only signal read (not write) readiness for listening sockets (#737)

This fixes the [reregister_different_interest_without_poll] test in mio.

[reregister_different_interest_without_poll]:
https://github.com/tokio-rs/mio/blob/66ac9fab79bf191218488c4f35c99d13935b7e12/tests/registering.rs#L118

Author: dicej

libc-bottom-half/sources/wasip2_tcp.c

@@ -1159,13 +1159,19 @@ int tcp_setsockopt(void *data, int level, int optname, const void *optval,
 static int tcp_poll_register(void *data, poll_state_t *state, short events) {
   tcp_socket_t *socket = (tcp_socket_t *)data;
   switch (socket->state.tag) {
-  case TCP_SOCKET_STATE_CONNECTING:
-  case TCP_SOCKET_STATE_LISTENING: {
+  case TCP_SOCKET_STATE_CONNECTING: {
     if ((events & (POLLRDNORM | POLLWRNORM)) != 0)
       return __wasilibc_poll_add(state, events, tcp_pollable(socket));
     break;
   }
 
+  case TCP_SOCKET_STATE_LISTENING: {
+    // Listening sockets can only be ready to read, not write.
+    if ((events & POLLRDNORM) != 0)
+      return __wasilibc_poll_add(state, events, tcp_pollable(socket));
+    break;
+  }
+
   case TCP_SOCKET_STATE_CONNECTED: {
     if ((events & POLLRDNORM) != 0) {
       if (__wasilibc_poll_add_input_stream(
@@ -1198,7 +1204,14 @@ static int tcp_poll_register(void *data, poll_state_t *state, short events) {
 static int tcp_poll_finish(void *data, poll_state_t *state, short events) {
   tcp_socket_t *socket = (tcp_socket_t *)data;
 
-  if (socket->state.tag != TCP_SOCKET_STATE_CONNECTING) {
+  switch (socket->state.tag) {
+  case TCP_SOCKET_STATE_CONNECTING:
+    break;
+  case TCP_SOCKET_STATE_LISTENING:
+    // Listening sockets can only be ready to read, not write.
+    __wasilibc_poll_ready(state, events & POLLRDNORM);
+    return 0;
+  default:
     __wasilibc_poll_ready(state, events);
     return 0;
   }

test/src/poll-connect.c

@@ -99,6 +99,9 @@ int main() {
     }
 
     if (server_client == -1 && fds[1].revents) {
+      // A listening socket should be readable but not writable:
+      ASSERT((fds[1].revents & POLLRDNORM) != 0);
+      ASSERT((fds[1].revents & POLLWRNORM) == 0);
       errno = 0;
       server_client =
           accept(server, (struct sockaddr *)&client_address, &client_len);