[Bug 260055] recvmsg / IP_RECVDSTADDR issue

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=260055

--- Comment #2 from Eugene Grosbein <eugen@freebsd.org> ---
(In reply to Konstantin Belousov from comment #1)

mpd5 uses additional threads to talk with RADIUS server only and these
additional threads are short-lived. The output of "thread apply all bt" in this
PR was not redacted, so there was only single thread 1 at the moment.

The socket is IPv4 UDP created by incoming L2TP over UDP request. It is in
blocking read mode.

> That said, GetSockDstAddress() is strange.  From its name, it seems
that the purpose of the function is to obtain the destination address,
as the control message. But it also tries to read some data from the
socket, and the data is discarded.

I coded the function GetSockDstAddress(). It is called just once after socket
creation in case L2TP server "self" address not specified in the mpd.conf

You are right, the purpose is to receive control message with destination
address from the kernel and it is my first attempt to programm this. I believed
this is right way to do that without reading payload data from the socket. 

Maybe there is some application logic error if some payload data is discarded,
but recvmsg() still has control message to return because of IP_RECVDSTADDR
socket option, hasn't it? So it should not block just after first UDP datagramm
arrived, I believe.

I am interested in fixing the problem and will add more information you
requested. However, I need also to minimize impact on this production server,
so please show exact command I should use to catch the kernel-side backtrace.
This my machine still uses FreeBSD 11.4-STABLE/i386 r365547 (September 2020).

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