client: handle empty watch responses elegantly

Even though current etcd does not time out
watches, the client could be running against
an old etcd version or the server may close
polling connection for other reasons.
This patch ignores successful (as in 200)
responses with emtpy bodies instead
of producing JSON errors.

client/keys.go

@@ -63,6 +63,7 @@ func (e Error) Error() string {
 
 var (
 	ErrInvalidJSON = errors.New("client: response is invalid json. The endpoint is probably not valid etcd cluster endpoint.")
+	ErrEmptyBody   = errors.New("client: response body is empty")
 )
 
 // PrevExistType is used to define an existence condition when setting
@@ -419,18 +420,23 @@ type httpWatcher struct {
 }
 
 func (hw *httpWatcher) Next(ctx context.Context) (*Response, error) {
-	httpresp, body, err := hw.client.Do(ctx, &hw.nextWait)
-	if err != nil {
-		return nil, err
-	}
+	for {
+		httpresp, body, err := hw.client.Do(ctx, &hw.nextWait)
+		if err != nil {
+			return nil, err
+		}
 
-	resp, err := unmarshalHTTPResponse(httpresp.StatusCode, httpresp.Header, body)
-	if err != nil {
-		return nil, err
-	}
+		resp, err := unmarshalHTTPResponse(httpresp.StatusCode, httpresp.Header, body)
+		if err != nil {
+			if err == ErrEmptyBody {
+				continue
+			}
+			return nil, err
+		}
 
-	hw.nextWait.WaitIndex = resp.Node.ModifiedIndex + 1
-	return resp, nil
+		hw.nextWait.WaitIndex = resp.Node.ModifiedIndex + 1
+		return resp, nil
+	}
 }
 
 // v2KeysURL forms a URL representing the location of a key.
@@ -590,6 +596,9 @@ func (a *createInOrderAction) HTTPRequest(ep url.URL) *http.Request {
 func unmarshalHTTPResponse(code int, header http.Header, body []byte) (res *Response, err error) {
 	switch code {
 	case http.StatusOK, http.StatusCreated:
+		if len(body) == 0 {
+			return nil, ErrEmptyBody
+		}
 		res, err = unmarshalSuccessfulKeysResponse(header, body)
 	default:
 		err = unmarshalFailedKeysResponse(body)