test: deflake test-permission-net-udp-handle

Sending a dgram socket over IPC duplicates the handle instead of moving
it: `dgram.Socket` has no `postSend` in the child_process conversion
table, and `bind()` calls `recvStart()` unconditionally, so the parent
copy keeps consuming datagrams after the child adopts the handle. Both
sockets then compete for every packet. In local testing the parent
consumed 110 consecutive packets before the child received one; on a
slower machine the race can keep the child from receiving anything
before the test timeout, which is how this failed in CI.

Close the parent copy as soon as the child reports it is receiving, so
the child is the only reader by the time the first datagram is sent.

Signed-off-by: Christian Aurich <christian.aurichzm@gmail.com>

Author: christianaurichzm

test/parallel/test-permission-net-udp-handle.js

@@ -60,6 +60,7 @@ if (mode === 'ipc-denied') {
   const droppedSocket = dgram.createSocket('udp4');
   const sender = dgram.createSocket('udp4');
   droppedSocket.bind(0, '127.0.0.1', common.mustCall(() => {
+    const { port } = droppedSocket.address();
     const child = fork(__filename, ['ipc-drop'], {
       execArgv: [
         '--permission',
@@ -73,15 +74,17 @@ if (mode === 'ipc-denied') {
       if (message === 'ready') {
         child.send('socket', droppedSocket);
       } else if (message === 'receiving') {
-        const { port } = droppedSocket.address();
+        // Sending the socket over IPC keeps this copy open and receiving, so
+        // both ends compete for incoming datagrams. Stop receiving before the
+        // first one goes out.
+        droppedSocket.close();
         timer = setInterval(() => {
           sender.send('after-drop', port, '127.0.0.1');
         }, 10);
       } else {
         assert.strictEqual(message, 'received');
         clearInterval(timer);
         sender.close();
-        droppedSocket.close();
         child.disconnect();
       }
     }, 3));