encoding/json: fails to ignore fields with more than one ignore tag

[siennathesane]

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

$ go version
go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

Yes. I can replicate this with go1.10.3 linux/amd64 and go1.10.2 windows/amd64.

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

GOARCH="amd64"
GOBIN="/mnt/p/go//bin"
GOCACHE="/home/mike/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/mnt/p/go/"
GORACE=""
GOROOT="/home/mike/.linuxbrew/Cellar/go/1.10.3/libexec"
GOTMPDIR=""
GOTOOLDIR="/home/mike/.linuxbrew/Cellar/go/1.10.3/libexec/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc-5"
CXX="g++-5"
CGO_ENABLED="1"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build284313999=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"github.com/globalsign/mgo/bson"
	"fmt"
	"encoding/json"
)

type T1 struct {
	Foo chan string `json:"-",bson:"-"`
}

type T2 struct {
	Bar chan string `bson:"-",json:"-"`
}

type T3 struct {
	Foo chan string `bson:"-"`
}

type T4 struct {
	Bar chan string `json:"-"`
}

func main() {
	_, err := bson.Marshal(&T1{Foo: make(chan string, 0)})
	if err != nil {
		fmt.Printf("json and bson fails due to multiple ignored tags: %s\n", err)
	}
	_, err = json.Marshal(&T2{Bar: make(chan string, 0)})
	if err != nil {
		fmt.Printf("bson and json fails due to multiple ignored tags: %s\n", err)
	}
	_, err = bson.Marshal(&T3{Foo: make(chan string, 0)})
	if err == nil {
		fmt.Println("bson passes due to one ignore tag.")
	}
	_, err = json.Marshal(&T4{Bar: make(chan string, 0)})
	if err == nil {
		fmt.Println("json passes due to one ignore tag.")
	}
}

What did you expect to see?

All four use cases should work. Struct field tags denoted by "-" should be ignored by all marshallers.

What did you see instead?

json and bson fails due to multiple ignored tags: Can't marshal chan string in a BSON document
bson and json fails due to multiple ignored tags: json: unsupported type: chan string
bson passes due to one ignore tag.
json passes due to one ignore tag.

My use case is having types pull double-duty as both MongoDB objects as well as JSON objects, but I want to be able to ignore unsupported types, such as channels, with struct tags and have both struct tags be ignored properly.

[josharian]

Try using a space to separate the struct tags instead of a comma. See https://golang.org/pkg/reflect/#StructTag.

[urandom]

This is another illustration of how confusing unstructured tags are: #23637

[FiloSottile]

The code works as expected when using spaces to separate tags. https://play.golang.org/p/0Fa7VU60y0G

[siennathesane]

Thanks @josharian & @FiloSottile, didn't realise it had to be spaces, thanks for the doc references as well!