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encoding/json: embedded *struct implementing UnmarshalJSON is nil and panics on Unmarshal #36716

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sabey opened this issue Jan 23, 2020 · 1 comment

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@sabey
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@sabey sabey commented Jan 23, 2020

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

$ go version
go version go1.13.6 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="/home/jackson/.cache/go-build"
GOENV="/home/jackson/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/jackson/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/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-build498302187=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/DZoQ9ro3jv7
my parent struct inherits UnmarshalJSON(data []byte) error from the embedded struct

https://github.com/golang/go/blob/master/src/encoding/json/decode.go#L492
in the indirect() function
if u, ok := v.Interface().(Unmarshaler); ok { is true, but since the embedded struct is not allocated it will cause a nil pointer exception

encoding/json.(*decodeState).object(0xc0002ccd10, 0x5e9bc0, 0xc000132f98, 0x16, 0xc0002ccd38, 0xc0003abe7b)
/usr/local/go/src/encoding/json/decode.go:667 +0x220b

What did you expect to see?

&{&{}}

What did you see instead?

&{<nil>}

Unmarshal should allocate the embedded struct so that we can call UnmarshalJSON()
this would only be relevant if there were only 1 embedded struct which implements UnmarshalJSON

@toothrot

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@toothrot toothrot commented Jan 23, 2020

Thanks for the thoughtful issue report. This is a duplicate of #30148. Feel free to discuss there if you have more to contribute.

@toothrot toothrot closed this Jan 23, 2020
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