encoding/json: unmarshaler doesn't find UnmarshalJSON on embedded anonymous struct

[fancl20]

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

$ go version
go version go1.15.9 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go-1.15"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.15/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build393930481=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/Xlxih58rSdC

Outer1 unmarshalled and Outer2 failed.

type Outer1 struct {
    Inner inner
}
type inner struct{ Mixin }

type Outer2 struct {
    Inner struct{ Mixin }
}

type Mixin string

func (r * Mixin) UnmarshalJSON(data []byte) error {
    return json.Unmarshal(data, (*string)(r))
}

What did you expect to see?

Outer2 unmarshalled as Outer1

What did you see instead?

panic: json: cannot unmarshal string into Go struct field Outer2.Inner of type struct { main.Mixin }

[mvdan]

At first glance this seems like a bug, thanks. There have been many bugs in the past involving struct embedding, but after a quick search I don't think this is a duplicate. Want to try working on a fix with a test?

cc @dsnet

[googollee]

It's because json.indirect() only tries to get a field address with a named type. See https://cs.opensource.google/go/go/+/refs/tags/go1.16.4:src/encoding/json/decode.go;l=443.

The type of Outer2.Inner is a struct without a name, so json.indirect() doesn't try its pointer and of course it's not compatible with json.UnmarshalJSONinterface.

I think at the entrance of json.indirect(), checking v.CanAddr() is enough. It needs not to check if v is a named type.

[dsnet]

@googollee, thank you for your analysis. I agree with your assessment of why this isn't triggering. It seems that this is a bug that's existed for 10 years :)

We could try fixing this for the Go1.18 release cycle, but I'm always wary of how fixing bugs breaks a number of users who already depend on the current behavior.

[mvdan]

We could always merge this early in the 1.18 cycle, and see if any users complain. I would hope that noone is depending on the existing inconsistent behavior - I've been wrong on that assumption before, but not always :)

[gopherbot]

Change https://golang.org/cl/324469 mentions this issue: encoding/json: call UnmarshalJSON() when decoding with a unamed struct.

[googollee]

Removing the check of named structs looses the limitation. I think it should be fine with existing codes.

I made a change with a unit test.