net/http: make responseAndError satisfy the net.Error interface

Allow clients to check for timeouts without relying on error substring
matching.

Fixes #6185.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/55470048

src/pkg/net/http/transport.go

@@ -869,6 +869,18 @@ type writeRequest struct {
 	ch  chan<- error
 }
 
+type httpError struct {
+	err     string
+	timeout bool
+}
+
+func (e *httpError) Error() string   { return e.err }
+func (e *httpError) Timeout() bool   { return e.timeout }
+func (e *httpError) Temporary() bool { return true }
+
+var errTimeout error = &httpError{err: "net/http: timeout awaiting response headers", timeout: true}
+var errClosed error = &httpError{err: "net/http: transport closed before response was received"}
+
 func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
 	pc.t.setReqConn(req.Request, pc)
 	pc.lk.Lock()
@@ -939,11 +951,11 @@ WaitResponse:
 			pconnDeadCh = nil                               // avoid spinning
 			failTicker = time.After(100 * time.Millisecond) // arbitrary time to wait for resc
 		case <-failTicker:
-			re = responseAndError{err: errors.New("net/http: transport closed before response was received")}
+			re = responseAndError{err: errClosed}
 			break WaitResponse
 		case <-respHeaderTimer:
 			pc.close()
-			re = responseAndError{err: errors.New("net/http: timeout awaiting response headers")}
+			re = responseAndError{err: errTimeout}
 			break WaitResponse
 		case re = <-resc:
 			break WaitResponse

src/pkg/net/http/transport_test.go

@@ -1237,6 +1237,20 @@ func TestTransportResponseHeaderTimeout(t *testing.T) {
 	for i, tt := range tests {
 		res, err := c.Get(ts.URL + tt.path)
 		if err != nil {
+			uerr, ok := err.(*url.Error)
+			if !ok {
+				t.Errorf("error is not an url.Error; got: %#v", err)
+				continue
+			}
+			nerr, ok := uerr.Err.(net.Error)
+			if !ok {
+				t.Errorf("error does not satisfy net.Error interface; got: %#v", err)
+				continue
+			}
+			if !nerr.Timeout() {
+				t.Errorf("want timeout error; got: %q", nerr)
+				continue
+			}
 			if strings.Contains(err.Error(), tt.wantErr) {
 				continue
 			}