Race on global variable: factorials

[nasermirzaei89]

We get a RACE error on running tests in the production with the flag -race enabled.
And the stack shows it comes from here:

I believe that the reason is multiple goroutines are calling method ExpTaylor and this method manipulates the shared variable factorials (https://github.com/shopspring/decimal/blob/master/decimal.go#L84).
As you used a global variable for caching factorials (#229), it's not goroutine safe and should have a lock.

[mwoss]

Hi @nasermirzaei89! Thanks for bringing this issue up.
I was aware of this potential race condition problem during the implementation of ExpTaylor, but I though my lock-less approach would be sufficient for - https://github.com/shopspring/decimal/blob/master/decimal.go#L1089:L1099. I guess I was wrong, I would need to take a closer look on that 👀

[mqzabin]

We opted to implement our own Taylor Series calculator for exponents in the 1/n format (n being integer). The calculator has its own factorial and Taylor terms cache to avoid RCs/locks/globals. This approach works pretty well, since we work with fixed exponents.

I'm working on moving it to a public repo. Benchmarks are promising:

goos: linux
goarch: amd64
pkg: taylor-bench
cpu: 12th Gen Intel(R) Core(TM) i7-1265U
BenchmarkTaylor/taylor_series_rate_calculator_-_30_digits-12               21740             53317 ns/op           54368 B/op       1476 allocs/op
BenchmarkTaylor/shopspring_-_30_digits-12                                    220           4984208 ns/op         4963830 B/op      10377 allocs/op
PASS
ok      taylor-bench    3.404s

EDIT: This is the repo, but the generics usage is probably affecting the benchmarks.