net/http: An empty reply causes false race conditions (go1.3beta1+)

[gopherbot]

by travis.bischel:

What does 'go version' print?
go version devel +f8b50ad4cac4 Mon Apr 21 17:00:27 2014 -0700 + linux/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

This can be done with three files:
http://play.golang.org/p/bxMjv7H1cw
1. create server.go and `go run` it
2. create race.go and race_test.go in a different directory
3. go test -race

What happened?
$ go test -race
==================
WARNING: DATA RACE
Write by goroutine 6:
  strings.(*Reader).Read()
      /usr/local/go/src/pkg/strings/reader.go:32 +0x6f
  go.(*struct { *strings.Reader; io.Closer }).Read()
      /usr/local/go/src/pkg/net/http/chunked.go:1 +0x90
  net/http.(*bodyEOFSignal).Read()
      /usr/local/go/src/pkg/net/http/transport.go:1123 +0x278
  io/ioutil.devNull.ReadFrom()
      /usr/local/go/src/pkg/io/ioutil/ioutil.go:151 +0xda
  io.Copy()
      /usr/local/go/src/pkg/io/io.go:349 +0x156
  _/home/twmb/testing.request()
      /home/twmb/testing/race.go:16 +0x1aa

Previous write by goroutine 7:
  strings.(*Reader).Read()
      /usr/local/go/src/pkg/strings/reader.go:32 +0x6f
  go.(*struct { *strings.Reader; io.Closer }).Read()
      /usr/local/go/src/pkg/net/http/chunked.go:1 +0x90
  net/http.(*bodyEOFSignal).Read()
      /usr/local/go/src/pkg/net/http/transport.go:1123 +0x278
  io/ioutil.devNull.ReadFrom()
      /usr/local/go/src/pkg/io/ioutil/ioutil.go:151 +0xda
  io.Copy()
      /usr/local/go/src/pkg/io/io.go:349 +0x156
  _/home/twmb/testing.request()
      /home/twmb/testing/race.go:16 +0x1aa

Goroutine 6 (running) created at:
  _/home/twmb/testing.RaceTest()
      /home/twmb/testing/race.go:28 +0x94
  _/home/twmb/testing.TestRaceTest()
      /home/twmb/testing/race_test.go:8 +0x2b
  testing.tRunner()
      /usr/local/go/src/pkg/testing/testing.go:422 +0x10f

Goroutine 7 (finished) created at:
  _/home/twmb/testing.RaceTest()
      /home/twmb/testing/race.go:29 +0xc3
  _/home/twmb/testing.TestRaceTest()
      /home/twmb/testing/race_test.go:8 +0x2b
  testing.tRunner()
      /usr/local/go/src/pkg/testing/testing.go:422 +0x10f
==================
PASS
Found 1 data race(s)
exit status 66
FAIL    _/home/twmb/testing 1.028s

What should have happened instead?
No race condition.

Please provide any additional information below.
https://code.google.com/p/go/source/browse/src/pkg/net/http/response.go#233
https://code.google.com/p/go/source/browse/src/pkg/net/http/server.go#1977

[gopherbot]

Comment 1 by travis.bischel:

This is specifically only for empty responses.

[ianlancetaylor]

Comment 2:

Labels changed: added repo-main, release-go1.3maybe.

[dvyukov]

Comment 3:

Labels changed: added threadsanitizer.

[bradfitz]

Comment 4:

This is a regression from https://code.google.com/p/go/source/detail?r=358e2b416518#
The Go http package contains this Body ReadCloser:
// eofReader is a non-nil io.ReadCloser that always returns EOF.
// It embeds a *strings.Reader so it still has a WriteTo method
// and io.Copy won't need a buffer.
var eofReader = &struct {
    *strings.Reader
    io.Closer
}{
    strings.NewReader(""),
    ioutil.NopCloser(nil),
}
That used to be safe to read from by multiple goroutines. Now that always-EOF Read is
also a write.

Owner changed to @bradfitz.

Status changed to Accepted.

[gopherbot]

Comment 5:

CL https://golang.org/cl/94740044 mentions this issue.

[bradfitz]

Comment 6:

This issue was closed by revision 13ea1fd.

Status changed to Fixed.