encoding/json: marshaling a zero value encoding.TextMarshaler interface struct field panics #34235
Description
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
The marshalerEncoder function checks that the following type assertion succeeds, or write null.
go/src/encoding/json/encode.go
Line 447 in 23f7398
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
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