Notice (2018-05-24): bugzilla.xamarin.com is now in
Please join us on
Visual Studio Developer Community and in the
Mono organizations on
GitHub to continue tracking issues. Bugzilla will remain
available for reference in read-only mode. We will continue to work
on open Bugzilla bugs, copy them to the new locations
as needed for follow-up, and add the new items under Related
Our sincere thanks to everyone who has contributed on this bug
tracker over the years. Thanks also for your understanding as we
make these adjustments and improvements for the future.
Please create a new report for Bug 39332 on
GitHub or Developer Community if you have new
information to add and do not yet see a matching new report.
If the latest results still closely match this report, you can use the
In special cases on GitHub you might also want the comments:
GitHub Markdown with public comments
Mono versions 220.127.116.11, 18.104.22.168, and 22.214.171.1247
We use some SimpleSNMP code to talk to an uninterrupt-able power supply, and in automated testing we have a simulated SNMP server class that listens to a UDP socket and responds to requests with simulated SNMP responses. Rarely, but reproducible, the client side SimpleSNMP code would hang in socket.ReceiveFrom(). Receive timeout is not respected and the client thread hangs.
Instead use socket.Connect(), then socket.Send() and socket.Receive(). If I use those variants then I cannot reproduce the problem. So I have a workaround for now, but I bet most people writing or porting UDP code are going to default to SendTo/ReceiveFrom for those kinds of sockets since Connect() is more of a TCP pattern.
The client thread is basically:
- create socket and set Send/Receive timeouts
- sock.SendTo() to send a 42 byte SNMP request packet
- sock.ReceiveFrom() to get the response
The server thread is basically:
- server = new UdpClient() and setup send/receive timeouts on the underlying Client socket
- await UdpClient.ReceiveAsync();
- for each request received, sends response with server.Send()
They communicate over a localhost port.
Sometimes I can loop this for 100,000 packets without issue. But more often we'll get the hang. Sometimes the hang occurs within just a few thousand iterations and it doesn't seem to fail more often with more iterations, feels more like it is racy / randomly distributed when it will hang.
Also oddly, when the problem occurs the server thread sees that same request packet every time it calls await UdpClient.ReceiveAsync() and basically spin locks getting the same packet over and over. So when the client wedges in this way, the receiver wedges in a different way.
The received packets on the server side have one difference once the problem occurs. Normally they would see the 42 byte long packets at the server, even though the buffer used with SendTo() was larger, because SendTo was called with a length param of 42. But once the problem occurs, the server starts to get packets that are the same size as the buffer used in SendTo() (e.g. 1024 bytes) in spite of the length parameter of 42. Changing the sent packet size to 42 to match the length parameter doesn't impact the problem, however. Just mentioning it in case that is a clue.
I'll attach a test application which reproduces the problem. Calling the command line app with param 100000 to run that many packets round trip usually reproduces the problem for us. The test application also allows an optional parameter to introduce a delay with each packet response, and that doesn't seem to impact the problem. Small delays in responding with the server thread will still reproduce the problem. Long delays will successfully trigger the client side receive timeout on the socket. So I can see that's working, except when the problem occurs where the client side receive will hang.
Created attachment 15241 [details]
Simple command line app to reproduce the problem
Usage for this command line app:
UdpMonoTest.exe <connect/sendto> <iterations> <optional delay_ms>
Calling it with "sendto" will use the SendTo/ReceiveFrom variants that repro the problem for me. Using "connect" uses the connect alternative pattern. Generally I would see the issue within 100000 iterations. And the delay_ms is on the server side, introducing a small delay between receiving a request packet and responding back to the client.