From 7fe5a954b48f5a868bf18ea3dbf300ab3f51680d Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Fri, 10 Apr 2026 07:22:04 -0700
Subject: [PATCH] tstest/tailmac: remove unnecessary socketpair relay for VZ
 network device
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Give VZ the dgram socket file descriptor directly, matching what
llmacstation does. The socketpair relay was added during debugging
but is unnecessary — the direct approach works fine.
---
 .../Swift/Common/TailMacConfigHelper.swift    | 37 ++-----------------
 1 file changed, 3 insertions(+), 34 deletions(-)

diff --git a/tstest/tailmac/Swift/Common/TailMacConfigHelper.swift b/tstest/tailmac/Swift/Common/TailMacConfigHelper.swift
index 796adf1cb..6c1db77fc 100644
--- a/tstest/tailmac/Swift/Common/TailMacConfigHelper.swift
+++ b/tstest/tailmac/Swift/Common/TailMacConfigHelper.swift
@@ -125,42 +125,11 @@ struct TailMacConfigHelper {
         print("Virtual if mac address is \(config.mac)")
         print("Client bound to \(clientSocket)")
         print("Connected to server at \(serverSocket)")
+        print("Socket fd is \(socket)")
 
-        // Use a socketpair between VZ and the relay. VZ reads/writes one end;
-        // background threads relay between the other end and the vnet dgram socket.
-        // This is more reliable than giving VZ the dgram socket directly.
-        var spFds: [Int32] = [0, 0]
-        guard socketpair(AF_UNIX, SOCK_DGRAM, 0, &spFds) == 0 else {
-            print("socketpair failed: \(String(cString: strerror(errno)))")
-            return networkDevice
-        }
-        let vzFd = spFds[0]
-        let relayFd = spFds[1]
-
-        let vzHandle = FileHandle(fileDescriptor: vzFd)
-        let device = VZFileHandleNetworkDeviceAttachment(fileHandle: vzHandle)
+        let handle = FileHandle(fileDescriptor: socket)
+        let device = VZFileHandleNetworkDeviceAttachment(fileHandle: handle)
         networkDevice.attachment = device
-
-        // Relay: guest→network (read from relayFd, write to dgram socket)
-        DispatchQueue.global().async {
-            var buf = [UInt8](repeating: 0, count: 65536)
-            while true {
-                let n = Darwin.read(relayFd, &buf, buf.count)
-                if n <= 0 { break }
-                Darwin.write(socket, buf, n)
-            }
-        }
-
-        // Relay: network→guest (read from dgram socket, write to relayFd)
-        DispatchQueue.global().async {
-            var buf = [UInt8](repeating: 0, count: 65536)
-            while true {
-                let n = Darwin.read(socket, &buf, buf.count)
-                if n <= 0 { break }
-                Darwin.write(relayFd, buf, n)
-            }
-        }
-
         return networkDevice
     }