net/http: occasional timeouts in TestTransportProxyHTTPSConnectTimeout

[bcmills]

net/http.TestTransportProxyHTTPSConnectTimeout is occasionally failing on multiple builders.

The test was newly added in CL 210286 (#28012). These failures are after CL 210738, so they aren't symptoms of the bug reported in #36070.

It's not obvious to me whether the problem is in the test itself, or in the code under test.

--- FAIL: TestTransportProxyHTTPSConnectTimeout (5.06s)
    transport_test.go:1496: timeout waiting for Transport to close its connection to the proxy
    transport_test.go:1457: Accept: accept tcp 127.0.0.1:34137: use of closed network connection
FAIL
FAIL	net/http	20.370s

2019-12-11T01:10:28-a1a67e6/linux-ppc64-buildlet
2019-12-11T00:01:17-9c8c27a/linux-arm64-packet
2019-12-10T21:11:20-207a0b7/linux-arm64-packet
2019-12-10T21:11:20-207a0b7/solaris-amd64-oraclerel

CC @bradfitz @ianlancetaylor

[bcmills]

I wonder if these builders are just too heavily loaded: perhaps the 5-second timeout is too short.

(The second log line is a secondary symptom of the test timeout: the listener is closed in a deferred call when the test function returns.)

[bcmills]

I'm unable to reproduce this locally using go test -run=TestTransportProxyHTTPSConnectTimeout -count=1000 on a linux/amd64 system with 6 cores.

[bcmills]

Ah, here's a theory: perhaps the 50ms timeout set on the Client in the test is too short. If I crank it down to 1ms, I can reproduce the failure locally.

Perhaps we just need to replace the timeout with synchronization and cancellation.

[gopherbot]

Change https://golang.org/cl/210857 mentions this issue: net/http: use cancellation instead of a timeout in TestTransportProxyHTTPSConnectTimeout