fix(apple): Enforce single Firezone instance (#10313)

show an alert to the user and ask to quit previous Firezone instance
manually before starting a new one.

Resolves: #10295

---------

Signed-off-by: Mariusz Klochowicz <mariusz@klochowicz.com>
Co-authored-by: Jamil <jamilbk@users.noreply.github.com>

swift/apple/Firezone/Application/FirezoneApp.swift

@@ -73,6 +73,11 @@ struct FirezoneApp: App {
     var menuBar: MenuBar?
     var store: Store?
 
+    func applicationWillFinishLaunching(_ notification: Notification) {
+      // Enforce single instance BEFORE the app fully launches
+      enforceSingleInstance()
+    }
+
     func applicationDidFinishLaunching(_: Notification) {
       if let store {
         menuBar = MenuBar(store: store)
@@ -86,6 +91,38 @@ struct FirezoneApp: App {
       maybeShowOutdatedAlert()
     }
 
+    private func enforceSingleInstance() {
+      // Get the actual bundle identifier from the running app
+      guard let bundleId = Bundle.main.bundleIdentifier else { return }
+
+      let runningApps = NSRunningApplication.runningApplications(
+        withBundleIdentifier: bundleId
+      )
+
+      guard runningApps.count > 1 else { return }
+
+      for app in runningApps where app != NSRunningApplication.current {
+        DispatchQueue.main.async {
+          let alert = NSAlert()
+          alert.messageText = "Another Firezone Instance Detected"
+          alert.informativeText = """
+            Another instance of Firezone is already running. \
+            Please quit the other instance from the menu bar to continue.
+
+            Location: \(app.bundleURL?.path ?? "Unknown")
+            """
+          alert.alertStyle = .warning
+          alert.addButton(withTitle: "OK")
+
+          // Show alert
+          alert.runModal()
+
+          // Exit this instance since we can't terminate the other one
+          NSApp.terminate(nil)
+        }
+      }
+    }
+
     private func maybeShowOutdatedAlert() {
       let osVersion = ProcessInfo.processInfo.operatingSystemVersion
 

website/src/components/Changelog/Apple.tsx

@@ -25,6 +25,10 @@ export default function Apple() {
     <Entries downloadLinks={downloadLinks} title="macOS / iOS">
       {/* When you cut a release, remove any solved issues from the "known issues" lists over in `client-apps`. This must not be done when the issue's PR merges. */}
       <Unreleased>
+        <ChangeItem pull="10313">
+          Fixes an issue where multiple concurrent Firezone macOS clients could
+          run simultaneously. We now enforce a single instance of the client.
+        </ChangeItem>
         <ChangeItem pull="10224">
           Fixes a minor DNS cache bug where newly-added DNS resources may not
           resolve for a few seconds after showing up in the Resource List.