cancel call
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}
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func (c *cancelCtx) Done() <-chan struct{} {
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return c.done
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}
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func (c *cancelCtx) Err() error {
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c.mu.Lock()
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defer c.mu.Unlock()
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return c.err
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}
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func (c *cancelCtx) String() string {
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return fmt.Sprintf("%v.WithCancel", c.Context)
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}
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// cancel closes c.done, cancels each of c's children, and, if
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// removeFromParent is true, removes c from its parent's children.
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func (c *cancelCtx) cancel(removeFromParent bool, err error) {
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if err == nil {
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panic("context: internal error: missing cancel error")
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}
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c.mu.Lock()
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if c.err != nil {
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c.mu.Unlock()
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return // already canceled
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}
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c.err = err
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close(c.done)
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for child := range c.children {
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// NOTE: acquiring the child's lock while holding parent's lock.
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child.cancel(false, err)
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}
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c.children = nil
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c.mu.Unlock()
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if removeFromParent {
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removeChild(c.Context, c)
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}
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}
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// WithDeadline returns a copy of the parent context with the deadline adjusted
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// to be no later than d. If the parent's deadline is already earlier than d,
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// WithDeadline(parent, d) is semantically equivalent to parent. The returned
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// context's Done channel is closed when the deadline expires, when the returned
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// cancel function is called, or when the parent context's Done channel is
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// closed, whichever happens first.
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//
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// Canceling this context releases resources associated with it, so code should
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// call cancel as soon as the operations running in this Context complete.
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func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
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if cur, ok := parent.Deadline(); ok && cur.Before(deadline) {
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// The current deadline is already sooner than the new one.
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return WithCancel(parent)
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}
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c := &timerCtx{
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cancelCtx: newCancelCtx(parent),
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deadline: deadline,
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}
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propagateCancel(parent, c)
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d := deadline.Sub(time.Now())
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if d <= 0 {
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c.cancel(true, DeadlineExceeded) // deadline has already passed
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return c, func() { c.cancel(true, Canceled) }
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}
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c.mu.Lock()
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defer c.mu.Unlock()
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if c.err == nil {
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c.timer = time.AfterFunc(d, func() {
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c.cancel(true, DeadlineExceeded)
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})
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}
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return c, func() { c.cancel(true, Canceled) }
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}
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// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
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// implement Done and Err. It implements cancel by stopping its timer then
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// delegating to cancelCtx.cancel.
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type timerCtx struct {
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cancelCtx
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timer *time.Timer // Under cancelCtx.mu.
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deadline time.Time
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}
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|
|
|
||
|
|
func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
|
||
|
|
return c.deadline, true
|
||
|
|
}
|
||
|
|
|
||
|
|
func (c *timerCtx) String() string {
|
||
|
|
return fmt.Sprintf("%v.WithDeadline(%s [%s])", c.cancelCtx.Context, c.deadline, c.deadline.Sub(time.Now()))
|
||
|
|
}
|
||
|
|
|
||
|
|
func (c *timerCtx) cancel(removeFromParent bool, err error) {
|
||
|
|
c.cancelCtx.cancel(false, err)
|
||
|
|
if removeFromParent {
|
||
|
|
// Remove this timerCtx from its parent cancelCtx's children.
|
||
|
|
removeChild(c.cancelCtx.Context, c)
|
||
|
|
}
|
||
|
|
c.mu.Lock()
|
||
|
|
if c.timer != nil {
|
||
|
|
c.timer.Stop()
|
||
|
|
c.timer = nil
|
||
|
|
}
|
||
|
|
c.mu.Unlock()
|
||
|
|
}
|
||
|
|
|
||
|
|
// 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)
|
||
|
|
// }
|
||
|
|
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
|
||
|
|
return WithDeadline(parent, time.Now().Add(timeout))
|
||
|
|
}
|
||
|
|
|
||
|
|
// 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.
|
||
|
|
func WithValue(parent Context, key interface{}, val interface{}) Context {
|
||
|
|
return &valueCtx{parent, key, val}
|
||
|
|
}
|
||
|
|
|
||
|
|
// A valueCtx carries a key-value pair. It implements Value for that key and
|
||
|
|
// delegates all other calls to the embedded Context.
|
||
|
|
type valueCtx struct {
|
||
|
|
Context
|
||
|
|
key, val interface{}
|
||
|
|
}
|
||
|
|
|
||
|
|
func (c *valueCtx) String() string {
|
||
|
|
return fmt.Sprintf("%v.WithValue(%#v, %#v)", c.Context, c.key, c.val)
|
||
|
|
}
|
||
|
|
|
||
|
|
func (c *valueCtx) Value(key interface{}) interface{} {
|
||
|
|
if c.key == key {
|
||
|
|
return c.val
|
||
|
|
}
|
||
|
|
return c.Context.Value(key)
|
||
|
|
}
|