net/smtp: TestTLSClient failures with "connection reset by peer" on freebsd

[dr2chase]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.8+ (development)

What operating system and processor architecture are you using (go env)?

freebsd-amd64-gce101

What did you do?

Tried to run Trybots for an unrelated change:
https://go-review.googlesource.com/c/37333/#message-ae31a67c5f6927959d8d507bc5ae951e01fbc4e7

Observed failure, followed instructions to check https://build.golang.org

In the first column for freebsd-x64 (the left-hand one), I clicked through some of the failures in that column, and noticed the repeated occurrence of

--- FAIL: TestTLSClient (0.00s)
	smtp_test.go:610: client error: close tcp 127.0.0.1:46828->127.0.0.1:46827: connection reset by peer
FAIL
FAIL	net/smtp	0.016s

which is also what besmirched my otherwise-clean TryBot run.

[josharian]

Happened again: https://storage.googleapis.com/go-build-log/82d5c3e9/freebsd-amd64-gce101_318ec227.log. Going to mail a CL disabling the test for now.

[gopherbot]

CL https://golang.org/cl/37653 mentions this issue.

[neelance]

This test is failing with my fake networking for js/wasm. It might be flaky on freebsd/amd64 for the same reason. Here is what I found:

1. client sends SMTP "QUIT"
2. server receives "QUIT", closes TLS connection and socket
3. socket "close" propagates to client, also closes the socket at the client
4. client wants to close TLS connection, which wants to send a TLS "close notify", but the socket got already closed
5. the error bubbles up and smtp.SendMail returns it