encoding/json: Sequential Key Casing

[andreidorin13]

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

go version go1.9.3 linux/amd64

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

GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOROOT="/usr/lib/go"
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build924240465=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
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"

What did you do?

package main

import (
	"fmt"
	"encoding/json"
)

type Test struct {
	Event string `json:"v"`
	Bytes json.RawMessage `json:"B"`
}

func main() {
	string := []byte(`{"v": "Test", "V": 1234, "B": {"key": "value"}}`)
	var test Test
	json.Unmarshal(string, &test)
	fmt.Printf("%+v\n", test)
}

Link: https://play.golang.org/p/FNjS_tks7Ps

What did you expect to see?

{Event:Test Struct:[123 34 107 101 121 34 58 32 34 118 97 108 117 101 34 125]}

What did you see instead?

{Event:Test Struct:[]}

Why does the lower case "v", followed by an upper case "V", cause the byte array to be empty?

[artyom]

@andreidorin13 don't ignore error from json.Unmarshal — it's non-nil here: json: cannot unmarshal number into Go struct field Test.v of type string.

As I see it the real caveat here is presence of v and V attributes in json object, and the following documented behavior of json.Unmarshal:

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match.

Please see the illustration here: https://play.golang.org/p/A9OXJRX4LlL

I believe that in presence of two or more attributes whose names differ only by case the preferring an exact match clause of json.Unmarshal doesn't hold — it seem to always use the value from the last attribute it encounters.

[bradfitz]

Duplicate of #14750.

Closing this one.