encoding/json: Inconsistent results with pointers and duplicated keys

[elwinar]

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

$ go version
go version go1.12.4 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
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/elwinar/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/elwinar/.go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build786320136=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/8ELiG37J-JI

What did you expect to see?

{<nil>} <nil>
{<nil>} <nil>

What did you see instead?

{0x40c1e8} <nil>
{<nil>} <nil>

---

As mentioned is dvyukov/go-fuzz-corpus#3, although the result itself is valid stricto sensu, it is inconsistent. We would expect either the first or the last indirection to be always nil, while it seems to depends on the previous value of the field.

[elwinar]

CC @mvdan @josharian .

[gopherbot]

Change https://golang.org/cl/174699 mentions this issue: encoding/json: check the type of a value when indirecting it