math/big: panic in big.Rat.SetString (`panic:impossible`)

[pventuzelo]

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

$ go version
go version go1.15.2 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=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/scop/.cache/go-build"
GOENV="/home/scop/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/scop/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/scop/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go-1.15"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.15/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
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-build861159730=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import (
	"math/big"
)

func main() {
	data := "9.9999999999999999999990000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000004000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
	new(big.Rat).SetString(data)
}

https://play.golang.org/p/NNhU-zb5KG8

What did you expect to see?

This should not panic.
Based on SetString documentation, If the operation failed, the value of z should be undefined and the returned value is nil.

What did you see instead?

panic: impossible

goroutine 1 [running]:
math/big.nat.divRecursiveStep(0xc0000d01c0, 0x33, 0x37, 0xc0000ea500, 0x96, 0x9c, 0xc0000ec000, 0x64, 0x1c9, 0x0, ...)
	/usr/local/go-faketime/src/math/big/nat.go:951 +0x139a
math/big.nat.divRecursive(0xc0000d01c0, 0x33, 0x37, 0xc0000ea500, 0x97, 0x9c, 0xc0000ec000, 0x64, 0x1c9)
	/usr/local/go-faketime/src/math/big/nat.go:828 +0x17e
math/big.nat.divLarge(0xc0000b20c0, 0x2, 0x6, 0xc0000ea500, 0x97, 0x9c, 0xc0000e3400, 0x96, 0x9b, 0xc0000e3900, ...)
	/usr/local/go-faketime/src/math/big/nat.go:727 +0x412
math/big.nat.div(0xc0000b20c0, 0x2, 0x6, 0xc0000ea500, 0x97, 0x9c, 0xc0000e3400, 0x96, 0x9b, 0xc0000e3900, ...)
	/usr/local/go-faketime/src/math/big/nat.go:672 +0x410
math/big.(*Int).QuoRem(0xc000093ca0, 0xc000093d20, 0xc000093d00, 0xc000093c80, 0xc000093ca0, 0xc000093c80)
	/usr/local/go-faketime/src/math/big/int.go:239 +0xbf
math/big.euclidUpdate(0xc000093d20, 0xc000093d00, 0x0, 0x0, 0xc000093ca0, 0xc000093c80, 0xc000093c60, 0xc000093c40, 0xc0000c6200)
	/usr/local/go-faketime/src/math/big/int.go:648 +0x67
math/big.(*Int).lehmerGCD(0xc000093df0, 0x0, 0x0, 0xc000093f38, 0xc000093f58, 0x168)
	/usr/local/go-faketime/src/math/big/int.go:718 +0x49c
math/big.(*Rat).norm(0xc000093f38, 0x6a)
	/usr/local/go-faketime/src/math/big/rat.go:445 +0x151
math/big.(*Rat).SetString(0xc00009ef38, 0x4dcfe3, 0xb5e, 0x563c40, 0xc000032778)
	/usr/local/go-faketime/src/math/big/ratconv.go:198 +0x6c7
main.main()
	/tmp/sandbox085893432/prog.go:9 +0x57

[ALTree]

Same as #42532.

[pventuzelo]

Oh sorry for the duplicate, problem with github.