Chapter Context

context/context.go

@@ -0,0 +1,21 @@
+package context
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+)
+
+type Store interface {
+	Fetch(ctx context.Context) (string, error)
+}
+
+func Server(store Store) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		data, err := store.Fetch(r.Context())
+		if err != nil {
+			return
+		}
+		fmt.Fprint(w, data)
+	}
+}

context/context_test.go

@@ -0,0 +1,97 @@
+package context
+
+import (
+	"context"
+	"errors"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+type SpyStore struct {
+	response string
+	t        *testing.T
+}
+
+func (s *SpyStore) Fetch(ctx context.Context) (string, error) {
+	data := make(chan string, 1)
+
+	go func() {
+		var result string
+		for _, c := range s.response {
+			select {
+			case <-ctx.Done():
+				log.Println("spy store got cancelled")
+				return
+			default:
+				time.Sleep(10 * time.Millisecond)
+				result += string(c)
+			}
+		}
+		data <- result
+	}()
+
+	select {
+	case <-ctx.Done():
+		return "", ctx.Err()
+	case res := <-data:
+		return res, nil
+	}
+}
+
+type SpyResponseWriter struct {
+	written bool
+}
+
+func (s *SpyResponseWriter) Header() http.Header {
+	s.written = true
+	return nil
+}
+
+func (s *SpyResponseWriter) Write([]byte) (int, error) {
+	s.written = true
+	return 0, errors.New("not implemented")
+}
+
+func (s *SpyResponseWriter) WriteHeader(statusCode int) {
+	s.written = true
+}
+
+func TestServer(t *testing.T) {
+	t.Run("returns data from store", func(t *testing.T) {
+		data := "hello, world"
+		store := &SpyStore{response: data, t: t}
+		srv := Server(store)
+
+		request := httptest.NewRequest(http.MethodGet, "/", nil)
+		response := httptest.NewRecorder()
+
+		srv.ServeHTTP(response, request)
+
+		if response.Body.String() != data {
+			t.Errorf("got %v, want %v", response.Body.String(), data)
+		}
+	})
+
+	t.Run("tells store to cancel work if request is cancelled", func(t *testing.T) {
+		data := "hello, world"
+		store := &SpyStore{response: data, t: t}
+		svr := Server(store)
+
+		request := httptest.NewRequest(http.MethodGet, "/", nil)
+
+		cancellingCtx, cancel := context.WithCancel(request.Context())
+		time.AfterFunc(5*time.Millisecond, cancel)
+		request = request.WithContext(cancellingCtx)
+
+		response := &SpyResponseWriter{}
+
+		svr.ServeHTTP(response, request)
+
+		if response.written {
+			t.Error("a response should not have been written")
+		}
+	})
+}