AF_UNIX/SOCK_DGRAM is dropping messages

sten.kristian.ivarsson@gmail.com sten.kristian.ivarsson@gmail.com
Thu Apr 8 19:47:28 GMT 2021


> > >>>>>>>> Using AF_UNIX/SOCK_DGRAM with current version (3.2.0) seems
> > to
> > >>>>>>>> drop messages or at least they are not received in the same
> > >>>>>>>> order they are  sent
> > >>>>
> > >>>> [snip]
> > >>>>
> > >>>>> Thanks for the test case.  I can confirm the problem.  I'm not
> > >>>>> familiar enough with the current AF_UNIX implementation to debug
> > >>>>> this easily.  I'd rather spend my time on the new implementation
> > >>>>> (on the topic/af_unix branch).  It turns out that your test case
> > >>>>> fails there too, but in a completely different way, due to a bug
> > >>>>> in sendto for datagrams.  I'll see if I can fix that bug and then try
> again.
> > >>>>>
> > >>>>> Ken
> > >>>>
> > >>>> Ok, too bad it wasn't our own code base but good that the "mystery"
> > >>>> is verified
> > >>>>
> > >>>> I finally succeed to build topic/af_unix (after finding out what
> > >>>> version of zlib was needed), but not with -D__WITH_AF_UNIX to
> > >>>> CXXFLAGS though and thus I haven’t tested it yet
> > >>>>
> > >>>> Is it sufficient to add the define to the "main" Makefile or do
> > >>>> you have to add it to all the Makefile:s ? I guess I can find out
> > >>>> though
> > >>>
> > >>> I do it on the configure line, like this:
> > >>>
> > >>>    ../af_unix/configure CXXFLAGS="-g -O0 -D__WITH_AF_UNIX" --
> > prefix=...
> > >>>
> > >>>> Is topic/af_unix fairly up to date with master branch ?
> > >>>
> > >>> Yes, I periodically cherry-pick commits from master to topic/af_unix.
> > >>> I'lldo that again right now.
> > >>>
> > >>>> Either way, I'll be glad to help out testing topic/af_unix
> > >>>
> > >>> Thanks!
> > >>
> > >> I've now pushed a fix for that sendto bug, and your test case runs
> > >> without error on the topic/af_unix branch.
> > >
> > > It seems like the test-case do work now with topic/af_unix in
> > > blocking mode, but when using non-blocking (with MSG_DONTWAIT) there
> > > are
> > some
> > > issues I think
> > >
> > > 1. When the queue is empty with non-blocking recv(), errno is set to
> > > EPIPE but I think it should be EAGAIN (or maybe the pipe is getting
> > > broken for real of some reason ?)
> > >
> > > 2. When using non-blocking recv() and no message is written at all,
> > > it seems like recv() blocks forever
> > >
> > > 3. Using non-blocking recv() where the "client" does send less than
> > > "count" messages, sometimes recv() blocks forever (as well)
> > >
> > >
> > > My naïve analysis of this is that for the first issue (if any) the
> > > wrong errno is set and for the second issue it blocks if no sendto()
> > > is done after the first recv(), i.e. nothing kicks the "reader thread"
> > > in the butt to realise the queue is empty. It is not super clear
> > > though what POSIX says about creating blocking descriptors and then
> > > using non-blocking-flags with recv(), but this works in Linux any
> > > way
> >
> > The explanation is actually much simpler.  In the recv code where a
> > bound datagram socket waits for a remote socket to connect to the
> > pipe, I simply forget to handle MSG_DONTWAIT.  I've pushed a fix.  Please
> retest.
> 
> I tested it and now it seems like we get EAGAIN when there's no msg on the
> queue, but it seems like the client is blocked as well and that it cannot write
> any more messages until it is consumed by the server, so the af_unix.cpp test-
> client end prematurely
> 
> If using sendto() with MSG_DONTWAIT as well, that is getting a EAGAIN, but
> the socket in it self is not a non-blocking socket, it is just the recv() that is done
> in a non-blocking fashion
> 
> As I said earlier, it's a bit fuzzy (or at least for me) what POSIX mean by
> non/blocking descriptors combined with non/blocking operations, but as far
> as I understand, it should be possible to use blocking sendto()and messages
> should be written (as long as some buffer is not filled) at the same time
> someone is doing non-blocking recv()
> 
> What is your take on this ?

I was thinking of this again and came to the conclusion that the fix semantically probably works ok

It was just me that didn't realise that only one message can be on the queue simultaneously even in blocking mode

The problem is not functional but merely a performance hog, that I guess you have already realised and you mentioned it in previous message but I guess I thought it was about some other issue


So, I guess the fix works ok (I haven't done any more tests than with the sample program), but I guess out of an throughput aspect I guess it would be a good idea to let more messages be written to the queue before the first is consumed or so (I guess you already have some thoughts about this?)

Keep up the good work,
Kristian


> > I should add that in all my work so far on the topic/af_unix branch,
> > I've thought mainly about stream sockets.  So there may still be
> > things remaining to be implemented for the datagram case.
> >
> > Ken



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