net/http: TestTransportMaxConnsPerHost flakes

[bradfitz]

I've seen this a few times now. @fraenkel, could you take a look?

https://storage.googleapis.com/go-build-log/b6c795d5/android-amd64-emu_95c3cb1f.log

--- FAIL: TestTransportMaxConnsPerHost (0.05s)
    transport_test.go:638: request failed: Get https://127.0.0.1:34180: write tcp 127.0.0.1:37680->127.0.0.1:34180: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45068: read tcp 127.0.0.1:36221->127.0.0.1:45068: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45070: read tcp 127.0.0.1:36221->127.0.0.1:45070: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45074: read tcp 127.0.0.1:36221->127.0.0.1:45074: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45066: read tcp 127.0.0.1:36221->127.0.0.1:45066: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45072: read tcp 127.0.0.1:36221->127.0.0.1:45072: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45069: read tcp 127.0.0.1:36221->127.0.0.1:45069: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45071: read tcp 127.0.0.1:36221->127.0.0.1:45071: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45073: read tcp 127.0.0.1:36221->127.0.0.1:45073: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45067: write tcp 127.0.0.1:36221->127.0.0.1:45067: use of closed network connection
2019/05/01 16:13:25 http: TLS handshake error from 127.0.0.1:45080: write tcp 127.0.0.1:36221->127.0.0.1:45080: use of closed network connection
FAIL
exitcode=1go_android_exec: adb exec-out rm -rf /data/local/tmp/go_android_exec/http.test-20892

[bradfitz]

Oh, and ignore the http: TLS handshake error from spam. That's an unrelated issue that needs fixing. (Tests shouldn't spam)

The concerning part here is only:

--- FAIL: TestTransportMaxConnsPerHost (0.05s)
    transport_test.go:638: request failed: Get https://127.0.0.1:34180: write tcp 127.0.0.1:37680->127.0.0.1:34180: use of closed network connection

[fraenkel]

on the plus side, I can reproduce it

$ ~/workspace/go/bin/go test -run TestTransportMaxConnsPerHost -count=100
--- FAIL: TestTransportMaxConnsPerHost (0.01s)
    transport_test.go:638: request failed: Get https://127.0.0.1:39561: write tcp 127.0.0.1:36518->127.0.0.1:39561: use of closed network connection

[fraenkel]

another oddity that randomly appears when the other paases

$ ~/workspace/go/bin/go test -run TestTransportMaxConnsPerHost -count=100 -race
--- FAIL: TestTransportMaxConnsPerHost (0.03s)
    transport_test.go:662: Too many get connections (http2): 2
    transport_test.go:676: Too many get connections (http2): 3
--- FAIL: TestTransportMaxConnsPerHost (0.03s)
    transport_test.go:662: Too many get connections (http2): 2
    transport_test.go:676: Too many get connections (http2): 3
FAIL

[gopherbot]

Change https://golang.org/cl/175097 mentions this issue: net/http: skip flaky TestTransportMaxConnsPerHost for now

[fraenkel]

@bradfitz I know how to make the test pass, but it is just papering over a potential issue.
The failing client is the 11th one. We have created a bit of a race between the current connection being closed and what connection the next client gets. It is quite a lot of code before MarkDead() is called in http2, and that window between Close() and MarkDead() allows any client to retrieve a connection that is closed.
The test case is simulating a network failure. I decided to try and make it cleaner by adding transport.CloseIdleConnections after the conn.Close(). Much to my surprise we end up in a hang at transport.go: line 1030.

[fraenkel]

I don't see how to fix the CloseIdleConnections which would have fixed the testcase.
The transport tracks host connection counts. When we close idle connections, the counts are not corrected for http2. We don't know the connect method key, nor do we know which connections are being closed on the http2 side.

[fraenkel]

I spoke too soon. We have the connection key and we can remove the host connection count. However, we really have no idea if the http2 connection is really closed or not. But we have marked the persistConn has broken regardless.

[fraenkel]

I have a fix coming. The definition of MaxConnPerHost once you CloseIdleConnections for http/2 is not always exact but it does attempt to do the right thing.

[gopherbot]

Change https://golang.org/cl/174950 mentions this issue: net/http: TestTransportMaxConnsPerHost flakes