check for WSAEMSGSIZE errors when receiving UDP packets on Windows

[marten-seemann]

Fixes #3956. @thepaul, could you give this patch a try?

[codecov]

Codecov Report

Merging #3982 (ab84587) into master (56cd866) will increase coverage by 0.07%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #3982      +/-   ##
==========================================
+ Coverage   82.92%   82.99%   +0.07%     
==========================================
  Files         146      146              
  Lines       14707    14801      +94     
==========================================
+ Hits        12195    12284      +89     
- Misses       2022     2030       +8     
+ Partials      490      487       -3     

Impacted Files	Coverage Δ
transport.go	63.43% <33.33%> (-0.48%)	⬇️
send_queue.go	100.00% <100.00%> (ø)
sys_conn_df_linux.go	41.51% <100.00%> (+1.12%)	⬆️
sys_conn_df_windows.go	62.50% <100.00%> (+5.36%)	⬆️

... and 3 files with indirect coverage changes

[sukunrt]

For windows this looks okay.
What happens on linux here? https://man7.org/linux/man-pages/man3/recvfrom.3p.html says

If a message is too long to fit in the supplied
buffer, and MSG_PEEK is not set in the flags argument, the excess
bytes shall be discarded.

Is this fine?

You might have to do handling for macOS since we merged #3946

[thepaul]

Fixes #3956. @thepaul, could you give this patch a try?

This does indeed fix the problem, according to my tests.

You might have to do handling for macOS since we merged #3946

No changes should be necessary- the DF bit on outgoing datagrams shouldn't affect recvfrom(). It would affect things if we were using recvmsg() instead, and able to receive MSG_TRUNC control messages, but quic-go isn't there yet I think.

[sukunrt]

@thepaul yes, there's no real change involved here. My comment was related to changes in sys_conn_df.go done in https://github.com/quic-go/quic-go/pull/3946/files

That commit changed the build flags on sys_conn_df.go and introduced sys_conn_df_darwin.go. An older version of these files is modified in this PR.

[thepaul]

Oh yes, I did read that change- sorry I wasn't clear! That change seems to amount to "set the DF bit on outgoing packets", and my claim is that that won't affect what is received with recvfrom().

[sukunrt]

This PR splits isMsgSizeErr into isSendMsgSizeErr and isRecvMsgSizeErr. That change needs to be made in this file too https://github.com/quic-go/quic-go/blob/master/sys_conn_df_darwin.go

[marten-seemann]

This PR splits isMsgSizeErr into isSendMsgSizeErr and isRecvMsgSizeErr. That change needs to be made in this file too https://github.com/quic-go/quic-go/blob/master/sys_conn_df_darwin.go

I see. This PR was based on an older commit, that's why CI succeeded on push, but failed on pull_request. I rebased and fixed.