Race with httputil.ReverseProxy

[raliste]

We are using prometheus.InstrumentHandler("test", handler) where handler is an httputil.ReverseProxy.

==================
WARNING: DATA RACE
Read by goroutine 36:
  github.com/prometheus/client_golang/prometheus.computeApproximateRequestSize()
      /Volumes/PreySecure/code/src/github.com/prometheus/client_golang/prometheus/http.go:176 +0x1eb

Previous write by goroutine 42:
  net/textproto.MIMEHeader.Set()
      /usr/local/go/src/net/textproto/header.go:22 +0x94
  net/http.Header.Set()
      /usr/local/go/src/net/http/header.go:31 +0x56
  net/http/httputil.(*ReverseProxy).ServeHTTP()
      /usr/local/go/src/net/http/httputil/reverseproxy.go:207 +0xb19
  net/http.(Handler).ServeHTTP-fm()
      /usr/local/go/src/net/http/h2_bundle.go:3847 +0x5d
  github.com/prometheus/client_golang/prometheus.InstrumentHandlerFuncWithOpts.func1()
      /Volumes/PreySecure/code/src/github.com/prometheus/client_golang/prometheus/http.go:158 +0x3c1
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/src/net/http/server.go:1618 +0x47
  main.(*Server).ServeHTTP()
      /Users/raliste/Work/PreySecure/code/src/github.preyhq.com/prey/net/upload/preyfrontend.go:204 +0x47e
  net/http.(*ServeMux).ServeHTTP()
      /usr/local/go/src/net/http/server.go:1910 +0x212
  github.preyhq.com/prey/net/frontend/go/server.(*Server).ServeHTTP()
      /Volumes/PreySecure/code/src/github.preyhq.com/prey/net/frontend/go/server/server.go:36 +0x398
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/src/net/http/server.go:2081 +0x206
  net/http.(*conn).serve()
      /usr/local/go/src/net/http/server.go:1472 +0x1565

Goroutine 36 (running) created at:
  github.com/prometheus/client_golang/prometheus.InstrumentHandlerFuncWithOpts.func1()
      /Volumes/PreySecure/code/src/github.com/prometheus/client_golang/prometheus/http.go:146 +0x1ff
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/go/src/net/http/server.go:1618 +0x47
  main.(*Server).ServeHTTP()
      /Users/raliste/Work/PreySecure/code/src/github.preyhq.com/prey/net/upload/preyfrontend.go:204 +0x47e
  net/http.(*ServeMux).ServeHTTP()
      /usr/local/go/src/net/http/server.go:1910 +0x212
  github.preyhq.com/prey/net/frontend/go/server.(*Server).ServeHTTP()
      /Volumes/PreySecure/code/src/github.preyhq.com/prey/net/frontend/go/server/server.go:36 +0x398
  net/http.serverHandler.ServeHTTP()
      /usr/local/go/src/net/http/server.go:2081 +0x206
  net/http.(*conn).serve()
      /usr/local/go/src/net/http/server.go:1472 +0x1565

Goroutine 42 (running) created at:
  net/http.(*Server).Serve()
      /usr/local/go/src/net/http/server.go:2137 +0x4d1
  net/http.(*Server).ListenAndServe()
      /usr/local/go/src/net/http/server.go:2098 +0x174
  github.preyhq.com/prey/net/frontend/go/server.ListenAndServe()
      /Volumes/PreySecure/code/src/github.preyhq.com/prey/net/frontend/go/server/server.go:50 +0x19f
  main.main()
      /Users/raliste/Work/PreySecure/code/src/github.preyhq.com/prey/net/upload/preyfrontend.go:214 +0xda
==================

[beorn7]

Thanks for reporting. This is kind of a known issue, but so far I thought it would be inappropriate to modify the header. Since the above is happening in the standard library, I cannot really claim that anymore. Looks like we are being too smart here, calculating the request size in the background. (Plus: Most people are probably interested in the timing metrics only, not in the request size metrics.)

I'll definitely document the current behavior, and I'm already working on breaking up the whole InstrumentHandler plumbing.

[beorn7]

Cf. #189