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
a115feab9c