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
@@ -161,11 +162,12 @@ func (rc *rawToCodeOceanWriter) Write(p []byte) (int, error) {
 
 // webSocketProxy is an encapsulation of logic for forwarding between Runners and CodeOcean.
 type webSocketProxy struct {
-	userExit   chan bool
+	userExit     chan bool
-	connection webSocketConnection
+	connection   webSocketConnection
-	Stdin      WebSocketReader
+	connectionMu sync.Mutex
-	Stdout     io.Writer
+	Stdin        WebSocketReader
-	Stderr     io.Writer
+	Stdout       io.Writer
+	Stderr       io.Writer
 }
 
 // upgradeConnection upgrades a connection to a websocket and returns a webSocketProxy for this connection.
@@ -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())