metsw24-max opened a new pull request, #50058:
URL: https://github.com/apache/arrow/pull/50058
### Rationale for this change
`Decimal{32,64,128,256}::FromString` parse the exponent as `int32_t`, so a
string like `0E-2147483648` reaches
`DecimalFromString`/`SimpleDecimalFromString` with `dec.exponent == INT32_MIN`
and negating it is UB:
```
decimal.cc: runtime error: negation of -2147483648 cannot be represented in
type 'int32_t' (aka 'int')
```
A near-`INT32_MIN` exponent overflows the `-exponent + fractional_digits`
addition the same way. Both helpers are reachable from the CSV/JSON readers
parsing decimal columns.
### What changes are included in this PR?
Compute the scale via `internal::SubtractWithOverflow` and reject the
unrepresentable exponents with `Status::Invalid`, in both helpers.
### Are these changes tested?
Yes, `0E-2147483648` and `1.0E-2147483647` added to
`DecimalFromStringTest.InvalidInput` (runs for Decimal32/64/128/256).
### Are there any user-facing changes?
No.
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