net/http: add Request.Context and Request.WithContext

Currently only used by the client. The server is not yet wired up.  A
TODO remains to document how it works server-side, once implemented.

Updates #14660

Change-Id: I27c2e74198872b2720995fa8271d91de200e23d5
Reviewed-on: https://go-review.googlesource.com/21496Reviewed-by: Andrew Gerrand <adg@golang.org>

src/go/build/deps_test.go

@@ -357,7 +357,8 @@ var pkgDeps = map[string][]string{
 	// HTTP, kingpin of dependencies.
 	"net/http": {
 		"L4", "NET", "OS",
-		"compress/gzip", "crypto/tls", "mime/multipart", "runtime/debug",
+		"context", "compress/gzip", "crypto/tls",
+		"mime/multipart", "runtime/debug",
 		"net/http/internal",
 		"golang.org/x/net/http2/hpack",
 	},

src/net/http/request.go

@@ -9,6 +9,7 @@ package http
 import (
 	"bufio"
 	"bytes"
+	"context"
 	"crypto/tls"
 	"encoding/base64"
 	"errors"
@@ -247,7 +248,43 @@ type Request struct {
 	// RoundTripper may support Cancel.
 	//
 	// For server requests, this field is not applicable.
+	//
+	// Deprecated: use the Context and WithContext methods
+	// instead. If a Request's Cancel field and context are both
+	// set, it is undefined whether Cancel is respected.
 	Cancel <-chan struct{}
+
+	// ctx is either the client or server context. It should only
+	// be modified via copying the whole Request using WithContext.
+	// It is unexported to prevent people from using Context wrong
+	// and mutating the contexts held by callers of the same request.
+	ctx context.Context
+}
+
+// Context returns the request's context. To change the context, use
+// WithContext.
+//
+// The returned context is always non-nil; it defaults to the
+// background context.
+func (r *Request) Context() context.Context {
+	// TODO(bradfitz): document above what Context means for server and client
+	// requests, once implemented.
+	if r.ctx != nil {
+		return r.ctx
+	}
+	return context.Background()
+}
+
+// WithContext returns a shallow copy of r with its context changed
+// to ctx. The provided ctx must be non-nil.
+func (r *Request) WithContext(ctx context.Context) *Request {
+	if ctx == nil {
+		panic("nil context")
+	}
+	r2 := new(Request)
+	*r2 = *r
+	r2.ctx = ctx
+	return r2
 }
 
 // ProtoAtLeast reports whether the HTTP protocol used

src/net/http/response_test.go

@@ -10,6 +10,7 @@ import (
 	"compress/gzip"
 	"crypto/rand"
 	"fmt"
+	"go/ast"
 	"io"
 	"io/ioutil"
 	"net/http/internal"
@@ -656,10 +657,14 @@ func diff(t *testing.T, prefix string, have, want interface{}) {
 		t.Errorf("%s: type mismatch %v want %v", prefix, hv.Type(), wv.Type())
 	}
 	for i := 0; i < hv.NumField(); i++ {
+		name := hv.Type().Field(i).Name
+		if !ast.IsExported(name) {
+			continue
+		}
 		hf := hv.Field(i).Interface()
 		wf := wv.Field(i).Interface()
 		if !reflect.DeepEqual(hf, wf) {
-			t.Errorf("%s: %s = %v want %v", prefix, hv.Type().Field(i).Name, hf, wf)
+			t.Errorf("%s: %s = %v want %v", prefix, name, hf, wf)
 		}
 	}
 }

src/net/http/transport.go

@@ -758,6 +758,9 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
 	case <-req.Cancel:
 		handlePendingDial()
 		return nil, errRequestCanceledConn
+	case <-req.Context().Done():
+		handlePendingDial()
+		return nil, errRequestCanceledConn
 	case <-cancelc:
 		handlePendingDial()
 		return nil, errRequestCanceledConn
@@ -1263,6 +1266,9 @@ func (pc *persistConn) readLoop() {
 		case <-rc.req.Cancel:
 			alive = false
 			pc.t.CancelRequest(rc.req)
+		case <-rc.req.Context().Done():
+			alive = false
+			pc.t.CancelRequest(rc.req)
 		case <-pc.closech:
 			alive = false
 		}
@@ -1567,6 +1573,9 @@ WaitResponse:
 		case <-cancelChan:
 			pc.t.CancelRequest(req.Request)
 			cancelChan = nil
+		case <-req.Context().Done():
+			pc.t.CancelRequest(req.Request)
+			cancelChan = nil
 		}
 	}
 

src/net/http/transport_test.go

@@ -13,6 +13,7 @@ import (
 	"bufio"
 	"bytes"
 	"compress/gzip"
+	"context"
 	"crypto/rand"
 	"crypto/tls"
 	"errors"
@@ -1625,7 +1626,13 @@ func TestCancelRequestWithChannel(t *testing.T) {
 	}
 }
 
-func TestCancelRequestWithChannelBeforeDo(t *testing.T) {
+func TestCancelRequestWithChannelBeforeDo_Cancel(t *testing.T) {
+	testCancelRequestWithChannelBeforeDo(t, false)
+}
+func TestCancelRequestWithChannelBeforeDo_Context(t *testing.T) {
+	testCancelRequestWithChannelBeforeDo(t, true)
+}
+func testCancelRequestWithChannelBeforeDo(t *testing.T, withCtx bool) {
 	setParallel(t)
 	defer afterTest(t)
 	unblockc := make(chan bool)
@@ -1646,9 +1653,15 @@ func TestCancelRequestWithChannelBeforeDo(t *testing.T) {
 	c := &Client{Transport: tr}
 
 	req, _ := NewRequest("GET", ts.URL, nil)
+	if withCtx {
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+		req = req.WithContext(ctx)
+	} else {
 		ch := make(chan struct{})
 		req.Cancel = ch
 		close(ch)
+	}
 
 	_, err := c.Do(req)
 	if err == nil || !strings.Contains(err.Error(), "canceled") {