On 4/13/2021 10:47 AM, Ken Brown via Cygwin wrote:
On 4/13/2021 10:06 AM, sten.kristian.ivars...@gmail.com wrote:
Hi Ken
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 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.
I finally got some time to test topic/af_unix in our "real" cygwin-application
(casual) and unfortunately very few of our unittests pass
The symptoms are that there's unexpected eternal blocking, sometimes there's
unexpected EADDRNOTAVAIL, sometimes it looks like some memory corruption (and
core-dumps)
Of course the memory corruption etc could be our self and the core-dumps might
be because of uncaught exceptions
Needles to say is that all unittests pass on Linux, but of course
cygwin-topic/af_unix could act according to POSIX-standard and the behaviour
couldbe due to our own misinterpretation of how POSIX works
More likely it's due to bugs in the topic/af_unix branch. This is still very
much a work in progress.
I will try to narrow down the quite complex logic and reproduce the problems
That would be ideal.
If you of some reason wanna try it with casual, I'd be glad to help you out
(it should be easier now that last time (but there might be some documentation
missing for Cygwin still))
https://bitbucket.org/casualcore/
I'm going on vacation in a few days, but I might do this when I get back.
Thanks for your testing.
By the way, if your code is using datagram sockets, then there are very serious
problems with our implementation (even aside from the performance issue that
we've already discussed). For example, I don't know of any reasonable way for
select to test whether such a socket is ready for writing. We'll need to solve
that somehow.
Ken
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