Fix Content Length Writer

that sends the HTTP Headers even if no data has to be written.

internal/api/runners.go

@@ -172,9 +172,12 @@ func (r *RunnerController) fileContent(writer http.ResponseWriter, request *http
 		err = targetRunner.GetFileContent(ctx, path, writer, privilegedExecution)
 	})
 	if errors.Is(err, runner.ErrFileNotFound) {
+		writer.Header().Del("Content-Length")
 		writeClientError(request.Context(), writer, err, http.StatusFailedDependency)
 		return
 	} else if err != nil {
+		// Otherwise, might assume that GetFileContent already wrote something and, therefore, the HTTP headers
+		// cannot be modified anymore.
 		log.WithContext(request.Context()).WithError(err).Error("Could not retrieve the requested file.")
 		writeInternalServerError(request.Context(), writer, err, dto.ErrorUnknown)
 		return

pkg/nullio/content_length.go

@@ -31,6 +31,9 @@ func (w *ContentLengthWriter) Write(p []byte) (count int, err error) {
 }
 
 func (w *ContentLengthWriter) handleDataForwarding(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
 	count, err := w.Target.Write(p)
 	if err != nil {
 		err = fmt.Errorf("could not write to target: %w", err)

tests/e2e/runners_test.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/gorilla/websocket"
 	"github.com/openHPI/poseidon/internal/api"
+	"github.com/openHPI/poseidon/internal/runner"
 	"github.com/openHPI/poseidon/pkg/dto"
 	"github.com/openHPI/poseidon/tests"
 	"github.com/openHPI/poseidon/tests/helpers"
@@ -429,6 +430,18 @@ func (s *E2ETestSuite) TestGetFileContent_Nomad() {
 		s.Require().NoError(err)
 		s.Equal(newFileContent, content)
 	})
+
+	s.Run("Permission Denied", func() {
+		getFileURL, err := url.Parse(helpers.BuildURL(api.BasePath, api.RunnersPath, runnerID, api.FileContentRawPath))
+		s.Require().NoError(err)
+		getFileURL.RawQuery = fmt.Sprintf("%s=%s", api.PathKey, "/proc/1/environ")
+		response, err := http.Get(getFileURL.String())
+		s.Require().NoError(err)
+		s.Equal(http.StatusFailedDependency, response.StatusCode)
+		content, err := io.ReadAll(response.Body)
+		s.Require().NoError(err)
+		s.Contains(string(content), runner.ErrFileNotFound.Error())
+	})
 }
 
 func (s *E2ETestSuite) TestRunnerGetsDestroyedAfterInactivityTimeout() {