math/big: UnmarshalText `<nil>` to *big.Float, *big.Int, *big.Rat caused panic

[hopehook]

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

$ go version
go version devel go1.18-8db9e1cebf Sat Feb 12 17:37:33 2022 +0800 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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/hopehook/Library/Caches/go-build"
GOENV="/Users/hopehook/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/hopehook/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/hopehook/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/hopehook/go-google/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/hopehook/go-google/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="devel go1.18-8db9e1cebf Sat Feb 12 17:37:33 2022 +0800"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/xc/x_k35ynn063c15716c0d_5pm0000gn/T/go-build3768576566=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

• Encoding *big.Float, *big.Int, *big.Rat nil pointers with MarshalText
• Unmarshal text <nil> to *big.Float, *big.Int, *big.Rat nil pointers with UnmarshalText

For example:
https://go.dev/play/p/oViLT_0fgBu

What did you expect to see?

• Encoding *big.Rat nil pointer will return text <nil>

  • Because of encoding *big.Float, *big.Int nil pointers with MarshalText will return the text <nil>

• Unmarshal text <nil> to *big.Float, *big.Int, *big.Rat nil pointers with UnmarshalText not panic

  • For consistency and reliability, UnmarshalText <nil> should also work fine, not panic.

What did you see instead?

panic like this:

• Encoding *big.Rat nil pointer with MarshalText

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x109abb8]

goroutine 1 [running]:
math/big.(*Rat).MarshalText(0xc000106f30?)
	/Users/hopehook/go-google/go/src/math/big/ratmarsh.go:62 +0x18
main.main()
	/Users/hopehook/lab/go/src/hello/main.go:13 +0x7e

• Unmarshal text <nil> to *big.Float nil pointer with UnmarshalText

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x108dc06]

goroutine 1 [running]:
math/big.(*Float).scan(0x0, {0x10cf578, 0xc000130000}, 0x0)
	/Users/hopehook/go-google/go/src/math/big/floatconv.go:34 +0x46
math/big.(*Float).Parse(0x0, {0xc000128007, 0x5}, 0x114af00?)
	/Users/hopehook/go-google/go/src/math/big/floatconv.go:273 +0x156
math/big.(*Float).UnmarshalText(0x114af00?, {0xc000128002, 0x5, 0x5})
	/Users/hopehook/go-google/go/src/math/big/floatmarsh.go:119 +0x4b
main.main()
	/Users/hopehook/lab/go/src/hello/main.go:11 +0x46

[gopherbot]

Change https://go.dev/cl/390174 mentions this issue: math/big: unmarshal text to struct big.Float caused panic

[odeke-em]

@hopehook thanks for filing this issue and thanks for the patience this long. So a few things:

MarshalText

• invoking (*big).(Float, Int, Rat)(nil).MarshalText() is invoking a method on a nil pointer: why shouldn't this panic? It should
• invoking (*big).(Float, Int, Rat)(nil).UnmarshalText on a nil pointer should also panic, why would you set to a nil pointer?

With the above, I don't think that we should do anything because the requests that you are asking are for special exceptions that differ from the norm of dereferencing objects, and this can cause unexpected results. Kindly please perhaps help me ith some rationale for why you need this exception, thank you!