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 #33792. Updates #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>
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