https://blog.golang.org/context

Go的併發模式：Context （上下文）

在Go的伺服器中，每個的進來的請求都是由獨立的goroutine來處理的。處理請求的goroutine中通常會啟動額外的goroutines去訪問資料庫和RPC服務。處同一個請求的這群goroutines通常需要訪問特定的請求值，例如：終端使用者的身份識別，授權令牌和請求的截止日期。當一個請求被取消或超時時，這個請求所有相關的goroutines都應該快速退出，以便系統回收相關資源。

谷歌開發了一個context的包，使得在一個處理請求中的所有goroutine中跨API邊界傳遞請求範圍內的值，取消訊號，截止時間很方便（makes it easy to pass request-scoped values, cancelation signals, and deadlines across API boundaries to all the goroutines involved in handling a request.）。

Context

// 一個Context包含一個 Deadline, cancelation signal, 和 request-scoped values
// across API boundaries. 它的方法是可供多個goroutines安全地同時使用
type Context interface {
    // Done返回一個channel，當Context取消時或超時時，它會被closed。
    Done() <-chan struct{}
​
    // Err indicates why this context was canceled, after the Done channel
    // is closed.
    Err() error
​
    // Deadline returns the time when this Context will be canceled, if any.
    Deadline() (deadline time.Time, ok bool)
​
    // Value returns the value associated with key or nil if none.
    Value(key interface{}) interface{}
}

Done方法返回一個channel，它作為Context中執行的相關函式的一個取消訊號，當channel關閉時，相關函式需要放棄它們的工作並返回。Err方法返回一個錯誤，代表Context為什麼取消。這時有篇文章介紹Done channel的更多細節。

Context沒有一個Cancel的方法，與Done channel是隻接收的原因是一樣的：接收取消訊號的函式通常不是傳送訊號函式。特別是，當父操作為子操作啟動goroutines時，這些子操作應該不能取消父操作。相反，WithCancel函式為取消一個新的Context值提供了一個途徑。

Context是多goroutine安全的。程式碼中可以將一個Context傳遞給多個goroutine，和取消Context以通知goroutine。

派生Derived contexts

context包提供了從現有context值中派生新的Context值的功能。它們形成一顆樹。當一個Context取消時，所有從它派生的Contexts都會取消。

// Background 返回一個空的(empty) Context. 它永遠不會被取消，沒有deadline,沒有values。
// Background is typically used in main, init, and tests,
// and as the top-level Context for incoming requests.
func Background() Context

Backgroud是任何的Context樹的根，永遠不會被取消。

WithCancel 和WithTimeout返回的派生的context可以比父context早點取消。當請求處理返回時，與這個請求相關的那個Context通常會被取消。WithCancel對於取消冗餘請求也很有用。WithTimeout對於設定對後端伺服器的請求的deadline很有用：

// WithCancel 返回父Context的一個拷貝，當父Context的Done被 關閉或cancel被呼叫時，子Context的Done也會盡可能快地被關閉。
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)
​
// A CancelFunc cancels a Context.
type CancelFunc func()
​
// WithTimeout returns a copy of parent whose Done channel is closed as soon as
// parent.Done is closed, cancel is called, or timeout elapses. The new
// Context's Deadline is the sooner of now+timeout and the parent's deadline, if
// any. If the timer is still running, the cancel function releases its
// resources.
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)
​
​
// WithValue returns a copy of parent whose Value method returns val for key.
func WithValue(parent Context, key interface{}, val interface{}) Context

例子：Google網頁搜尋

我們這個例子中是一個http服務端，處理 /search?q=golang&timeout=1s 這樣的URLs，通過轉發查詢關鍵詞"golang"給 Google Web Search API ，然後渲染結果。超時引數告訴伺服器在該持續時間過去之後取消請求。

程式碼跨三個包（The code is split across three packages:）

• server 提供main函式和/search的處理函式（handler）

• userip 提供從請求中提取使用者IP地址並將其與上下文關聯的功能。

• google 提供Search函式，將查詢發給Google。

Server程式

package main
​
import (
    "context"
    "html/template"
    "log"
    "net/http"
    "time"
​
    "golang.org/x/blog/content/context/google"
    "golang.org/x/blog/content/context/userip"
)
​
func main() {
    http.HandleFunc("/search", handleSearch)
    log.Fatal(http.ListenAndServe(":8080", nil))
}
​
// handleSearch handles URLs like /search?q=golang&timeout=1s by forwarding the
// query to google.Search. If the query param includes timeout, the search is
// canceled after that duration elapses.
func handleSearch(w http.ResponseWriter, req *http.Request) {
    // ctx is the Context for this handler. Calling cancel closes the
    // ctx.Done channel, which is the cancellation signal for requests
    // started by this handler.
    var (
        ctx    context.Context
        cancel context.CancelFunc
    )
    timeout, err := time.ParseDuration(req.FormValue("timeout"))
    if err == nil {
        // The request has a timeout, so create a context that is
        // canceled automatically when the timeout expires.
        ctx, cancel = context.WithTimeout(context.Background(), timeout)
    } else {
        ctx, cancel = context.WithCancel(context.Background())
    }
    defer cancel() // Cancel ctx as soon as handleSearch returns.
​
    // Check the search query.
    query := req.FormValue("q")
    if query == "" {
        http.Error(w, "no query", http.StatusBadRequest)
        return
    }
​
    // Store the user IP in ctx for use by code in other packages.
    userIP, err := userip.FromRequest(req)
    if err != nil {
        http.Error(w, err.Error(), http.StatusBadRequest)
        return
    }
    ctx = userip.NewContext(ctx, userIP)
​
    // Run the Google search and print the results.
    start := time.Now()
    results, err := google.Search(ctx, query)
    elapsed := time.Since(start)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    if err := resultsTemplate.Execute(w, struct {
        Results          google.Results
        Timeout, Elapsed time.Duration
    }{
        Results: results,
        Timeout: timeout,
        Elapsed: elapsed,
    }); err != nil {
        log.Print(err)
        return
    }
}
​
var resultsTemplate = template.Must(template.New("results").Parse(`
<html>
<head/>
<body>
  <ol>
  {{range .Results}}
    <li>{{.Title}} - <a href="{{.URL}}">{{.URL}}</a></li>
  {{end}}
  </ol>
  <p>{{len .Results}} results in {{.Elapsed}}; timeout {{.Timeout}}</p>
</body>
</html>
`))

userip包

package userip
​
import (
    "context"
    "fmt"
    "net"
    "net/http"
)
​
// FromRequest 從req中提取使用者IP地址, 如果有的話
func FromRequest(req *http.Request) (net.IP, error) {
    ip, _, err := net.SplitHostPort(req.RemoteAddr)
    if err != nil {
        return nil, fmt.Errorf("userip: %q is not IP:port", req.RemoteAddr)
    }
​
    userIP := net.ParseIP(ip)
    if userIP == nil {
        return nil, fmt.Errorf("userip: %q is not IP:port", req.RemoteAddr)
    }
    return userIP, nil
}
​
// key的型別不匯出，通過這樣防止其它包中使用同樣的key而引起衝突。got it
type key int
​
// userIPkey 是使用者IP地址的context key, 它的零值是任意的。
// 如果當前包內定義其它context keys, 它們會有不同的整形值。
const userIPKey key = 0
​
// NewContext returns a new Context carrying userIP.
func NewContext(ctx context.Context, userIP net.IP) context.Context {
    return context.WithValue(ctx, userIPKey, userIP)
}
​
// FromContext extracts the user IP address from ctx, if present.
func FromContext(ctx context.Context) (net.IP, bool) {
    // ctx.Value returns nil if ctx has no value for the key;
    // the net.IP type assertion returns ok=false for nil.
    userIP, ok := ctx.Value(userIPKey).(net.IP)
    return userIP, ok
}

google包

package google
​
import (
    "context"
    "encoding/json"
    "net/http"
​
    "golang.org/x/blog/content/context/userip"
)
​
// Results is an ordered list of search results.
type Results []Result
​
// A Result contains the title and URL of a search result.
type Result struct {
    Title, URL string
}
​
// Search sends query to Google search and returns the results.
func Search(ctx context.Context, query string) (Results, error) {
    // Prepare the Google Search API request.
    req, err := http.NewRequest("GET", "https://ajax.googleapis.com/ajax/services/search/web?v=1.0", nil)
    if err != nil {
        return nil, err
    }
    q := req.URL.Query()
    q.Set("q", query)
​
    // If ctx is carrying the user IP address, forward it to the server.
    // Google APIs use the user IP to distinguish server-initiated requests
    // from end-user requests.
    if userIP, ok := userip.FromContext(ctx); ok {
        q.Set("userip", userIP.String())
    }
    req.URL.RawQuery = q.Encode()
​
    // Issue the HTTP request and handle the response. The httpDo function
    // cancels the request if ctx.Done is closed.
    var results Results
    err = httpDo(ctx, req, func(resp *http.Response, err error) error {
        if err != nil {
            return err
        }
        defer resp.Body.Close()
​
        // Parse the JSON search result.
        // https://developers.google.com/web-search/docs/#fonje
        var data struct {
            ResponseData struct {
                Results []struct {
                    TitleNoFormatting string
                    URL               string
                }
            }
        }
        if err := json.NewDecoder(resp.Body).Decode(&data); err != nil {
            return err
        }
        for _, res := range data.ResponseData.Results {
            results = append(results, Result{Title: res.TitleNoFormatting, URL: res.URL})
        }
        return nil
    })
    // httpDo waits for the closure we provided to return, so it's safe to
    // read results here.
    return results, err
}
​
// httpDo issues the HTTP request and calls f with the response. If ctx.Done is
// closed while the request or f is running, httpDo cancels the request, waits
// for f to exit, and returns ctx.Err. Otherwise, httpDo returns f's error.
func httpDo(ctx context.Context, req *http.Request, f func(*http.Response, error) error) error {
    // Run the HTTP request in a goroutine and pass the response to f.
    c := make(chan error, 1)
    req = req.WithContext(ctx)
    go func() { c <- f(http.DefaultClient.Do(req)) }()
    select {
    case <-ctx.Done():
        <-c // Wait for f to return.
        return ctx.Err()
    case err := <-c:
        return err
    }
}

總結

在Google，我們要求Go程式設計師，在傳入和傳出請求之間的呼叫路徑上，將Context作為每個函式中的第一個引數傳遞。這使許多不同團隊開發的Go程式碼可以很好地進行互操作。它提供對超時和取消的簡單控制，並確保諸如安全性憑證之類的關鍵值正確地傳遞Go程式。