前言 - context 源码
可以先了解官方 context.go 轮廓. 这里捎带保存一份当前 context 版本备份.
// Copyright 2014 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. // 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. // // 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. package context import ( "errors" "internal/reflectlite" "sync" "time" ) // A Context carries a deadline, a cancellation signal, and other values across // API boundaries. // // Context's methods may be called by multiple goroutines simultaneously. type Context interface { // Deadline returns the time when work done on behalf of this context // should be canceled. Deadline returns ok==false when no deadline is // set. Successive calls to Deadline return the same results. Deadline() (deadline time.Time, ok bool) // Done returns a channel that's closed when work done on behalf of this // context should be canceled. Done may return nil if this context can // never be canceled. Successive calls to Done return the same value. // // WithCancel arranges for Done to be closed when cancel is called; // WithDeadline arranges for Done to be closed when the deadline // expires; WithTimeout arranges for Done to be closed when the timeout // elapses. // // Done is provided for use in select statements: // // // Stream generates values with DoSomething and se