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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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+
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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 // import "golang.org/x/net/context"
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+func (*emptyCtx) Done() <-chan struct{} {
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+ return nil
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+}
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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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+
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+func (*emptyCtx) Value(key interface{}) interface{} {
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+ return nil
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+}
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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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+
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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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+
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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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+
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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's 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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+ done chan struct{} // closed by the first cancel call.
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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) }
|
|
|
|
+ }
|
|
|
|
+ c.mu.Lock()
|
|
|
|
+ defer c.mu.Unlock()
|
|
|
|
+ if c.err == nil {
|
|
|
|
+ c.timer = time.AfterFunc(d, func() {
|
|
|
|
+ c.cancel(true, DeadlineExceeded)
|
|
|
|
+ })
|
|
|
|
+ }
|
|
|
|
+ return c, func() { c.cancel(true, Canceled) }
|
|
|
|
+}
|
|
|
|
+
|
|
|
|
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
|
|
|
|
+// implement Done and Err. It implements cancel by stopping its timer then
|
|
|
|
+// delegating to cancelCtx.cancel.
|
|
|
|
+type timerCtx struct {
|
|
|
|
+ cancelCtx
|
|
|
|
+ timer *time.Timer // Under cancelCtx.mu.
|
|
|
|
+
|
|
|
|
+ deadline time.Time
|
|
|
|
+}
|
|
|
|
+
|
|
|
|
+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)
|
|
|
|
+}
|