聊聊Golang自带的HttpClient超时机制

// 来自 src/context/example_test.go
func ExampleWithValue() {
	type favContextKey string

	f := func(ctx context.Context, k favContextKey) {
		if v := ctx.Value(k); v != nil {
			fmt.Println("found value:", v)
			return
		}
		fmt.Println("key not found:", k)
	}

	k := favContextKey("language")
	ctx := context.WithValue(context.Background(), k, "Go")

	f(ctx, k)
	f(ctx, favContextKey("color"))

	// Output:
	// found value: Go
	// key not found: color
}

// 来自 src/context/example_test.go
gen := func(ctx context.Context) <-chan int {
		dst := make(chan int)
		n := 1
		go func() {
			for {
				select {
				case <-ctx.Done():
					return // returning not to leak the goroutine
				case dst <- n:
					n++
				}
			}
		}()
		return dst
	}

// 来自 src/context/example_test.go
ctx, cancel := context.WithCancel(context.Background())
defer cancel() // cancel when we are finished consuming integers

for n := range gen(ctx) {
	fmt.Println(n)
	if n == 5 {
		break
	}
}
// Output:
// 1
// 2
// 3
// 4
// 5

// 来自 src/context/example_test.go
func ExampleWithTimeout() {
	// Pass a context with a timeout to tell a blocking function that it
	// should abandon its work after the timeout elapses.
	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
	defer cancel()

	select {
	case <-time.After(1 * time.Second):
		fmt.Println("overslept")
	case <-ctx.Done():
		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
	}

	// Output:
	// context deadline exceeded
}

	client := http.Client{
		Timeout: 10 * time.Second,
	}
	
	// 来自 src/net/http/client.go
	type Client struct {
	// ... 省略其他字段
	// Timeout specifies a time limit for requests made by this
	// Client. The timeout includes connection time, any
	// redirects, and reading the response body. The timer remains
	// running after Get, Head, Post, or Do return and will
	// interrupt reading of the Response.Body.
	//
	// A Timeout of zero means no timeout.
	//
	// The Client cancels requests to the underlying Transport
	// as if the Request's Context ended.
	//
	// For compatibility, the Client will also use the deprecated
	// CancelRequest method on Transport if found. New
	// RoundTripper implementations should use the Request's Context
	// for cancellation instead of implementing CancelRequest.
	Timeout time.Duration
}

	client := http.Client{
		Timeout: 10 * time.Minute,
	}
	resp, err := client.Get("http://127.0.0.1:81/hello")

// 来自 src/net/http/client.go
deadline = c.deadline()

// 来自 src/net/http/client.go
stopTimer, didTimeout := setRequestCancel(req, rt, deadline)

// 来自 src/net/http/client.go
var cancelCtx func()
if oldCtx := req.Context(); timeBeforeContextDeadline(deadline, oldCtx) {
	req.ctx, cancelCtx = context.WithDeadline(oldCtx, deadline)
}

// 来自 src/net/http/client.go
timer := time.NewTimer(time.Until(deadline))
var timedOut atomicBool

go func() {
	select {
	case <-initialReqCancel:
		doCancel()
		timer.Stop()
	case <-timer.C:
		timedOut.setTrue()
		doCancel()
	case <-stopTimerCh:
		timer.Stop()
	}
}()

// 位于 src/net/http/transport.go
// CancelRequest cancels an in-flight request by closing its connection.
// CancelRequest should only be called after RoundTrip has returned.
func (t *Transport) CancelRequest(req *Request) {
	t.cancelRequest(cancelKey{req}, errRequestCanceled)
}

// 位于 src/net/http/transport.go
for {
	select {
	case <-ctx.Done():
		req.closeBody()
		return nil, ctx.Err()
	default:
	}

	// ...
	pconn, err := t.getConn(treq, cm)
	// ...
}

// 位于 src/net/http/transport.go
// Queue for idle connection.
if delivered := t.queueForIdleConn(w); delivered {
	// ...
	return pc, nil
}

// 位于 src/net/http/transport.go
// Queue for permission to dial.
t.queueForDial(w)

// 位于 src/net/http/transport.go
// Wait for completion or cancellation.
select {
case <-w.ready:
	// ...
	return w.pc, w.err
case <-req.Cancel:
	return nil, errRequestCanceledConn
case <-req.Context().Done():
	return nil, req.Context().Err()
case err := <-cancelc:
	if err == errRequestCanceled {
		err = errRequestCanceledConn
	}
	return nil, err
}

// 位于 src/net/http/transport.go
go pconn.readLoop()
go pconn.writeLoop()

// 位于 src/net/http/transport.go
func (pc *persistConn) writeLoop() {
	defer close(pc.writeLoopDone)
	for {
		select {
		case wr := <-pc.writech:
			startBytesWritten := pc.nwrite
			err := wr.req.Request.write(pc.bw, pc.isProxy, wr.req.extra, pc.waitForContinue(wr.continueCh))
			// ... 
			if err != nil {
				pc.close(err)
				return
			}
		case <-pc.closech:
			return
		}
	}
}