Go - Mock http.Response body with a file - go

Question

I'm trying to test a Go function which performs a call to an external service. Here's the function:
func (gs *EuGameService) retrieveGames(client model.HTTPClient) (model.EuGamesResponse, error) {
req, err := http.NewRequest(http.MethodGet, gs.getGamesEndpoint, nil)
if err != nil {
log.Fatal("Error while creating request ", err)
return nil, err
}
resp, err := client.Do(req)
if err != nil {
log.Fatal("Error while retrieving EU games", err)
return nil, err
}
var euGames model.EuGamesResponse
decoder := json.NewDecoder(resp.Body)
decoder.Decode(&euGames)
return euGames, nil
}
to properly test it, I'm trying to inject a mock client.
type HTTPClient interface {
Do(req *http.Request) (*http.Response, error)
}
type mockClient struct{}
func (mc *mockClient) Do(req *http.Request) (*http.Response, error) {
mock, _ := os.Open("../stubs/eugames.json")
defer mock.Close()
r := ioutil.NopCloser(bufio.NewReader(mock))
return &http.Response{
Status: string(http.StatusOK),
StatusCode: http.StatusOK,
Body: r,
}, nil
}
the file eugames.json contains a couple of games. But for some reason, the body is always empty! What am I missing here? I tried to use a constant with the file content and it works, games are decoded correctly. So I'm assuming there's a problem with my use of the file.