net/http: if context is canceled, return its error

This permits the error message to distinguish between a context that was
canceled and a context that timed out.

Updates #16381.

Change-Id: I3994b98e32952abcd7ddb5fee08fa1535999be6d
Reviewed-on: https://go-review.googlesource.com/24978
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/client_test.go

@@ -313,8 +313,8 @@ func TestClientRedirectContext(t *testing.T) {
 	if !ok {
 		t.Fatalf("got error %T; want *url.Error", err)
 	}
-	if ue.Err != ExportErrRequestCanceled && ue.Err != ExportErrRequestCanceledConn {
-		t.Errorf("url.Error.Err = %v; want errRequestCanceled or errRequestCanceledConn", ue.Err)
+	if ue.Err != context.Canceled {
+		t.Errorf("url.Error.Err = %v; want %v", ue.Err, context.Canceled)
 	}
 }
 

src/net/http/transport.go

@@ -76,7 +76,7 @@ type Transport struct {
 	idleLRU    connLRU
 
 	reqMu       sync.Mutex
-	reqCanceler map[*Request]func()
+	reqCanceler map[*Request]func(error)
 
 	altMu    sync.RWMutex
 	altProto map[string]RoundTripper // nil or map of URI scheme => RoundTripper
@@ -498,12 +498,17 @@ func (t *Transport) CloseIdleConnections() {
 // cancelable context instead. CancelRequest cannot cancel HTTP/2
 // requests.
 func (t *Transport) CancelRequest(req *Request) {
+	t.cancelRequest(req, errRequestCanceled)
+}
+
+// Cancel an in-flight request, recording the error value.
+func (t *Transport) cancelRequest(req *Request, err error) {
 	t.reqMu.Lock()
 	cancel := t.reqCanceler[req]
 	delete(t.reqCanceler, req)
 	t.reqMu.Unlock()
 	if cancel != nil {
-		cancel()
+		cancel(err)
 	}
 }
 
@@ -783,11 +788,11 @@ func (t *Transport) removeIdleConnLocked(pconn *persistConn) {
 	}
 }
 
-func (t *Transport) setReqCanceler(r *Request, fn func()) {
+func (t *Transport) setReqCanceler(r *Request, fn func(error)) {
 	t.reqMu.Lock()
 	defer t.reqMu.Unlock()
 	if t.reqCanceler == nil {
-		t.reqCanceler = make(map[*Request]func())
+		t.reqCanceler = make(map[*Request]func(error))
 	}
 	if fn != nil {
 		t.reqCanceler[r] = fn
@@ -800,7 +805,7 @@ func (t *Transport) setReqCanceler(r *Request, fn func()) {
 // for the request, we don't set the function and return false.
 // Since CancelRequest will clear the canceler, we can use the return value to detect if
 // the request was canceled since the last setReqCancel call.
-func (t *Transport) replaceReqCanceler(r *Request, fn func()) bool {
+func (t *Transport) replaceReqCanceler(r *Request, fn func(error)) bool {
 	t.reqMu.Lock()
 	defer t.reqMu.Unlock()
 	_, ok := t.reqCanceler[r]
@@ -849,7 +854,7 @@ func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (*persistC
 		// set request canceler to some non-nil function so we
 		// can detect whether it was cleared between now and when
 		// we enter roundTrip
-		t.setReqCanceler(req, func() {})
+		t.setReqCanceler(req, func(error) {})
 		return pc, nil
 	}
 
@@ -874,8 +879,8 @@ func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (*persistC
 		}()
 	}
 
-	cancelc := make(chan struct{})
-	t.setReqCanceler(req, func() { close(cancelc) })
+	cancelc := make(chan error, 1)
+	t.setReqCanceler(req, func(err error) { cancelc <- err })
 
 	go func() {
 		pc, err := t.dialConn(ctx, cm)
@@ -897,7 +902,12 @@ func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (*persistC
 		select {
 		case <-req.Cancel:
 		case <-req.Context().Done():
-		case <-cancelc:
+			return nil, req.Context().Err()
+		case err := <-cancelc:
+			if err == errRequestCanceled {
+				err = errRequestCanceledConn
+			}
+			return nil, err
 		default:
 			// It wasn't an error due to cancelation, so
 			// return the original error message:
@@ -922,10 +932,13 @@ func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (*persistC
 		return nil, errRequestCanceledConn
 	case <-req.Context().Done():
 		handlePendingDial()
-		return nil, errRequestCanceledConn
-	case <-cancelc:
+		return nil, req.Context().Err()
+	case err := <-cancelc:
 		handlePendingDial()
-		return nil, errRequestCanceledConn
+		if err == errRequestCanceled {
+			err = errRequestCanceledConn
+		}
+		return nil, err
 	}
 }
 
@@ -1231,8 +1244,8 @@ type persistConn struct {
 	mu                   sync.Mutex // guards following fields
 	numExpectedResponses int
 	closed               error // set non-nil when conn is closed, before closech is closed
+	canceledErr          error // set non-nil if conn is canceled
 	broken               bool  // an error has happened on this connection; marked broken so it's not reused.
-	canceled             bool  // whether this conn was broken due a CancelRequest
 	reused               bool  // whether conn has had successful request/response and is being reused.
 	// mutateHeaderFunc is an optional func to modify extra
 	// headers on each outbound request before it's written. (the
@@ -1270,11 +1283,12 @@ func (pc *persistConn) isBroken() bool {
 	return b
 }
 
-// isCanceled reports whether this connection was closed due to CancelRequest.
-func (pc *persistConn) isCanceled() bool {
+// canceled returns non-nil if the connection was closed due to
+// CancelRequest or due to context cancelation.
+func (pc *persistConn) canceled() error {
 	pc.mu.Lock()
 	defer pc.mu.Unlock()
-	return pc.canceled
+	return pc.canceledErr
 }
 
 // isReused reports whether this connection is in a known broken state.
@@ -1297,10 +1311,10 @@ func (pc *persistConn) gotIdleConnTrace(idleAt time.Time) (t httptrace.GotConnIn
 	return
 }
 
-func (pc *persistConn) cancelRequest() {
+func (pc *persistConn) cancelRequest(err error) {
 	pc.mu.Lock()
 	defer pc.mu.Unlock()
-	pc.canceled = true
+	pc.canceledErr = err
 	pc.closeLocked(errRequestCanceled)
 }
 
@@ -1328,8 +1342,8 @@ func (pc *persistConn) mapRoundTripErrorFromReadLoop(startBytesWritten int64, er
 	if err == nil {
 		return nil
 	}
-	if pc.isCanceled() {
-		return errRequestCanceled
+	if err := pc.canceled(); err != nil {
+		return err
 	}
 	if err == errServerClosedIdle {
 		return err
@@ -1351,8 +1365,8 @@ func (pc *persistConn) mapRoundTripErrorFromReadLoop(startBytesWritten int64, er
 // its pc.closech channel close, indicating the persistConn is dead.
 // (after closech is closed, pc.closed is valid).
 func (pc *persistConn) mapRoundTripErrorAfterClosed(startBytesWritten int64) error {
-	if pc.isCanceled() {
-		return errRequestCanceled
+	if err := pc.canceled(); err != nil {
+		return err
 	}
 	err := pc.closed
 	if err == errServerClosedIdle {
@@ -1509,8 +1523,10 @@ func (pc *persistConn) readLoop() {
 				waitForBodyRead <- isEOF
 				if isEOF {
 					<-eofc // see comment above eofc declaration
-				} else if err != nil && pc.isCanceled() {
-					return errRequestCanceled
+				} else if err != nil {
+					if cerr := pc.canceled(); cerr != nil {
+						return cerr
+					}
 				}
 				return err
 			},
@@ -1550,7 +1566,7 @@ func (pc *persistConn) readLoop() {
 			pc.t.CancelRequest(rc.req)
 		case <-rc.req.Context().Done():
 			alive = false
-			pc.t.CancelRequest(rc.req)
+			pc.t.cancelRequest(rc.req, rc.req.Context().Err())
 		case <-pc.closech:
 			alive = false
 		}
@@ -1836,8 +1852,8 @@ WaitResponse:
 		select {
 		case err := <-writeErrCh:
 			if err != nil {
-				if pc.isCanceled() {
-					err = errRequestCanceled
+				if cerr := pc.canceled(); cerr != nil {
+					err = cerr
 				}
 				re = responseAndError{err: err}
 				pc.close(fmt.Errorf("write error: %v", err))
@@ -1861,9 +1877,8 @@ WaitResponse:
 		case <-cancelChan:
 			pc.t.CancelRequest(req.Request)
 			cancelChan = nil
-			ctxDoneChan = nil
 		case <-ctxDoneChan:
-			pc.t.CancelRequest(req.Request)
+			pc.t.cancelRequest(req.Request, req.Context().Err())
 			cancelChan = nil
 			ctxDoneChan = nil
 		}

src/net/http/transport_test.go

@@ -1718,9 +1718,18 @@ func testCancelRequestWithChannelBeforeDo(t *testing.T, withCtx bool) {
 	}
 
 	_, err := c.Do(req)
+	if ue, ok := err.(*url.Error); ok {
+		err = ue.Err
+	}
+	if withCtx {
+		if err != context.Canceled {
+			t.Errorf("Do error = %v; want %v", err, context.Canceled)
+		}
+	} else {
 		if err == nil || !strings.Contains(err.Error(), "canceled") {
 			t.Errorf("Do error = %v; want cancelation", err)
 		}
+	}
 }
 
 // Issue 11020. The returned error message should be errRequestCanceled