net/http: flaky TestTransportHeadChunkedResponse

[bradfitz]

Noticed at http://build.golang.org/log/c4295893036b8f63339f2fdcfa6a7f052edbdfa4

--- FAIL: TestTransportHeadChunkedResponse-4 (0.02s)
    transport_test.go:611: ip/ports differed between head requests: "127.0.0.1:54643" vs "127.0.0.1:54644"
FAIL
FAIL    net/http    10.889s

The test is simply:

        defer afterTest(t)
        ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
                if r.Method != "HEAD" {
                        panic("expected HEAD; got " + r.Method)
                }
                w.Header().Set("Transfer-Encoding", "chunked") // client should ignore                                                     
                w.Header().Set("x-client-ipport", r.RemoteAddr)
                w.WriteHeader(200)
        }))
        defer ts.Close()

        tr := &Transport{DisableKeepAlives: false}
        c := &Client{Transport: tr}

        res1, err := c.Head(ts.URL)
        if err != nil {
                t.Fatalf("request 1 error: %v", err)
        }
        res2, err := c.Head(ts.URL)
        if err != nil {
                t.Fatalf("request 2 error: %v", err)
        }
        if v1, v2 := res1.Header.Get("x-client-ipport"), res2.Header.Get("x-client-ipport"); v1 != v2 {
                t.Errorf("ip/ports differed between head requests: %q vs %q", v1, v2)
        }
}

Perhaps the first Head returns but the second Head call starts before the background Transport goroutines get scheduled and register that the connection is idle.

[tzneal]

Review at https://go-review.googlesource.com/#/c/11170/

[rsc]

Dup of #11250, closed.