math/big: underflow panic in roundShortest

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Description

	f := new(big.Float).SetPrec(big.MaxPrec).SetFloat64(1)

	b := f.Append(nil, 'g', -1)

panic: underflow

goroutine 1 [running]:
math/big.nat.sub(0x0, 0x0, 0x0, 0x104402c0, 0x0, 0x6, 0x180010, 0x1, 0x1, 0x0, ...)
	/usr/local/go/src/math/big/nat.go:129 +0x3e0
math/big.roundShortest(0x10429f38, 0x10429f90)
	/usr/local/go/src/math/big/ftoa.go:195 +0x1c0
math/big.(*Float).Append(0x10429f90, 0x0, 0x0, 0x0, 0x10429f67, 0xffffffff, 0x104340b0, 0x371f, 0x0, 0x1040a130)
	/usr/local/go/src/math/big/ftoa.go:97 +0x680
main.main()
	/tmp/sandbox082194187/main.go:11 +0xc0

Contributor guide

Research direction

Investigate the roundShortest function in math/big/ftoa.go. The panic indicates an underflow in nat.sub when rounding the shortest decimal representation. Trace the logic for 'g' format with precision 1 and very high precision (MaxPrec). Consider if the issue is due to a missing check for zero mantissa or exponent underflow. Examine the conditions under which sub is called and ensure proper handling when the subtraction results in a borrow that leads to underflow.

Tech stack

go

Domain

backend

Issue type

Bug

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

Half day

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

Active

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

Clear

Prerequisites

Go

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

60