big int panic on text conversion

[Poonai]

big int panics during text conversion randomly 

stack trace:

panic: runtime error: index out of range [1] with length 1

goroutine 2088184 [running]:
math/big.nat.itoa({0xc01db71500, 0x1, 0x199?}, 0x0, 0xa)
        /usr/local/go/src/math/big/natconv.go:340 +0x3d2
math/big.(*Int).Text(...)
        /usr/local/go/src/math/big/intconv.go:25
math/big.(*Int).String(...)
        /usr/local/go/src/math/big/intconv.go:40

It shows up randomly, help me debug

[Poonai]

I'm not able to reproduce as well

[Dan Kortschak]

I think you have probably ended up with a zero big.Int with a non-zero
length abs field. This is likely either from unsafe manipulation or a
race.

The crash can be replicated thus https://go.dev/play/p/WV78xvsbn4m

[Kurtis Rader]

Maybe provide a minimal reproducible example (https://stackoverflow.com/help/minimal-reproducible-example)?

While it is theoretically possible there is a bug in the `big` package that could result in such panics it is much more likely the bug is in your code. So showing us how you are using the `big` package will help us help you.

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Kurtis Rader

Caretaker of the exceptional canines Junior and Hank

[Brian Candler]

Have you tried running your entire code under the race detector?

https://go.dev/blog/race-detector

[Poonai]

Thanks all,

figured out the issue:

func main() {

    a := big.NewInt(1)

    b := *a

    c := *a

    c.Sub(&c, big.NewInt(1))

    fmt.Println(b.String())

    fmt.Println(c.String())

    fmt.Println(a.String())

}

reproducible code. don't know what causing it tho :(

[Dan Kortschak]

On Wed, 2024-02-14 at 02:07 -0800, Poonai wrote:
> Thanks all,
>
> figured out the issue:
>
> func main() {
>     a := big.NewInt(1)
>     b := *a
>     c := *a
>     c.Sub(&c, big.NewInt(1))
>     fmt.Println(b.String())
>     fmt.Println(c.String())
>     fmt.Println(a.String())
> }
>
>
> reproducible code. don't know what causing it tho :(

Minimised: https://go.dev/play/p/oL17vkcjaEl

This is happening because the sum in a doesn't result in a
normalisation of b, https://go.dev/play/p/cIBDbRXFnAT (in this
situation where the abs field represents a zero, it should be zero
length).

Given that this can happen without a race or unsafe modifications it
might be worth filing a issue for.

[Jan Mercl]

On Wed, Feb 14, 2024 at 12:14 PM 'Dan Kortschak' via golang-nuts
<golan...@googlegroups.com> wrote:

> Given that this can happen without a race or unsafe modifications it
> might be worth filing a issue for.

I think this is expected. Quoting from the big.Int docs
https://pkg.go.dev/math/big#Int

----
To "copy" an Int value, an existing (or newly allocated) Int must be
set to a new value using the Int.Set method; shallow copies of Ints
are not supported and may lead to errors.
----

But OP's 'b := *a' is creating a shallow copy.

[Dan Kortschak]

You are absolutely right. I looked for this kind of warning, but
obviously not hard enough.