The missing UDP packets

[Raimo Niskanen]

I can reproduce it on a month old 'master'.
We'll take a look at it...

Thank you for a pinpointed bug report!
/ Raimo


On Fri, Feb 07, 2020 at 04:50:54PM +0300, Alexander Petrovsky wrote:
> >
> > From: Valentin Micic
> >
> > I’ve run a couple of tests on my own, and managed to reproduce your
> > “second! and only second!!” packet problem.
> >
> > It happens under following circumstances:
> >
> > 1) Socket (Rx) is instructed to provide a single packet as it arrives ( as
> > per {active, once} );
> >
> >
> > 2) After receiving this packet, a call to gen_udp:recv( Rx, 0, 0 ) is
> > executed before another packet has ARRIVED to the kernel buffer;
> >
> > Then the following diagram illustrates the problem:
> >
> > Note:
> >     - Tx is sending socket;
> >     - Rx is receiving socket;
> >
> > *Please note that sequence diagrams below are not completely accurate, but
> > it is reasonably illustrative of what may have happened.*
> >
> >
> >
> > The next sequence diagram depicts a situation when a non-zero value for
> > gen_udp:recv/3, e.g. gen_udp:recv( Rx, 0, *10* ):
> >
> >
> >
> >
> > Thus, it appears that gen_udp:recv( *Socket, Length*, *Timeout* ) is
> > broken when *Timeout* is set to *0*.
> > For all positive non-zero values for Timeout, it behaves properly (or as
> > intuitively expected).
> >
> > Therefore, this should either be fixed, or it should be documented in
> > order to indicate that 0 must *not* be used as a value for the Timeout in
> > gen_udp:recv/3 call.
> >
> > Aside, I fail to see a benefit of using Timeout of value 0 when combined
> > with {active, once}. The only time when zero value may appear to be useful
> > is when server does need to process other messages, e.g. messages not
> > originated by the socket, which would be the case when gen_server is used
> > to support the socket. In my view (which is supported by substantial
> > empirical evidence), gen_server is not the most optimal behaviour to use
> > when implementing support for sockets.
> >
> > Kind regards
> >
> > V/
> >
> >
> So, seems like there is some buggy behavior in the erlang VM for reading
> UDP messages between switching from {active, once} to gen_udp:recv(socket,
> 0, 0)? Lukas, Raimo could you help with that?
> 
> -- 
> Петровский Александр / Alexander Petrovsky,
> 
> Skype: askjuise
> Phone: +7 931 9877991




-- 

/ Raimo Niskanen, Erlang/OTP, Ericsson AB