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 Unix poll() may yield
POLLOUT on the first call (for select) but *not* the second (for 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 the (E)POLLOUT condition 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 retransmission and control packets (e.g. MTU probing). Such
packets share the same buffer with application-initiated packets,
and thus counted in sk_wmem_queued_add just like application data.
See also: sk_wmem_queued_add() callers (Linux 5.19).
2. 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 the nonblocking
flag is set.
Note: for diagnosis, `getsockopt(fd, SOL_SOCKET, SO_MEMINFO, ...)` can be
used to retrieve both sk_wmem_queued (the amount of bytes queued in the
send buffer) and sk_sndbuf (the size of the send buffer itself, which
can also be retrieved via SO_SNDBUF).
--
https://gitlab.winehq.org/wine/wine/-/merge_requests/617
Signed-off-by: Nikolay Sivov <nsivov(a)codeweavers.com>
--
v2: evr/tests: Add a test to show that IMFVideoDisplayControl is accessible.
evr: Implement lazy initialization with mixer and presenter instances.
evr: Add IMFVideoRenderer stub to the filter.
evr: Add IMFGetService stub.
https://gitlab.winehq.org/wine/wine/-/merge_requests/681