ThreadSanitizer says:

WARNING: DATA RACE
Write by goroutine 374:
  net/http_test.BenchmarkServerHijack()
      src/pkg/net/http/serve_test.go:2413 +0x34d
  testing.(*B).runN()
      src/pkg/testing/benchmark.go:119 +0xc0
  testing.(*B).launch()
      src/pkg/testing/benchmark.go:207 +0x1ba

Previous read by goroutine 110:
  net/http_test.(*oneConnListener).Accept()
      src/pkg/net/http/serve_test.go:42 +0x59
  net/http.(*Server).Serve()
      src/pkg/net/http/server.go:1618 +0xa1
  net/http.Serve()
      src/pkg/net/http/server.go:1557 +0xac

Goroutine 374 (running) created at:
  testing.(*B).run()
      src/pkg/testing/benchmark.go:170 +0x3c
  testing.RunBenchmarks()
      src/pkg/testing/benchmark.go:303 +0x6ba
  testing.Main()
      src/pkg/testing/testing.go:411 +0x216
  main.main()
      /tmp/builder/go-build794329701/net/http/_test/_testmain.go:425 +0xdc

Goroutine 110 (finished) created at:
  net/http_test.BenchmarkServerHijack()
      src/pkg/net/http/serve_test.go:2414 +0x419
  testing.(*B).runN()
      src/pkg/testing/benchmark.go:119 +0xc0
  testing.(*B).launch()
      src/pkg/testing/benchmark.go:207 +0x1ba

http://build.golang.org/log/3de092081861c2a60d12cb21a264c24566d5e57c