More accurately replicate Linux's Unix domain socket event notification.

The documentation for EPOLLET (which repeatedly instructs users to read/write
until EAGAIN or short read) suggests that its intent is to only cause an epoll
notification when an FD registered with EPOLLET transitions from not-readable
to readable, or not-writable to writable. In practice, however, the actual
implementation for at least Unix domain sockets will trigger an epoll
notification on *every* send (cf. `net/unix/af_unix.c:unix_stream_sendmsg()` =>
"`other->sk_data_ready(other)`" => `net/core/sock.c:sock_def_readable()`), and
nginx (which registers Unix domain sockets with EPOLLET and - apparently
incorrectly - does *not* read from them until EAGAIN or short read) depends on
this property.

PiperOrigin-RevId: 601911295

pkg/sentry/socket/unix/transport/queue.go

@@ -167,7 +167,7 @@ func (q *queue) Enqueue(ctx context.Context, data [][]byte, c ControlMessages, f
 		b = b[n:]
 	}
 
-	notify = q.dataList.Front() == nil
+	notify = true
 	q.used += l
 	q.dataList.PushBack(&message{
 		Data:    v,
@@ -200,13 +200,11 @@ func (q *queue) Dequeue() (e *message, notify bool, err *syserr.Error) {
 		return nil, false, err
 	}
 
-	notify = !q.bufWritable()
-
 	e = q.dataList.Front()
 	q.dataList.Remove(e)
 	q.used -= e.Length()
 
-	notify = notify && q.bufWritable()
+	notify = q.bufWritable()
 
 	q.mu.Unlock()