fix: client prompt return on context cancellation

[umegbewe]

This fixes the issue where the client blocks for the entire duration of the response stream, even when the provided context had already been canceled or times out

Relevant issue #1698

[bwplotka]

Thanks! Some questions, not sure if this will work. My understanding was that we have to manually chunk reads from the resp.Body.

[bwplotka]

This assumes that close will somehow cancel _, err := buf.ReadFrom(resp.Body). Is it true? How?

Can we have a unit tests that perhaps shows this? 🤔

[umegbewe]

Yes, calling resp.Body.Close() will close the underlying connection, which unblocks any pending read and returns an error.

see unit test, if okay i will add this to client_test.go

func TestDoContextCancellation(t *testing.T) {
	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("partial"))
		if f, ok := w.(http.Flusher); ok {
			f.Flush()
		}

		time.Sleep(500 * time.Millisecond) // simulate delay
		w.Write([]byte(" remaining"))
	}))
	defer ts.Close()

	client, err := NewClient(Config{
		Address: ts.URL,
	})
	if err != nil {
		t.Fatalf("failed to create client: %v", err)
	}

	req, err := http.NewRequest("GET", ts.URL, nil)
	if err != nil {
		t.Fatalf("failed to create request: %v", err)
	}

	ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
	defer cancel()

	start := time.Now()
	resp, body, err := client.Do(ctx, req)
	elapsed := time.Since(start)

	if err != context.DeadlineExceeded {
		t.Errorf("expected error %v, got: %v", context.DeadlineExceeded, err)
	}
	if body != nil {
		t.Errorf("expected no body due to cancellation, got: %q", string(body))
	}
	if elapsed > 200*time.Millisecond {
		t.Errorf("Do did not return promptly on cancellation: took %v", elapsed)
	}

	if resp != nil && resp.Body != nil {
		resp.Body.Close()
	}
}