Network Connection Logic Enhancement

Nucleic-Session: 93A70648-2057-4092-9372-860E1BA151D8
Co-authored-by: Nucleic <[email protected]>

NucleicRemote/NucleicRemote/Models/HostConnection.swift

@@ -689,6 +689,72 @@ final class HostConnection {
         if !connectivity.isLive, pathMonitor.isSatisfied {
             reconnectAttempts = 0
             reconnect(to: host)
+            return
+        }
+        // Live over a fallback transport (tailnet/relay) and a LAN path just reappeared (rejoined the
+        // Mac's Wi-Fi) → the return leg of the switch above. The tunnel survives the interface change
+        // on its own, so nothing else would ever re-plan back down to the direct path — probe LAN and
+        // swap to it if the Mac actually answers here.
+        maybeUpgradeToLAN()
+    }
+
+    /// Probe whether the pinned Mac is reachable over LAN right now and, if so, switch back to the
+    /// direct connection — the tailnet/relay → LAN return leg. Only meaningful while live over a
+    /// non-LAN transport with a LAN-capable path available; a bare TCP connect to the pinned LAN
+    /// endpoint that reaches `.ready` is the "the Mac is on this network" signal. The probe runs
+    /// entirely off the live connection, so joining a *foreign* Wi-Fi (no Mac here) costs one short,
+    /// silent connect attempt and leaves the working tunnel untouched.
+    private func maybeUpgradeToLAN() {
+        guard let host = pinnedHost, connectivity.isLive, activeTransport != .lan,
+              pathMonitor.canUseLAN, lanUpgradeProbe == nil,
+              let endpoint = discovery.endpoint(
+                forFingerprint: host.fingerprint, lanHost: host.lanHost, lanPort: host.lanPort)
+        else { return }
+        lanUpgradeProbe = Task { [weak self] in
+            let reachable = await Self.probeReachable(endpoint)
+            guard let self else { return }
+            self.lanUpgradeProbe = nil
+            // Re-check the world didn't move under us during the probe (still live, still not on LAN,
+            // LAN still available) before tearing a working connection down.
+            guard !Task.isCancelled, reachable, self.connectivity.isLive,
+                  self.activeTransport != .lan, self.pathMonitor.canUseLAN,
+                  let host = self.pinnedHost
+            else { return }
+            self.reconnectAttempts = 0
+            self.reconnect(to: host)  // rebuilds the candidate chain LAN-first
+        }
+    }
+
+    /// Bare TCP reachability check for `maybeUpgradeToLAN`: does `endpoint` accept a connection within
+    /// `timeoutSeconds`? Tears the probe socket down regardless — it only tests reachability, never
+    /// carries traffic. A short timeout so a foreign Wi-Fi (Mac absent) doesn't stall the check.
+    private static func probeReachable(_ endpoint: NWEndpoint, timeoutSeconds: Double = 3) async -> Bool {
+        let params = NWParameters.tcp
+        params.includePeerToPeer = true
+        let connection = NWConnection(to: endpoint, using: params)
+        let queue = DispatchQueue(label: "nucleic.remote.lanprobe")
+        let once = ProbeOnce()
+        return await withTaskCancellationHandler {
+            await withCheckedContinuation { (cont: CheckedContinuation<Bool, Never>) in
+                connection.stateUpdateHandler = { state in
+                    switch state {
+                    case .ready:
+                        if once.take() { cont.resume(returning: true) }
+                        connection.cancel()
+                    case .failed, .cancelled:
+                        if once.take() { cont.resume(returning: false) }
+                    default:
+                        break
+                    }
+                }
+                queue.asyncAfter(deadline: .now() + timeoutSeconds) {
+                    if once.take() { cont.resume(returning: false) }
+                    connection.cancel()
+                }
+                connection.start(queue: queue)
+            }
+        } onCancel: {
+            connection.cancel()
         }
     }