Find available port when starting backend plugin

CURA-11067

cura/BackendPlugin.py

@@ -1,5 +1,6 @@
 # Copyright (c) 2023 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
+import socket
 import subprocess
 from typing import Optional, List
 
@@ -42,6 +43,15 @@ class BackendPlugin(AdditionalSettingDefinitionsAppender, PluginObject):
     def getAddress(self) -> str:
         return self._plugin_address
 
+    def setAvailablePort(self) -> None:
+        """
+        Sets the port to a random available port.
+        """
+        sock = socket.socket()
+        sock.bind((self.getAddress(), 0))
+        port = sock.getsockname()[1]
+        self.setPort(port)
+
     def _validatePluginCommand(self) -> list[str]:
         """
         Validate the plugin command and add the port parameter if it is missing.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -83,7 +83,6 @@ class CuraEngineBackend(QObject, Backend):
             os.path.join(CuraApplication.getInstallPrefix(), "bin"),
             os.path.dirname(os.path.abspath(sys.executable)),
         ]
-        self._last_backend_plugin_port = self._port + 1000
         for path in search_path:
             engine_path = os.path.join(path, executable_name)
             if os.path.isfile(engine_path):
@@ -205,8 +204,7 @@ class CuraEngineBackend(QObject, Backend):
         for backend_plugin in backend_plugins:
             # Set the port to prevent plugins from using the same one.
             if backend_plugin.getPort() < 1:
-                backend_plugin.setPort(self._last_backend_plugin_port)
-                self._last_backend_plugin_port += 1
+                backend_plugin.setAvailablePort()
             backend_plugin.start()
 
     def stopPlugins(self) -> None: