Mock context.Done() in unit test

Question

I have a HTTP handler that sets a context deadline on each request: ``` func submitHandler(stream chan data) http.HandlerFunc { return func(w http.ResponseWriter, r *http.Request) { ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second) defer cancel() // read request body, etc. select { case stream <- req: w.WriteHeader(http.StatusNoContent) case <-ctx.Done(): err := ctx.Err() if err == context.DeadlineExceeded { w.WriteHeader(http.StatusRequestTimeout) } log.Printf("context done: %v", err) } } } ``` I am easily able to test the `http.StatusNoContent` header, but I am unsure about how to test the `<-ctx.Done()` case in the select statement. In my test case I have built a mock `context.Context` and passed it to the `req.WithContext()` method on my mock `http.Request`, however, the status code returned is always `http.StatusNoContent` which leads me to believe the `select` statement is always falling into the first case in my test. ``` type mockContext struct{} func (ctx mockCon…

Accepted Answer

So, after much trial and error I figured out what I was doing wrong. Instead of trying to create a mock `context.Context`, I created a new one with an expired deadline and immediately called the returned `cancelFunc`. I then passed this to `req.WithContext()` and now it works like a charm!

```
func TestHandler(t *testing.T) {
    stream := make(chan data, 1)
    defer close(stream)

handler := submitHandler(stream)
    req, err := http.NewRequest(http.MethodPost, "/submit", nil)
    if err != nil {
        t.Fatal(err)
    }

stream <- data{}
    ctx, cancel := context.WithDeadline(req.Context(), time.Now().Add(-7*time.Hour))
    cancel()
    req = req.WithContext(ctx)

rec := httptest.NewRecorder()
    handler.ServeHTTP(rec, req)

if rec.Code != http.StatusRequestTimeout {
        t.Errorf("expected status code: %d, got: %d", http.StatusRequestTimeout, rec.Code)
    }
}
```

Mock context.Done() in unit test

Solution

Problem