server: Always prefer synchronous I/O in nonblocking mode.

foobar2000.exe's UPnP Media Renderer component (foo_out_upnp.dll)
expects that, if a select() call completes successfully with a non-empty
writefds set, any immediately following send() call on a socket in the
writefds set never fails with WSAEWOULDBLOCK.

On Wine, the Winsock select() and send() implementations both call the
Unix poll(2) under the hood to test if I/O is possible on the socket.
As it turns out, it's entirely possible that Linux poll() may yield
POLLOUT on the first call (by select) but *not* the second (by send),
even if no send() call has been made in the meanwhile.

On Linux (as of v5.19), a connected (ESTABLISHED) TCP socket that has
not been shut down indicates (E)POLLOUT only if the ratio of
sk_wmem_queued (the amount of bytes queued in the send buffer) to
sk_sndbuf (the size of send buffer size itself, which can be retrieved
via SO_SNDBUF) is below a certain threshold.  Therefore, a falling edge
in POLLOUT can be triggered due to a number of reasons:

1. TCP fragmentation.  Once a TCP packet is split out from a larger
   sk_buff, it incurs extra bookkeeping overhead (e.g. sk_buff header)
   that is counted in sk_wmem_queued alongside application data.
   See also: tcp_fragment(), tso_fragment() (Linux 5.19).

2. Control packets (e.g. MTU probing).  Such packets share the same
   buffer with application-initiated packets, and thus counted in
   sk_wmem_queued.
   See also: sk_wmem_queued_add() callers (Linux 5.19).

3. Memory pressure.  This causes sk_sndbuf to shrink.
   See also: sk_stream_moderate_sndbuf() callers (Linux 5.19).

Fix this by always attempting synchronous I/O first if req->force_async
is unset and the nonblocking flag is set.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=53486

server/sock.c

@@ -3468,11 +3468,19 @@ DECL_HANDLER(recv_socket)
          * asyncs will not consume all available data; if there's no data
          * available, the current request won't be immediately satiable.
          */
-        if (check_fd_events( sock->fd, req->oob && !is_oobinline( sock ) ? POLLPRI : POLLIN ))
+        if ((!req->force_async && sock->nonblocking) ||
+            check_fd_events( sock->fd, req->oob && !is_oobinline( sock ) ? POLLPRI : POLLIN ))
         {
             /* Give the client opportunity to complete synchronously.
              * If it turns out that the I/O request is not actually immediately satiable,
-             * the client may then choose to re-queue the async (with STATUS_PENDING). */
+             * the client may then choose to re-queue the async (with STATUS_PENDING).
+             *
+             * Note: If the nonblocking flag is set, we don't poll the socket
+             * here and always opt for synchronous completion first.  This is
+             * because the application has probably seen POLLIN already from a
+             * preceding select()/poll() call before it requested to receive
+             * data.
+             */
             status = STATUS_ALERTED;
         }
     }
@@ -3564,11 +3572,27 @@ DECL_HANDLER(send_socket)
          * asyncs will not consume all available space; if there's no space
          * available, the current request won't be immediately satiable.
          */
-        if (check_fd_events( sock->fd, POLLOUT ))
+        if ((!req->force_async && sock->nonblocking) || check_fd_events( sock->fd, POLLOUT ))
         {
             /* Give the client opportunity to complete synchronously.
              * If it turns out that the I/O request is not actually immediately satiable,
-             * the client may then choose to re-queue the async (with STATUS_PENDING). */
+             * the client may then choose to re-queue the async (with STATUS_PENDING).
+             *
+             * Note: If the nonblocking flag is set, we don't poll the socket
+             * here and always opt for synchronous completion first.  This is
+             * because the application has probably seen POLLOUT already from a
+             * preceding select()/poll() call before it requested to send data.
+             *
+             * Furthermore, some applications expect that any send() call on a
+             * socket that has indicated POLLOUT beforehand never fails with
+             * WSAEWOULDBLOCK.  It's possible that Linux poll() may yield
+             * POLLOUT on the first call but not the second, even if no send()
+             * call has been made in the meanwhile.  This can happen for a
+             * number of reasons; for example, TCP fragmentation may consume
+             * extra buffer space for each packet that has been split out, or
+             * the TCP/IP networking stack may decide to shrink the send buffer
+             * due to memory pressure.
+             */
             status = STATUS_ALERTED;
         }
     }