fix: pick a free port for the desktop backend when 30001 is taken (#1663)

electron/local-files.cjs

@@ -330,7 +330,11 @@ const AUTH_TOKEN_PATTERN = /^[A-Za-z0-9_-]+\.[A-Za-z0-9_-]+\.[A-Za-z0-9_-]+$/;
 // path, so nothing here names it. Only a /plugin-api/<id> path is accepted,
 // which keeps every transfer on the backend's plugin routes.
 const API_PATH_PATTERN = /^\/plugin-api\/[a-z0-9][a-z0-9-]*$/;
-const LOCAL_BACKEND = "http://localhost:30001";
+let localBackendPort = 30001;
+
+function setLocalBackendPort(port) {
+  localBackendPort = port;
+}
 let registeredApiPath = null;
 
 function setTransferApiPath(apiPath) {
@@ -403,7 +407,8 @@ function createTargetResolver({ localBaseUrl, getLinkedServer, apiPath }) {
       }
       return {
         url: `${normalizeHttpBase(
-          localBaseUrl ?? `${LOCAL_BACKEND}${resolveApiPath()}`,
+          localBaseUrl ??
+            `http://localhost:${localBackendPort}${resolveApiPath()}`,
           "Local backend URL",
         )}${routePath}`,
         headers,
@@ -1050,6 +1055,7 @@ module.exports = {
   createLocalFileHandlers,
   createTargetResolver,
   setTransferApiPath,
+  setLocalBackendPort,
   publishDownload,
   defaultPublishFs,
   assertWithinRoot,

electron/main.cjs

@@ -35,7 +35,10 @@ const { isCloseActiveTabInput } = require("./keyboard-shortcuts.cjs");
 const { quitApp } = require("./app-quit.cjs");
 const { selectLinuxPasswordStore } = require("./linux-password-store.cjs");
 const { resolveLocalShell, repairSpawnHelpers } = require("./local-shell.cjs");
-const { registerLocalFileHandlers } = require("./local-files.cjs");
+const {
+  registerLocalFileHandlers,
+  setLocalBackendPort,
+} = require("./local-files.cjs");
 const {
   EXTERNAL_EDITOR_SETTING,
   isLaunchableFileName,
@@ -1124,6 +1127,35 @@ function reapOrphanedBackendProcess() {
   }
 }
 
+const DEFAULT_BACKEND_PORT = 30001;
+let backendPort = DEFAULT_BACKEND_PORT;
+
+function canListenOn(port) {
+  return new Promise((resolve) => {
+    const probe = net.createServer();
+    probe.once("error", () => resolve(null));
+    probe.listen(port, "127.0.0.1", () => {
+      const { port: bound } = probe.address();
+      probe.close(() => resolve(bound));
+    });
+  });
+}
+
+// 30001 when it is free, otherwise any free port. Only the desktop app talks
+// to this backend, so the number itself does not matter.
+async function findBackendPort() {
+  const preferred = await canListenOn(DEFAULT_BACKEND_PORT);
+  if (preferred) return preferred;
+  const fallback = await canListenOn(0);
+  if (fallback) {
+    logToFile(
+      `Port ${DEFAULT_BACKEND_PORT} is in use, starting the backend on ${fallback}`,
+    );
+    return fallback;
+  }
+  return DEFAULT_BACKEND_PORT;
+}
+
 function startBackendServer() {
   reapOrphanedBackendProcess();
   backendFailure = null;
@@ -1164,7 +1196,8 @@ function startBackendServer() {
         DATA_DIR: dataDir,
         NODE_ENV: "production",
         ELECTRON_EMBEDDED: "true",
-        PORT: "30001",
+        PORT: String(backendPort),
+        EMBEDDED_BACKEND_PORT: String(backendPort),
         VERSION: app.getVersion(),
       },
       stdio: ["pipe", "pipe", "pipe", "ipc"],
@@ -1429,6 +1462,7 @@ function createWindow() {
       sandbox: true,
       webSecurity: true,
       preload: path.join(__dirname, "preload.js"),
+      additionalArguments: [`--termix-backend-port=${backendPort}`],
       partition: termixSessionPartition,
       allowRunningInsecureContent: false,
       webviewTag: false,
@@ -3814,6 +3848,8 @@ app.whenReady().then(async () => {
   await clearElectronJwtCookiesAtStartup();
 
   if (!isDev) {
+    backendPort = await findBackendPort();
+    setLocalBackendPort(backendPort);
     const result = await startBackendServer();
     logToFile("startBackendServer result:", result);
   } else {

electron/preload.js

@@ -14,7 +14,16 @@ function invokeAllowed(channel, ...args) {
   return ipcRenderer.invoke(channel, ...args);
 }
 
+// The embedded backend's port, picked by the main process at startup.
+const backendPortArg = process.argv.find((arg) =>
+  arg.startsWith("--termix-backend-port="),
+);
+const backendPort = backendPortArg
+  ? Number(backendPortArg.split("=")[1])
+  : undefined;
+
 contextBridge.exposeInMainWorld("electronAPI", {
+  backendPort,
   getAppVersion: () => ipcRenderer.invoke("get-app-version"),
   // Kept so the interface size can reset a zoom an earlier build applied.
   setZoomFactor: (factor) => {