transport: hide Stream string when stored in context

[heyitsanthony]

It is not thread-safe to call context.String() on any context with a
stream value since valueCtx will use %#v to access all of the Stream
fields without holding a lock. Instead, use "" as the GoString.

[heyitsanthony]

The data race that's showing up:

WARNING: DATA RACE
Write at 0x00c423dbbcf9 by goroutine 683:
  google.golang.org/grpc/transport.(*http2Server).handleData()
      /home/jenkins/workspace/etcd-proxy/gopath/src/google.golang.org/grpc/transport/http2_server.go:379 +0x3aa
  google.golang.org/grpc/transport.(*http2Server).HandleStreams()
      /home/jenkins/workspace/etcd-proxy/gopath/src/google.golang.org/grpc/transport/http2_server.go:287 +0xa15
  google.golang.org/grpc.(*Server).serveStreams()
      /home/jenkins/workspace/etcd-proxy/gopath/src/google.golang.org/grpc/server.go:451 +0x1db
  google.golang.org/grpc.(*Server).serveNewHTTP2Transport()
      /home/jenkins/workspace/etcd-proxy/gopath/src/google.golang.org/grpc/server.go:438 +0x4e3
  google.golang.org/grpc.(*Server).handleRawConn()
      /home/jenkins/workspace/etcd-proxy/gopath/src/google.golang.org/grpc/server.go:415 +0x5e7

Previous read at 0x00c423dbbcf8 by goroutine 422:
  reflect.typedmemmove()
      /usr/local/go/src/runtime/mbarrier.go:253 +0x0
  reflect.packEface()
      /usr/local/go/src/reflect/value.go:112 +0x11c
  reflect.valueInterface()
      /usr/local/go/src/reflect/value.go:949 +0x18c
  reflect.Value.Interface()
      /usr/local/go/src/reflect/value.go:919 +0x51
  fmt.(*pp).printValue()
      /usr/local/go/src/fmt/print.go:694 +0x3aea
  fmt.(*pp).printValue()
      /usr/local/go/src/fmt/print.go:848 +0x27dc
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:682 +0x1b1
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:998 +0x1cad
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x77
  context.(*valueCtx).String()
      /usr/local/go/src/context/context.go:471 +0x16a
  fmt.(*pp).handleMethods()
      /usr/local/go/src/fmt/print.go:596 +0x40a
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:679 +0x132
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:998 +0x1cad
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x77
  context.(*valueCtx).String()
      /usr/local/go/src/context/context.go:471 +0x16a
  fmt.(*pp).handleMethods()
      /usr/local/go/src/fmt/print.go:596 +0x40a
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:679 +0x132
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:998 +0x1cad
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x77
  context.(*valueCtx).String()
      /usr/local/go/src/context/context.go:471 +0x16a
  fmt.(*pp).handleMethods()
      /usr/local/go/src/fmt/print.go:596 +0x40a
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:679 +0x132
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:998 +0x1cad
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x77
  context.(*cancelCtx).String()
      /usr/local/go/src/context/context.go:332 +0xcc
  fmt.(*pp).handleMethods()
      /usr/local/go/src/fmt/print.go:596 +0x40a
  fmt.(*pp).printArg()
      /usr/local/go/src/fmt/print.go:679 +0x132
  fmt.(*pp).doPrintf()
      /usr/local/go/src/fmt/print.go:998 +0x1cad
  fmt.Sprintf()
      /usr/local/go/src/fmt/print.go:196 +0x77
  github.com/coreos/etcd/clientv3.(*watcher).Watch()
      /home/jenkins/workspace/etcd-proxy/gopath/src/github.com/coreos/etcd/clientv3/watch.go:250 +0x48c
  github.com/coreos/etcd/clientv3/concurrency.(*Election).observe()
      /home/jenkins/workspace/etcd-proxy/gopath/src/github.com/coreos/etcd/clientv3/concurrency/election.go:178 +0x461

[xiang90]

/cc @menghanl @MakMukhi

can you take a look?

[menghanl]

How about implementing GoString() on Stream?
It can return the pointer and the method name. Something like

func (s *Stream) GoString() string {
    return fmt.Sprintf("<stream: %p, %v>", s, s.method)
}

[heyitsanthony]

@menghanl OK, all fixed.

[menghanl]

Thanks. LGTM.

[xiang90]

@menghanl thank you so much for the quick review.

[xiang90]

@menghanl can we get a release soon? this currently blocks our CI to get things done :P

[menghanl]

@xiang90 Just did a v1.2.1 release on v1.2.x branch. This fix in included in that release.