staticfiles: Signal that redirection headers have been written (#2792)

The Handler interface expects a first return value of 0 if headers have
already been written.
(cf. https://godoc.org/github.com/caddyserver/caddy/caddyhttp/httpserver#Handler)

When using http.Redirect, this is the case as http.Redirect does write
headers. When using Caddy with nested handlers, returning
http.StatusMovedPermanently could cause a wrong (200) response on a
non-slashy request on a directory name. Returning 0 ensures the
redirection will always take place.

caddyhttp/staticfiles/fileserver.go

@@ -116,7 +116,7 @@ func (fs FileServer) serveFile(w http.ResponseWriter, r *http.Request) (int, err
 			}
 			urlCopy.Path += "/"
 			http.Redirect(w, r, urlCopy.String(), http.StatusMovedPermanently)
-			return http.StatusMovedPermanently, nil
+			return 0, nil
 		}
 	} else {
 		// ensure no trailing slash
@@ -143,7 +143,7 @@ func (fs FileServer) serveFile(w http.ResponseWriter, r *http.Request) (int, err
 				urlCopy.Path = strings.TrimPrefix(urlCopy.Path, "/")
 			}
 			http.Redirect(w, r, urlCopy.String(), http.StatusMovedPermanently)
-			return http.StatusMovedPermanently, nil
+			return 0, nil
 		}
 	}
 

caddyhttp/staticfiles/fileserver_test.go

@@ -301,6 +301,9 @@ func TestServeHTTP(t *testing.T) {
 
 		// perform the test
 		status, err := fileServer.ServeHTTP(responseRecorder, request)
+		if status == 0 {
+			status = responseRecorder.Code
+		}
 		etag := responseRecorder.Header().Get("Etag")
 		body := responseRecorder.Body.String()
 		vary := responseRecorder.Header().Get("Vary")