Avoid concurrent writes to the websocket connection

Previously, the server sometimes crashed due to concurrent writes
to the websocket connection. Now, we ensure that only one concurrent
function writes to the websocket at a time by enclosing the WriteMessage
function with a mutex.

internal/api/websocket.go

@@ -10,6 +10,7 @@ import (
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
 	"io"
 	"net/http"
+	"sync"
 )
 
 const CodeOceanToRawReaderBufferSize = 1024
@@ -163,6 +164,7 @@ func (rc *rawToCodeOceanWriter) Write(p []byte) (int, error) {
 type webSocketProxy struct {
 	userExit     chan bool
 	connection   webSocketConnection
+	connectionMu sync.Mutex
 	Stdin        WebSocketReader
 	Stdout       io.Writer
 	Stderr       io.Writer
@@ -261,7 +263,7 @@ func (wp *webSocketProxy) sendToClient(message dto.WebSocketMessage) error {
 		wp.closeWithError("Error creating message")
 		return fmt.Errorf("error marshaling WebSocket message: %w", err)
 	}
-	err = wp.connection.WriteMessage(websocket.TextMessage, encodedMessage)
+	err = wp.writeMessage(websocket.TextMessage, encodedMessage)
 	if err != nil {
 		errorMessage := "Error writing the exit message"
 		log.WithError(err).Warn(errorMessage)
@@ -280,13 +282,19 @@ func (wp *webSocketProxy) closeNormal() {
 }
 
 func (wp *webSocketProxy) close(message []byte) {
-	err := wp.connection.WriteMessage(websocket.CloseMessage, message)
+	err := wp.writeMessage(websocket.CloseMessage, message)
 	_ = wp.connection.Close()
 	if err != nil {
 		log.WithError(err).Warn("Error during websocket close")
 	}
 }
 
+func (wp *webSocketProxy) writeMessage(messageType int, data []byte) error {
+	wp.connectionMu.Lock()
+	defer wp.connectionMu.Unlock()
+	return wp.connection.WriteMessage(messageType, data) //nolint:wrapcheck // Wrap the original WriteMessage in a mutex.
+}
+
 // connectToRunner is the endpoint for websocket connections.
 func (r *RunnerController) connectToRunner(writer http.ResponseWriter, request *http.Request) {
 	targetRunner, _ := runner.FromContext(request.Context())