From 611cd4aad523666d6c5f0cecae5f1050e93ee69c Mon Sep 17 00:00:00 2001
From: Tom Butcher <thomasbutcher15@gmail.com>
Date: Sun, 2 Aug 2026 16:25:09 +0100
Subject: [PATCH] Implement single instance management and enhance deep link
 handling

- Added single instance server functionality to prevent multiple app instances.
- Introduced `ensureSingleInstanceLock` and `closeSingleInstanceServer` methods for managing instance locks.
- Updated main application entry point to handle deep links and focus events.
- Enhanced window management by showing the main window on focus and handling deep links appropriately.
---
 src/bun/index.js               |  18 +++++-
 src/desktop/single-instance.js | 111 +++++++++++++++++++++++++++++++++
 src/desktop/window.js          |   5 ++
 3 files changed, 133 insertions(+), 1 deletion(-)
 create mode 100644 src/desktop/single-instance.js

diff --git a/src/bun/index.js b/src/bun/index.js
index eb827ca..6bbdf99 100644
--- a/src/bun/index.js
+++ b/src/bun/index.js
@@ -1,15 +1,30 @@
 import { createAppRpc } from '../desktop/rpc.js'
+import {
+  closeSingleInstanceServer,
+  ensureSingleInstanceLock
+} from '../desktop/single-instance.js'
 import {
   registerGlobalShortcuts,
   unregisterGlobalShortcuts
 } from '../desktop/spotlight.js'
 import {
   createMainWindow,
+  handleDeepLink,
   handleDeepLinkFromArgv,
   setupDevAuthServer,
-  setupNavigationGestures
+  setupNavigationGestures,
+  showMainWindow
 } from '../desktop/window.js'
 
+const gotSingleInstanceLock = await ensureSingleInstanceLock({
+  onDeepLink: handleDeepLink,
+  onFocus: showMainWindow
+})
+
+if (!gotSingleInstanceLock) {
+  process.exit(0)
+}
+
 const rpc = createAppRpc()
 const mainWindow = await createMainWindow(rpc)
 
@@ -20,4 +35,5 @@ handleDeepLinkFromArgv()
 
 process.on('exit', () => {
   unregisterGlobalShortcuts()
+  closeSingleInstanceServer()
 })
diff --git a/src/desktop/single-instance.js b/src/desktop/single-instance.js
new file mode 100644
index 0000000..13ad9f5
--- /dev/null
+++ b/src/desktop/single-instance.js
@@ -0,0 +1,111 @@
+const PROTOCOL_PREFIX = 'farmcontrol://'
+const SINGLE_INSTANCE_HOST = '127.0.0.1'
+
+// Stable port derived from the app identifier so multiple apps don't collide.
+function getSingleInstancePort() {
+  const identifier = 'com.tombutcher.farmcontrol'
+  let hash = 0
+
+  for (const char of identifier) {
+    hash = (hash * 31 + char.charCodeAt(0)) | 0
+  }
+
+  return 49152 + (Math.abs(hash) % 16383)
+}
+
+function findProtocolUrl(args) {
+  return args.find(
+    (arg) => typeof arg === 'string' && arg.startsWith(PROTOCOL_PREFIX)
+  )
+}
+
+function buildSecondInstanceMessage() {
+  const url = findProtocolUrl(process.argv)
+  return url ? { type: 'deeplink', url } : { type: 'focus' }
+}
+
+function parseIncomingMessage(data) {
+  const text = data.toString().trim()
+  if (!text) return null
+
+  try {
+    return JSON.parse(text)
+  } catch {
+    return null
+  }
+}
+
+async function forwardToPrimaryInstance(message) {
+  await new Promise((resolve, reject) => {
+    Bun.connect({
+      hostname: SINGLE_INSTANCE_HOST,
+      port: getSingleInstancePort(),
+      socket: {
+        open(socket) {
+          socket.write(`${JSON.stringify(message)}\n`)
+          socket.end()
+          resolve()
+        },
+        data() {},
+        error(_socket, error) {
+          reject(error)
+        },
+        close() {
+          resolve()
+        }
+      }
+    }).catch(reject)
+  })
+}
+
+let server = null
+
+function startPrimaryInstanceServer({ onDeepLink, onFocus }) {
+  server = Bun.listen({
+    hostname: SINGLE_INSTANCE_HOST,
+    port: getSingleInstancePort(),
+    socket: {
+      data(socket, data) {
+        const message = parseIncomingMessage(data)
+        socket.end()
+
+        if (!message) return
+
+        if (message.type === 'deeplink' && message.url) {
+          onDeepLink?.(message.url)
+        } else if (message.type === 'focus') {
+          onFocus?.()
+        }
+      }
+    }
+  })
+}
+
+export async function ensureSingleInstanceLock({ onDeepLink, onFocus }) {
+  if (process.platform === 'darwin') {
+    return true
+  }
+
+  try {
+    startPrimaryInstanceServer({ onDeepLink, onFocus })
+    return true
+  } catch (error) {
+    if (error?.code !== 'EADDRINUSE') {
+      console.warn('Single instance lock failed:', error)
+      return true
+    }
+
+    try {
+      await forwardToPrimaryInstance(buildSecondInstanceMessage())
+    } catch (forwardError) {
+      console.warn('Failed to forward to running instance:', forwardError)
+    }
+
+    return false
+  }
+}
+
+export function closeSingleInstanceServer() {
+  server?.stop?.()
+  server = null
+}
diff --git a/src/desktop/window.js b/src/desktop/window.js
index 8941bda..ab828b8 100644
--- a/src/desktop/window.js
+++ b/src/desktop/window.js
@@ -20,6 +20,11 @@ export function getMainWindow() {
   return mainWindow
 }
 
+export function showMainWindow() {
+  mainWindow?.show?.()
+  mainWindow?.activate?.()
+}
+
 export async function getMainViewUrl() {
   const channel = await Updater.localInfo.channel()
   if (channel === 'dev' || process.env.NODE_ENV === 'development') {