tree cc601ce584e4b7a2dec82a685cc543f700873810
parent 7f1ef49347e6e1f5a51c9ca3e4f4f617c2317042
author Robert Griesemer <gri@golang.org> 1571874291 -0700
committer Dmitri Shuralyov <dmitshur@golang.org> 1590620006 +0000

[release-branch.go1.13] math/big: make Rat.Denom side-effect free

A Rat is represented via a quotient a/b where a and b are Int values.
To make it possible to use an uninitialized Rat value (with a and b
uninitialized and thus == 0), the implementation treats a 0 denominator
as 1.

Rat.Num and Rat.Denom return pointers to these values a and b. Because
b may be 0, Rat.Denom used to first initialize it to 1 and thus produce
an undesirable side-effect (by changing the Rat's denominator).

This CL changes Denom to return a new (not shared) *Int with value 1
in the rare case where the Rat was not initialized. This eliminates
the side effect and returns the correct denominator value.

While this is changing behavior of the API, the impact should now be
minor because together with (prior) CL https://golang.org/cl/202997,
which initializes Rats ASAP, Denom is unlikely used to access the
denominator of an uninitialized (and thus 0) Rat. Any operation that
will somehow set a Rat value will ensure that the denominator is not 0.

Fixes #36689.
For #33792.
For #3521.

Change-Id: I0bf15ac60513cf52162bfb62440817ba36f0c3fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/203059
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/233323
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>