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[87.249.172.112]) by smtp.gmail.com with ESMTPSA id u6sm302366lfo.188.2021.04.08.01.37.28 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Thu, 08 Apr 2021 01:37:28 -0700 (PDT) From: To: "'Ken Brown'" , References: <04cc01d71ffa$7d1e6cf0$775b46d0$@gmail.com> <00d901d7208e$97c05c50$c74114f0$@gmail.com> <860668bf-8cf9-0969-6a01-7fbf8b782db1@cornell.edu> <000901d72607$55dc5a90$01950fb0$@gmail.com> <3346cd1c-b93f-83c4-ff26-553ac95ec692@cornell.edu> <7c21a430-9609-7fd4-1a02-8b7c1978d2f8@cornell.edu> <001901d72af4$4009cd50$c01d67f0$@gmail.com> <134074c1-4c0b-0842-b88b-536a1ed4aefe@cornell.edu> In-Reply-To: <134074c1-4c0b-0842-b88b-536a1ed4aefe@cornell.edu> Subject: RE: AF_UNIX/SOCK_DGRAM is dropping messages Date: Thu, 8 Apr 2021 10:37:29 +0200 Message-ID: <002101d72c52$695ea630$3c1bf290$@gmail.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_NextPart_000_0022_01D72C63.2CE83980" X-Mailer: Microsoft Outlook 16.0 Content-Language: en-se Thread-Index: AQIPffBCgY7dkx32YYBd4buxXBOzegICwCl2At957CQCAh4QbgK/qZQ0Aiflzi4DDsW9ugMOPnyiAg8iLcGpmeMO0A== X-Spam-Status: No, score=-1.7 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: cygwin@cygwin.com X-Mailman-Version: 2.1.29 Precedence: list List-Id: General Cygwin discussions and problem reports List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 08 Apr 2021 08:37:32 -0000 This is a multipart message in MIME format. ------=_NextPart_000_0022_01D72C63.2CE83980 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable > >>>>>>>> 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=E2=80=99t 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=3D"-g -O0 -D__WITH_AF_UNIX" -- > prefix=3D... > >>> > >>>> 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 na=C3=AFve 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 >=20 > 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 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. >=20 > Ken ------=_NextPart_000_0022_01D72C63.2CE83980 Content-Type: text/plain; name="af_unix.cpp" Content-Transfer-Encoding: quoted-printable Content-Disposition: attachment; filename="af_unix.cpp" #include #include #undef AF_UNIX #define AF_UNIX 31 #include #include #include #include #include #include // $ g++ --std=3Dgnu++17 af_unix.cpp const char* const path =3D "address"; const int count =3D 10; const int size =3D BUFSIZ * 1; int client() { const int fd =3D socket( AF_UNIX, SOCK_DGRAM, 0); if( fd =3D=3D -1) { perror( "socket error"); return -1; } struct sockaddr_un address{}; strcpy( address.sun_path, path); address.sun_family =3D AF_UNIX; char buffer[size] =3D {}; for( int idx =3D 0; idx < count; ++idx) { memcpy( buffer, &idx, sizeof idx); fprintf( stdout, "client idx: %d\n", idx); =20 const ssize_t result =3D sendto( fd, buffer, size, 0, (struct = sockaddr*)&address, sizeof address); // Assume the whole chunk can be sent if( result !=3D size) { perror( "sendto error"); return -1; } } close( fd); return 0; } int server() { const int fd =3D socket( AF_UNIX, SOCK_DGRAM, 0); if( fd =3D=3D -1) { perror( "socket error"); return -1; } struct sockaddr_un address{}; strcpy( address.sun_path, path); address.sun_family =3D AF_UNIX; const int result =3D bind( fd, (struct sockaddr*)&address, sizeof = address); if( result =3D=3D -1) { perror( "bind error"); return -1; } return fd; } int main( int argc, char* argv[]) { const int fd =3D server( ); if( fd !=3D -1) { fprintf( stdout, "%d\tnumber of packages\n", count); fprintf( stdout, "%d\tbytes per package\n", size); std::thread{ [&](){client( );}}.detach(); std::this_thread::sleep_for( std::chrono::microseconds( 250)); =20 char buffer[size] =3D {}; for( int idx =3D 0; idx < count; ++idx) { fprintf( stdout, "server idx: %d\n", idx); const ssize_t result =3D recv( fd, buffer, size, = MSG_DONTWAIT); // Assume the whole chunk can be read if( result !=3D size) { perror("recv error"); unlink( path); return -1; } int index =3D 0; memcpy( &index, buffer, sizeof idx); if( index !=3D idx) { fprintf( stderr, "expected %d but got %d\n", idx, = index); unlink( path); return -1; } } close( fd); unlink( path); } return 0; } ------=_NextPart_000_0022_01D72C63.2CE83980--