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net/http/httputil: ReverseProxy FlushInterval no longer flushes headers in go1.12 #31125

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liggitt opened this issue Mar 29, 2019 · 6 comments

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@liggitt
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commented Mar 29, 2019

What version of Go are you using (go version)?

$ go version
go version go1.12.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/liggitt/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/liggitt/go"
GOPROXY=""
GORACE=""
GOROOT="/Users/liggitt/.gvm/gos/go1.12.1"
GOTMPDIR=""
GOTOOLDIR="/Users/liggitt/.gvm/gos/go1.12.1/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/h8/3dzrjpfj76d93vbhc5f2r8tw00kjgb/T/go-build963504992=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

  1. Set up a reverse proxy with a flush interval.
  2. Proxied to a backend that writes and flushes headers, then streams response chunks (which can be arbitrarily delayed from the original header write/flush).

What did you expect to see?

Clients making requests to the proxy should see the headers from the backend no later than the FlushInterval

What did you see instead?

Clients making requests to the proxy receive no data until the first body chunk is written.

https://play.golang.org/p/Y8KQO8-57MR

  • go1.11 works properly
  • go1.12 hangs

The change in 5440bfc#diff-d863507a61be206d112f6e00e6d812a2L343 removed the goroutine that ensured the FlushInterval was honored while blocking in copyResponse, even prior to the first Write() call being encountered. The time.After() ticker it sets up doesn't get set up until the first Write() call is made.

@gopherbot

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commented Mar 29, 2019

Change https://golang.org/cl/170066 mentions this issue: net/http/httputil: make ReverseProxy flush headers on FlushInterval

@thaJeztah

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commented Mar 29, 2019

Could this be tracked for Go 1.12.2 ? (is a separate tracking issue needed for that?)

@ALTree ALTree changed the title httputil.ReverseProxy FlushInterval no longer flushes headers in go1.12 net/http/httputil: ReverseProxy FlushInterval no longer flushes headers in go1.12 Mar 29, 2019

@ALTree

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commented Mar 29, 2019

cc @bradfitz for investigation and decision regarding a possible backport

is a separate tracking issue needed for that?

In general, after the issue has been investigated, a team member will decide if it qualifies for a backport. Then a backport issue will be opened.

@PaulSD

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commented Mar 29, 2019

Work-around for Go 1.12:

func HttpHandler(w http.ResponseWriter, req *http.Request) {
  ...
  proxy.ServeHTTP(NewFlushResponseWriter(w), req)
}

type flushResponseWriter struct {
  http.ResponseWriter
  http.Flusher
  http.Hijacker
  http.CloseNotifier
}
func NewFlushResponseWriter(wrap http.ResponseWriter) (http.ResponseWriter) {
  f, _ := wrap.(http.Flusher)
  h, _ := wrap.(http.Hijacker)
  cn, _ := wrap.(http.CloseNotifier)
  if f == nil || h == nil || cn == nil {
    log.Printf("WARNING: http.ResponseWriter doesn't implement all flushResponseWriter interfaces, will not flush response headers")
    return wrap
  }
  return flushResponseWriter{wrap, f, h, cn}
}
func (self flushResponseWriter) WriteHeader(statusCode int) {
  self.ResponseWriter.WriteHeader(statusCode)
  self.Flusher.Flush()
}
@bradfitz

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commented Mar 29, 2019

@gopherbot, please backport to Go 1.12.

@gopherbot

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commented Mar 29, 2019

Backport issue(s) opened: #31144 (for 1.12).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.

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