feat: allow forcing cache control header

Author: albaad

README.md

@@ -64,8 +64,31 @@ func main() {
 	srv = cache.Middleware(srv)
 	// ... do more things
 }
-````
+```
 
 Doing it, Gqlgen write the lowest max-age defined in cacheControl extensions.
 
 For more informations, see `_example` folder.
+
+### Force cache control in case of error
+
+By default, any existing cache hints will not result in a Cache-Control header in case of an error. 
+
+It's possible to force it by adding a `forceCacheControl` key with a `true` value. 
+
+
+```go
+func (h *handler) GraphqlHandler() gin.HandlerFunc {
+	// ... setup gqlgen/graphql/handler
+	// ... setup server
+	srv.Use(cache.Extension{})
+	cachedServer := cache.Middleware(srv)
+
+	return func(c *gin.Context) {
+		ctx := context.WithValue(c.Request.Context(), "forceCacheControl", true)
+		c.Request = c.Request.WithContext(ctx)
+
+		cachedServer.ServeHTTP(c.Writer, c.Request)
+	}
+}
+````

cache/context.go

@@ -0,0 +1,12 @@
+package cache
+
+import "context"
+
+type contextKey string
+
+const forceCacheControlKey = contextKey("forceCacheControl")
+
+// ContextWithForceCacheControl creates a new context with the forceCacheControl option.
+func ContextWithForceCacheControl(ctx context.Context, forceCacheControl bool) context.Context {
+	return context.WithValue(ctx, forceCacheControlKey, forceCacheControl)
+}

cache/context_test.go

@@ -0,0 +1,37 @@
+package cache
+
+import (
+	"context"
+	"reflect"
+	"testing"
+)
+
+func TestContextWithForceCacheControl(t *testing.T) {
+	type args struct {
+		ctx               context.Context
+		forceCacheControl bool
+	}
+	tests := []struct {
+		name string
+		args args
+		want context.Context
+	}{
+		{
+			"forceCacheControl is false",
+			args{context.Background(), false},
+			context.WithValue(context.Background(), forceCacheControlKey, false),
+		},
+		{
+			"valid key and value forceCacheControl",
+			args{context.Background(), true},
+			context.WithValue(context.Background(), forceCacheControlKey, true),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ContextWithForceCacheControl(tt.args.ctx, tt.args.forceCacheControl); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("ContextWithForceCacheControl() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}

cache/middleware.go

@@ -40,11 +40,13 @@ func (c responseWriter) Header() http.Header {
 }
 
 func (c responseWriter) Write(bytes []byte) (int, error) {
+	forceCacheControl, _ := c.r.Context().Value(forceCacheControlKey).(bool)
+
 	if c.w.Header().Get("Cache-Control") == "" {
 		resp := graphql.Response{}
 		err := json.Unmarshal(bytes, &resp)
 		if err == nil {
-			writeCacheControl(c.r.Context(), c.w, &resp)
+			writeCacheControl(c.r.Context(), c.w, &resp, forceCacheControl)
 		}
 	}
 
@@ -55,8 +57,8 @@ func (c responseWriter) WriteHeader(statusCode int) {
 	c.w.WriteHeader(statusCode)
 }
 
-func writeCacheControl(ctx context.Context, w http.ResponseWriter, response *graphql.Response) {
-	if len(response.Errors) > 0 {
+func writeCacheControl(ctx context.Context, w http.ResponseWriter, response *graphql.Response, forceCacheControl bool) {
+	if len(response.Errors) > 0 && !forceCacheControl {
 		return
 	}