encoding/binary: panic when Write a nil.

[laushunyu]

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

$ go version
go version go1.20.5 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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/macoo/.cache/go-build"
GOENV="/home/macoo/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/macoo/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/macoo/go"
GOPRIVATE=""
GOPROXY="https://goproxy.cn,direct"
GOROOT="/home/macoo/go/go1.20.5"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/macoo/go/go1.20.5/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.5"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/macoo/GolandProjects/binbug/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3159415642=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"bytes"
	"crypto/md5"
	"encoding/binary"
	"fmt"
)

type MyStruct struct {
	Name string
}

func main() {
	var a *MyStruct
	hash := md5.New()
	binary.Write(bytes.NewBuffer(nil), binary.LittleEndian, a)
	fmt.Printf("%x", hash.Sum(nil))
}

then:

$ go run .

What did you expect to see?

nothing will be write to bytes.NewBuffer(nil)

What did you see instead?

panic: reflect: call of reflect.Value.Type on zero Value

goroutine 1 [running]:
reflect.Value.typeSlow({0x0?, 0x0?, 0x524400?})
        /home/macoo/go/go1.20.5/src/reflect/value.go:2610 +0x12e
reflect.Value.Type(...)
        /home/macoo/go/go1.20.5/src/reflect/value.go:2605
encoding/binary.dataSize({0x0?, 0x0?, 0xc00004a5c0?})
        /home/macoo/go/go1.20.5/src/encoding/binary/binary.go:490 +0x17f
encoding/binary.Write({0x4c06a8, 0xc00007e150}, {0x4c0ae8, 0x567230}, {0x48dd80?, 0x0})
        /home/macoo/go/go1.20.5/src/encoding/binary/binary.go:448 +0xea5
main.main()
        /home/macoo/GolandProjects/binbug/main.go:17 +0xb5

[laushunyu]

The struct pointed to by the data pointer may be acceptable by dataSize,
but since this pointer does not point to any data, no data should be written to it and no error should be returned(maybe?).

[gopherbot]

Change https://go.dev/cl/503695 mentions this issue: encoding/binary: fix panic when Write a nil

[bcmills]

The documentation at https://pkg.go.dev/encoding/binary#Write says:

Data must be a fixed-size value or a slice of fixed-size values, or a pointer to such data.

The nil interface value is not any of those things, so this is a user error that should have been caught during testing and/or code review; a panic is appropriate in that case.

[bcmills]

but since this pointer does not point to any data, no data should be written to it and no error should be returned(maybe?).

The nil interface value is not a pointer.

[ianlancetaylor]

The standard library does not do explicit checks for all possible incorrect values. Panicking on an invalid value is acceptable behavior.