math/big: Rat.Denom has side effects

[ericlagergren]

From here: ericlagergren/decimal#138

My package divides (*Rat).Num by (*Rat).Denom to produce a decimal number. It turns out that (*Rat).Denom lazily sets its denominator, meaning it cannot be used concurrently.

Since the receiver is x not z, I assumed that the method simply read the denominator.

The receiver name should be updated to indicate side effects and/or the documentation for big.Rat should mention this side effect. I assume that the 7 years this has spent in the tree is too long to be reverted 😄. And, at any rate, not materializing the denominator can save allocations for whole numbers, which is a good thing.

I can send a CL if you want.

[bcmills]

CC @griesemer

[griesemer]

@ericlagergren Please send a CL. Assign to me for review. Thanks.

[gopherbot]

Change https://golang.org/cl/192017 mentions this issue: math/big: document that Rat.Denom might modify the receiver

[griesemer]

Reopening due to #34919
Should Denom() be allowed to have side-effects in the first place?
If we change this, we may break existing programs. See also #3521 and the respective test cases.

[gopherbot]

Change https://golang.org/cl/201205 mentions this issue: math/big: make Rat accessors safe for concurrent use