encoding/json: Unmarshalling *websocket.Conn over a websocket with JSON crashes

[dolanor]

1. What version of Go are you using (go version)?
   go version go1.6 linux/amd64
2. What operating system and processor architecture are you using (go env)?
   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="linux"
   GOOS="linux"
   GOPATH="/home/dolanor/.local/gopath/"
   GORACE=""
   GOROOT="/usr/local/go"
   GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
   GO15VENDOREXPERIMENT="1"
   CC="gcc"
   GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
   CXX="g++"
   CGO_ENABLED="1"
3. What did you do?
   I sent a *websocket.Conn on a websocket in JSON (by pure laziness, without caring about having it on the other side).
4. What did you expect to see?
   I expected to get unmarshalling kind of working, even if the *websocket.Conn wouldn't work: I didn't really wanted to get this information on the other side, but it was part of a struct I wanted to send over the wire.

If I put this *websocket.Conn to nil, then I don't get problems on the other side.

1. What did you see instead?

    panic: reflect: reflect.Value.Set using value obtained using unexported field [recovered]
        panic: interface conversion: string is not error: missing method Error

    goroutine 7 [running]:
    panic(0x752c40, 0xc820010680)
        /usr/local/go/src/runtime/panic.go:464 +0x3e6
    encoding/json.(*decodeState).unmarshal.func1(0xc8200dbd98)
        /usr/local/go/src/encoding/json/decode.go:156 +0xba
    panic(0x6c2ba0, 0xc8200e8650)
        /usr/local/go/src/runtime/panic.go:426 +0x4e9
    reflect.flag.mustBeAssignable(0x1d4)
        /usr/local/go/src/reflect/value.go:226 +0x161
    reflect.Value.Set(0x762fa0, 0xc82008c430, 0x1d4, 0x762fa0, 0xc8200e8640, 0x94)
        /usr/local/go/src/reflect/value.go:1328 +0x25
    encoding/json.(*decodeState).literalStore(0xc820106000, 0xc8201040f9, 0x4, 0x507, 0x762fa0, 0xc82008c430, 0x1d4, 0xc8200da700)
        /usr/local/go/src/encoding/json/decode.go:786 +0x3549
    encoding/json.(*decodeState).literal(0xc820106000, 0x762fa0, 0xc82008c430, 0x1d4)
        /usr/local/go/src/encoding/json/decode.go:717 +0xe9
    encoding/json.(*decodeState).value(0xc820106000, 0x762fa0, 0xc82008c430, 0x1d4)
        /usr/local/go/src/encoding/json/decode.go:370 +0x37b
    encoding/json.(*decodeState).object(0xc820106000, 0x7a7320, 0xc82008c3f0, 0x199)
        /usr/local/go/src/encoding/json/decode.go:684 +0x116a
    encoding/json.(*decodeState).value(0xc820106000, 0x7a9260, 0xc820010648, 0x196)
        /usr/local/go/src/encoding/json/decode.go:367 +0x3a1
    encoding/json.(*decodeState).object(0xc820106000, 0x795d40, 0xc820010640, 0x199)
        /usr/local/go/src/encoding/json/decode.go:684 +0x116a
    encoding/json.(*decodeState).value(0xc820106000, 0x7a19e0, 0xc8200c5da0, 0x196)
        /usr/local/go/src/encoding/json/decode.go:367 +0x3a1
    encoding/json.(*decodeState).array(0xc820106000, 0x6c3220, 0xc8200c5d60, 0x197)
        /usr/local/go/src/encoding/json/decode.go:518 +0xa6b
    encoding/json.(*decodeState).value(0xc820106000, 0x6c3220, 0xc8200c5d60, 0x197)
        /usr/local/go/src/encoding/json/decode.go:364 +0x3c1
    encoding/json.(*decodeState).object(0xc820106000, 0x741a80, 0xc8200c5d60, 0x199)
        /usr/local/go/src/encoding/json/decode.go:684 +0x116a
    encoding/json.(*decodeState).value(0xc820106000, 0x6a6400, 0xc820056b78, 0x196)
        /usr/local/go/src/encoding/json/decode.go:367 +0x3a1
    encoding/json.(*decodeState).object(0xc820106000, 0x79e600, 0xc820056b60, 0x199)
        /usr/local/go/src/encoding/json/decode.go:684 +0x116a
    encoding/json.(*decodeState).value(0xc820106000, 0x6a61c0, 0xc820056b60, 0x16)
        /usr/local/go/src/encoding/json/decode.go:367 +0x3a1
    encoding/json.(*decodeState).unmarshal(0xc820106000, 0x6a61c0, 0xc820056b60, 0x0, 0x0)
        /usr/local/go/src/encoding/json/decode.go:168 +0x196
    encoding/json.Unmarshal(0xc820104000, 0x23b, 0x600, 0x6a61c0, 0xc820056b60, 0x0, 0x0)
        /usr/local/go/src/encoding/json/decode.go:96 +0x12b
    golang.org/x/net/websocket.jsonUnmarshal(0xc820104000, 0x23b, 0x600, 0x201, 0x6a61c0, 0xc820056b60, 0x0, 0x0)
        /home/dolanor/.local/gopath/src/golang.org/x/net/websocket/websocket.go:390 +0x57
    golang.org/x/net/websocket.Codec.Receive(0x861560, 0x861568, 0xc82008c240, 0x6a61c0, 0xc820056b60, 0x0, 0x0)
        /home/dolanor/.local/gopath/src/golang.org/x/net/websocket/websocket.go:331 +0x34d

[mikioh]

I have no strong opinion on how unexposed, non-assignable fields should be treated in encoding/json package.

[OneOfOne]

the json package maybe should just check if a field is exported or not for the internal type cache?

[bradfitz]

I am unable to reproduce this.

Do you have a repro?

I tried:

package main

import (
        "encoding/json"
        "fmt"
        "log"

        "golang.org/x/net/websocket"
)

func main() {
        type T struct {
                Foo  string
                conn *websocket.Conn
        }
        t := &T{"bar", new(websocket.Conn)}
        enc, err := json.Marshal(t)
        if err != nil {
                log.Fatalf("Marshal: %v", err)
        }
        var t2 T
        if err := json.Unmarshal(enc, &t2); err != nil {
                log.Fatalf("Unmarshal: %v", err)
        }
        fmt.Printf("%#v\n", t2)
}

But it works at tip, Go 1.6, and Go 1.5 just fine with:

main.T{Foo:"bar", conn:(*websocket.Conn)(nil)}

Let me know and I can reopen this.