encoding/json: marshaling a zero value encoding.TextMarshaler interface struct field panics

[wI2L]

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

$ go version
go version go1.13 darwin/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="/Users/will/Library/Caches/go-build"
GOENV="/Users/will/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/will/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/opt/go/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/opt/go/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/will/github/wI2L/jettison/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/rw/cyt9bjxx34qglfg2jsfm27jr0000gn/T/go-build698090917=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Given the following test program, that attempt to marshal a struct containing a nil instance of encoding.TextMarshaler, json.Marshal panic.
https://play.golang.org/p/ypv_Qj-QFOf

What did you expect to see?

The foollowing JSON string: {"M":null}

What did you see instead?

panic: interface conversion: interface is nil, not encoding.TextMarshaler [recovered]
	panic: interface conversion: interface is nil, not encoding.TextMarshaler

goroutine 1 [running]:
encoding/json.(*encodeState).marshal.func1(0x450f08, 0x8)
	/usr/local/go/src/encoding/json/encode.go:305 +0xc0
panic(0x137ca0, 0x43e3a0)
	/usr/local/go/src/runtime/panic.go:679 +0x240
encoding/json.textMarshalerEncoder(0x45a060, 0x1359e0, 0x40c158, 0x94, 0x100, 0x0)
	/usr/local/go/src/encoding/json/encode.go:485 +0xe0
encoding/json.structEncoder.encode(0x4580f0, 0x1, 0x1, 0x43e340, 0x45a060, 0x1342c0, 0x40c158, 0x99, 0x190100, 0x1342c0)
	/usr/local/go/src/encoding/json/encode.go:664 +0x340
encoding/json.(*encodeState).reflectValue(0x45a060, 0x1342c0, 0x40c158, 0x99, 0x20100, 0x4af2)
	/usr/local/go/src/encoding/json/encode.go:337 +0xa0
encoding/json.(*encodeState).marshal(0x45a060, 0x1342c0, 0x40c158, 0x100, 0x0, 0x0)
	/usr/local/go/src/encoding/json/encode.go:309 +0x120
encoding/json.Marshal(0x1342c0, 0x40c158, 0x1342c0, 0x40c158, 0x3, 0x0, 0x43c0c0, 0x1359e0)
	/usr/local/go/src/encoding/json/encode.go:161 +0x60
main.main()
	/tmp/sandbox165513751/prog.go:12 +0x60

Notes

The program reproducing the panic is an adaptation of an existing testcase of the encoding/json package, that can be found at the following line.

go/src/encoding/json/encode_test.go

Line 480 in 23f7398

{v: struct{ M Marshaler }{}, want: `{"M":null}`},

The marshalerEncoder function checks that the following type assertion succeeds, or write null.

go/src/encoding/json/encode.go

Line 447 in 23f7398

m, ok := v.Interface().(Marshaler)

If we rewrite the program reproducing the panic to use the json.Marshaler interface instead, we observe that the panic no longer occurs.
https://play.golang.org/p/cZVFIU7N216

The following line of the textMarshalerEncoder function should use the same check.

go/src/encoding/json/encode.go

Line 484 in 23f7398

m := v.Interface().(encoding.TextMarshaler)

I am surprised that the TestNilMarshal test is limited to the json.Marshal interface, and that the marshalerEncoder and textMarshalerEncoder functions are not equivalent in behavior. I can't think of ant reasonable reason for this, so I can only guess this is an unfortunate mistake.

The original issue that reported the panic for the json.Marshaler interface is #16042.

/cc @mvdan

[mvdan]

Doesn't seem like a recent regression. It looks like you've figured out the fix - want to send a CL?

[wI2L]

@mvdan Found it the same way as the issue in #33675, by writing some tests for my custom JSON encoder. I guess there's good sides on rewriting the wheel.
I'll send a CL asap.
Edit: What do you think of duplicating the test cases of TestNilMarshaler for encoding.TextMarshaler while we're at it ?

[gopherbot]

Change https://golang.org/cl/194642 mentions this issue: encoding/json: encode nil encoding.TextMarshaler instance as "null"