net: TestUnixAndUnixpacketServer flaky on linux

[bcmills]

https://build.golang.org/log/09fce21c31d8c4435fb99a4f7c0e8a1bb2bf900f and
https://build.golang.org/log/faa4560674d8e7d406b7031bfe985db237b9860a:

--- FAIL: TestUnixAndUnixpacketServer (0.00s)
    server_test.go:197: #3: EOF
FAIL
FAIL	net	8.796s

See previously #13205 (CC @bradfitz).

[bradfitz]

/cc @mikioh

[bcmills]

Here's one on linux-amd64-jessie:
https://build.golang.org/log/3845a3972f455aadd7d2a606928e7ba68d8fd2fd

[bcmills]

linux-s390x-ibm: https://build.golang.org/log/895e752f114ea5e0b3e13598c5615957274122a6

[bigmikes]

TestUnixAndUnixpacketServer makes use of transponder function for the echo server. After some investigation I noticed that the test become stable if connection is not closed. I basically commented the below defer c.Close() line.

go/src/net/mockserver_test.go

Lines 222 to 226 in a6a7b14

	return
	}
	defer c.Close()
	network := ln.Addr().Network()

I am not sure why this is happening. In theory the Close should take care of flushing all the data of socket before actually closing it.

Is it possible that for really unlucky cases the connection gets closed before the client is able to read the data?